LTC3868 [Linear]

Low IQ, Dual 2-Phase Synchronous Step-Down Controller; 低IQ ,双两相同步降压型控制器
LTC3868
型号: LTC3868
厂家: Linear    Linear
描述:

Low IQ, Dual 2-Phase Synchronous Step-Down Controller
低IQ ,双两相同步降压型控制器

控制器
文件: 总38页 (文件大小:574K)
中文:  中文翻译
下载:  下载PDF数据表文档文件
LTC3868  
Low I , Dual  
Q
2-Phase Synchronous  
Step-Down Controller  
FeaTures  
DescripTion  
Theꢀ LTC®3868ꢀ isꢀ aꢀ highꢀ performanceꢀ dualꢀ step-downꢀ  
switchingregulatorcontrollerthatdrivesallN-channelꢀ  
synchronouspowerMOSFETstages.Aconstantfrequencyꢀ  
currentmodearchitectureallowsaphase-lockablefre-  
quencyꢀofꢀupꢀtoꢀ850kHz.ꢀPowerꢀlossꢀandꢀnoiseꢀdueꢀtoꢀtheꢀ  
inputꢀcapacitorꢀESRꢀareꢀminimizedꢀbyꢀoperatingꢀtheꢀtwoꢀ  
controllerꢀoutputsꢀoutꢀofꢀphase.  
n
Low Operating I : 170µA (One Channel On)  
Q
n
n
n
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Wide Output Voltage Range: 0.8V ≤ V  
≤ 14V  
OUT  
Wide V Range: 4V to 24V  
IN  
R  
or DCR Current Sensing  
SENSE  
ꢀ Out-of-PhaseꢀControllersꢀReduceꢀRequiredꢀInputꢀ  
CapacitanceꢀandꢀPowerꢀSupplyꢀInducedꢀNoise  
®
n
n
n
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ꢀ OPTI-LOOP ꢀCompensationꢀMinimizesꢀC  
OUT  
ꢀ Phase-LockableꢀFrequencyꢀ(75kHzꢀtoꢀ850kHz)  
ꢀ ProgrammableꢀFixedꢀFrequencyꢀ(50kHzꢀtoꢀ900kHz)  
ꢀ SelectableꢀContinuous,ꢀPulse-Skippingꢀorꢀꢀ  
BurstꢀMode®ꢀOperationꢀatꢀLightꢀLoads  
Theꢀ170μAꢀno-loadꢀquiescentꢀcurrentꢀextendsꢀoperatingꢀ  
lifeꢀinꢀbattery-poweredꢀsystems.ꢀOPTI-LOOPꢀcompensa-  
tionꢀallowsꢀtheꢀtransientꢀresponseꢀtoꢀbeꢀoptimizedꢀoverꢀ  
aꢀwideꢀrangeꢀofꢀoutputꢀcapacitanceꢀandꢀESRꢀvalues.ꢀTheꢀ  
LTC3868ꢀfeaturesꢀaꢀprecisionꢀ0.8Vꢀreferenceꢀandꢀaꢀpowerꢀ  
goodoutputindicator.Awide4Vto24Vinputsupplyrangeꢀ  
encompassesꢀaꢀwideꢀrangeꢀofꢀintermediateꢀbusꢀvoltagesꢀ  
andꢀbatteryꢀchemistries.  
n
n
n
n
n
n
n
n
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ꢀ VeryꢀLowꢀDropoutꢀOperation:ꢀ99%ꢀDutyꢀCycle  
ꢀ AdjustableꢀOutputꢀVoltageꢀSoft-Start  
ꢀ PowerꢀGoodꢀOutputꢀVoltageꢀMonitor  
ꢀ OutputꢀOvervoltageꢀProtection  
ꢀ OutputꢀLatchoffꢀProtectionꢀDuringꢀShortꢀCircuit  
Independentꢀsoft-startꢀpinsꢀforꢀeachꢀcontrollerꢀrampꢀtheꢀ  
outputꢀvoltagesꢀduringꢀstart-up.ꢀCurrentꢀfoldbackꢀlimitsꢀ  
MOSFETꢀheatꢀdissipationꢀduringꢀshort-circuitꢀconditions.ꢀ  
Theꢀoutputꢀshort-circuitꢀlatchoffꢀfeatureꢀfurtherꢀprotectsꢀ  
theꢀcircuitꢀinꢀshort-circuitꢀconditions.  
ꢀ LowꢀShutdownꢀI :ꢀ8µA  
Q
ꢀ InternalꢀLDOꢀPowersꢀGateꢀDriveꢀfromꢀV ꢀorꢀEXTV  
IN  
CC  
ꢀ NoꢀCurrentꢀFoldbackꢀDuringꢀStart-Up  
ꢀ Smallꢀ5mmꢀ×ꢀ5mmꢀQFNꢀPackage  
applicaTions  
Forꢀaꢀleadedꢀ28-leadꢀSSOPꢀpackageꢀwithꢀaꢀfixedꢀcurrentꢀ  
limitꢀandꢀoneꢀPGOODꢀoutput,ꢀwithoutꢀphaseꢀmodulationꢀ  
orꢀaꢀclockꢀoutput,ꢀseeꢀtheꢀLTC3868-1ꢀdataꢀsheet.ꢀ  
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ꢀ NotebookꢀandꢀPalmtopꢀComputers  
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ꢀ PortableꢀInstruments  
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L,LT,LTC,LTM,BurstMode,OPTI-LOOP,PolyPhase,µModule,LinearTechnologyandtheLinearꢀ  
ꢀ BatteryꢀOperatedꢀDigitalꢀDevices  
logoareregisteredtrademarksandNoR ꢀandUltraFastaretrademarksofLinearTechnologyꢀ  
SENSE  
n
ꢀ DistributedꢀDCꢀPowerꢀSystems  
Corporation.Allothertrademarksarethepropertyoftheirrespectiveowners.ProtectedbyU.S.ꢀ  
Patents,including5481178,5705919,5929620,6100678,6144194,6177787,6304066,6580258.  
Typical applicaTion  
High Efficiency Dual 8.5V/3.3V Step-Down Converter  
V
Efficiency and Power Loss  
IN  
9V TO 24V  
22µF  
50V  
vs Load Current  
4.7µF  
V
INTV  
CC  
100  
90  
10000  
1000  
100  
10  
IN  
TG1  
TG2  
0.1µF  
0.1µF  
BOOST1  
SW1  
BOOST2  
SW2  
3.3µH  
7.2µH  
80  
EFFICIENCY  
70  
BG1  
BG2  
60  
50  
LTC3868  
POWER LOSS  
PGND  
+
+
40  
30  
20  
10  
0
SENSE1  
SENSE1  
SENSE2  
0.01Ω  
193k  
0.007Ω  
1
V
8.5V  
3.5A  
SENSE2  
OUT2  
V
V
IN  
V
= 12V  
= 3.3V  
OUT1  
3.3V  
5A  
V
V
FB1  
FB2  
OUT  
FIGURE 12 CIRCUIT  
62.5k  
I
I
TH2  
TH1  
SS1  
0.1  
150µF  
680pF  
15k  
680pF  
150µF  
SGND  
SS2  
0.0001 0.001  
0.01  
0.1 10  
1
20k  
OUTPUT CURRENT (A)  
20k  
15k  
0.1µF  
0.1µF  
3868 TA01b  
3868 TA01  
3868fb  
LTC3868  
absoluTe MaxiMuM raTings  
pin conFiguraTion  
(Note 1)  
TOP VIEW  
InputꢀSupplyꢀVoltageꢀ(V )......................... –0.3Vꢀtoꢀ28V  
IN  
TopsideꢀDriverꢀVoltagesꢀ  
ꢀ BOOST1,ꢀBOOST2ꢀ................................. –0.3Vꢀtoꢀ34V  
SwitchꢀVoltageꢀ(SW1,ꢀSW2)ꢀꢀ........................ –5Vꢀtoꢀ28V  
(BOOST1-SW1),ꢀ(BOOST2-SW2)ꢀ................ –0.3Vꢀtoꢀ6V  
RUN1,ꢀRUN2................................................ –0.3Vꢀtoꢀ8V  
ꢀ MaximumꢀCurrentꢀSourcedꢀintoꢀPinꢀfromꢀ  
32 31 30 29 28 27 26 25  
SENSE1  
FREQ  
1
2
3
4
5
6
7
8
24 BOOST1  
23 BG1  
PHASMD  
CLKOUT  
PLLIN/MODE  
SGND  
V
IN  
22  
21  
PGND  
33  
SGND  
20 EXTV  
ꢀ Sourceꢀ>8Vꢀ......................................................100µA  
CC  
CC  
+
+
INTV  
19  
18 BG2  
17 BOOST2  
SENSE1 ,ꢀSENSE2 ,ꢀSENSE1  
RUN1  
SENSE2 ꢀVoltages...................................... –0.3Vꢀtoꢀ16V  
RUN2  
PLLIN/MODE,ꢀFREQꢀVoltagesꢀꢀ.............. –0.3VꢀtoꢀINTV  
CC  
CC  
9
10 11 12 13 14 15 16  
I
,ꢀPHASMDꢀVoltagesꢀꢀ....................... –0.3VꢀtoꢀINTV  
LIM  
EXTV ꢀ...................................................... –0.3Vꢀtoꢀ14V  
CC  
I
,ꢀI ,V ,ꢀV ꢀVoltagesꢀ...................... –0.3Vꢀtoꢀ6V  
TH1 TH2 FB1 FB2  
UH PACKAGE  
32-LEAD (5mm s 5mm) PLASTIC QFN  
PGOOD1,ꢀPGOOD2ꢀVoltagesꢀ....................... –0.3Vꢀtoꢀ6V  
T ꢀ=ꢀ125°C,ꢀθ ꢀ=ꢀ34°C/W  
JMAX JA  
SS1,ꢀSS2,ꢀINTV ꢀVoltagesꢀ......................... –0.3Vꢀtoꢀ6V  
CC  
EXPOSEDꢀPADꢀ(PINꢀ33)ꢀISꢀSGND,ꢀMUSTꢀBEꢀSOLDEREDꢀTOꢀPCB  
OperatingꢀTemperatureꢀRangeꢀ(Noteꢀ2)ꢀ... –40°Cꢀtoꢀ85°C  
JunctionꢀTemperatureꢀ(Noteꢀ3)ꢀ............................. 125°C  
StorageꢀTemperatureꢀRange................... –65°Cꢀtoꢀ150°C  
orDer inForMaTion  
LEAD FREE FINISH  
LTC3868EUH#PBF  
LTC3868IUH#PBF  
TAPE AND REEL  
PART MARKING*  
3868  
PACKAGE DESCRIPTION  
TEMPERATURE RANGE  
–40°Cꢀtoꢀ85°C  
LTC3868EUH#TRPBF  
LTC3868IUH#TRPBF  
32-Leadꢀ(5mmꢀ×ꢀ5mm)ꢀPlasticꢀQFN  
32-Leadꢀ(5mmꢀ×ꢀ5mm)ꢀPlasticꢀQFN  
3868  
–40°Cꢀtoꢀ85°C  
ConsultꢀLTCꢀMarketingꢀforꢀpartsꢀspecifiedꢀwithꢀwiderꢀoperatingꢀtemperatureꢀranges.ꢀꢀ*Theꢀtemperatureꢀgradeꢀisꢀidentifiedꢀbyꢀaꢀlabelꢀonꢀtheꢀshippingꢀ  
container.ConsultꢀLTCꢀMarketingꢀforꢀinformationꢀonꢀnon-standardꢀleadꢀbasedꢀfinishꢀparts.  
Forꢀmoreꢀinformationꢀonꢀleadꢀfreeꢀpartꢀmarking,ꢀgoꢀto:ꢀhttp://www.linear.com/leadfree/ꢀꢀ  
Forꢀmoreꢀinformationꢀonꢀtapeꢀandꢀreelꢀspecifications,ꢀgoꢀto:ꢀhttp://www.linear.com/tapeandreel/  
elecTrical characTerisTics The l denotes the specifications which apply over the full operating  
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VRUN1,2 = 5V, EXTVCC = 0V unless otherwise noted.  
SYMBOL  
PARAMETER  
CONDITIONS  
MIN  
4
TYP  
MAX  
24  
UNITS  
V
V
V
InputꢀSupplyꢀOperatingꢀVoltageꢀRange  
RegulatedꢀFeedbackꢀVoltage  
FeedbackꢀCurrent  
IN  
l
(Noteꢀ4)ꢀI  
(Noteꢀ4)  
ꢀVoltageꢀ=ꢀ1.2V  
TH1,2  
0.788  
0.8  
5
0.812  
50  
V
FB1,2  
FB1,2  
I
nA  
V
V
ReferenceꢀVoltageꢀLineꢀRegulation  
OutputꢀVoltageꢀLoadꢀRegulation  
(Noteꢀ4)ꢀV ꢀ=ꢀ4.5Vꢀtoꢀ24V  
0.002  
0.02  
%/V  
REFLNREG  
LOADREG  
IN  
(Note4)ꢀ  
%
l
l
MeasuredꢀinꢀServoꢀLoop,ꢀꢀ  
0.01  
0.1  
I ꢀVoltageꢀ=ꢀ1.2Vꢀtoꢀ0.7V  
TH  
(Note4)ꢀ  
%
MeasuredꢀinꢀServoꢀLoop,ꢀꢀ  
–0.01  
–0.1  
I ꢀVoltageꢀ=ꢀ1.2Vꢀtoꢀ2V  
TH  
3868fb  
LTC3868  
elecTrical characTerisTics The l denotes the specifications which apply over the full operating  
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VRUN1,2 = 5V, EXTVCC = 0V unless otherwise noted.  
SYMBOL  
PARAMETER  
CONDITIONS  
MIN  
TYP  
MAX  
UNITS  
g
TransconductanceꢀAmplifierꢀg  
InputꢀDCꢀSupplyꢀCurrent  
(Noteꢀ4)ꢀI  
(Noteꢀ5)  
ꢀ=ꢀ1.2V,ꢀSink/Sourceꢀ=ꢀ5µA  
TH1,2  
2
mmho  
m1,2  
m
I
Q
Pulse-SkippingꢀorꢀForcedꢀContinuousꢀ  
Modeꢀ  
RUN1ꢀ=ꢀ5VꢀandꢀRUN2ꢀ=ꢀ0Vꢀorꢀꢀ  
RUN1ꢀ=ꢀ0VꢀandꢀRUN2ꢀ=ꢀ5V,ꢀꢀ  
1.3  
2
mA  
mA  
µA  
(OneꢀChannelꢀOn)  
V
ꢀ=ꢀ0.83Vꢀ(NoꢀLoad)  
FB1  
Pulse-SkippingꢀorꢀForcedꢀContinuousꢀ  
Modeꢀ  
(BothꢀChannelsꢀOn)  
RUN1,2ꢀ=ꢀ5V,ꢀV  
ꢀ=ꢀ0.83Vꢀ(NoꢀLoad)  
FB1,2  
SleepꢀModeꢀ(OneꢀChannelꢀOn)  
RUN1ꢀ=ꢀ5VꢀandꢀRUN2ꢀ=ꢀ0Vꢀorꢀꢀ  
RUN1ꢀ=ꢀ0VꢀandꢀRUN2ꢀ=ꢀ5V,ꢀꢀ  
170  
250  
V
ꢀ=ꢀ0.83Vꢀ(NoꢀLoad)  
FB1  
SleepꢀModeꢀ(BothꢀChannelsꢀOn)  
Shutdown  
RUN1,2ꢀ=ꢀ5V,ꢀV  
RUN1,2ꢀ=ꢀ0V  
ꢀ=ꢀ0.83Vꢀ(NoꢀLoad)  
300  
8
450  
25  
µA  
µA  
FB1,2  
l
l
UVLO  
UndervoltageꢀLockout  
INTV ꢀRampingꢀUpꢀ  
4ꢀ  
3.8  
4.2ꢀ  
4
Vꢀ  
V
CC  
INTV ꢀRampingꢀDown  
3.6  
CC  
V
FeedbackꢀOvervoltageꢀProtection  
MeasuredꢀatꢀV  
EachꢀChannel  
EachꢀChannelꢀ  
,ꢀRelativeꢀtoꢀRegulatedꢀV  
FB1,2  
7
10  
13  
1
%
OVL  
FB1,2  
+
+
I
I
SENSE ꢀPinsꢀCurrent  
µA  
SENSE  
SENSE  
SENSE ꢀPinsꢀCurrent  
µAꢀ  
µA  
V
V
ꢀ<ꢀINTV ꢀ–ꢀ0.5Vꢀ  
1ꢀ  
OUT1,2  
OUT1,2  
CC  
CC  
ꢀ>ꢀINTV ꢀ+ꢀ0.5V  
540  
700  
DF  
MaximumꢀDutyꢀFactor  
Soft-StartꢀChargeꢀCurrent  
RUNꢀPinꢀOnꢀThreshold  
InꢀDropout,ꢀFREQꢀ=ꢀ0V  
98  
0.7  
99.4  
1.0  
1.26  
50  
%
µA  
V
MAX  
I
V
V
ꢀ=ꢀ0V  
SS1,2  
1.4  
SS1,2  
l
V
V
V
V
ꢀOn  
,ꢀV ꢀRising  
RUN1 RUN2  
1.21  
1.31  
RUN1,2  
RUN1,2  
ꢀHyst RUNꢀPinꢀHysteresis  
SSꢀPinꢀLatchoffꢀArmingꢀThreshold  
mV  
V
ꢀLA  
V
V
,ꢀV ꢀRisingꢀfromꢀ1V  
SS1 SS2  
1.9  
1.3  
7
2
2.1  
1.7  
13  
SS1,2  
SS1,2  
ꢀLT  
SSꢀPinꢀLatchoffꢀThreshold  
SSꢀDischargeꢀCurrent  
,ꢀV ꢀFallingꢀfromꢀ2V  
SS1 SS2  
1.5  
10  
V
I
ꢀLT  
Short-CircuitꢀConditionꢀV  
ꢀ=ꢀ0.5V,ꢀꢀ  
FB1,2  
µA  
DSC1,2  
V
ꢀ=ꢀ4.5V  
SS1,2  
V
MaximumꢀCurrentꢀSenseꢀThreshold  
V
V
V
ꢀ=ꢀ0.7V,ꢀV  
ꢀ=ꢀ0.7V,ꢀV  
ꢀ=ꢀ0.7V,ꢀV  
–, –ꢀ=ꢀ3.3V,ꢀI ꢀ=ꢀ0ꢀ  
22ꢀ  
43ꢀ  
64  
30ꢀ  
50ꢀ  
75  
36ꢀ  
57ꢀ  
86  
mVꢀ  
mVꢀ  
mV  
SENSE(MAX)  
FB1,2  
FB1,2  
FB1,2  
SENSE1  
SENSE1  
SENSE1  
2
LIM  
–, –ꢀ=ꢀ3.3V,ꢀI ꢀ=ꢀFLOATꢀ  
2
LIM  
–, –ꢀ=ꢀ3.3V,ꢀI ꢀ=ꢀINTV  
2
LIM  
CC  
Gate Driver  
TG1,2  
Pull-UpꢀOn-Resistanceꢀ  
Pull-DownꢀOn-Resistance  
2.5ꢀ  
1.5  
Ωꢀ  
Ω
BG1,2  
Pull-UpꢀOn-Resistanceꢀ  
Pull-DownꢀOn-Resistance  
2.4ꢀ  
1.1  
Ωꢀ  
Ω
TGꢀTransitionꢀTime:ꢀ  
ꢀꢀꢀRiseꢀTimeꢀ  
ꢀꢀꢀFallꢀTime  
(Noteꢀ6)ꢀ  
nsꢀ  
ns  
TG1,2ꢀt ꢀ  
C
C
ꢀ=ꢀ3300pFꢀ  
25ꢀ  
16  
r
LOAD  
LOAD  
TG1,2ꢀt  
ꢀ=ꢀ3300pF  
f
BGꢀTransitionꢀTime:ꢀ  
ꢀꢀꢀRiseꢀTimeꢀ  
ꢀꢀꢀFallꢀTime  
(Noteꢀ6)ꢀ  
LOAD  
LOAD  
nsꢀ  
ns  
BG1,2ꢀt ꢀ  
C
C
ꢀ=ꢀ3300pFꢀ  
ꢀ=ꢀ3300pF  
28ꢀ  
13  
r
BG1,2ꢀt  
f
TG/BGꢀt  
TopꢀGateꢀOffꢀtoꢀBottomꢀGateꢀOnꢀDelayꢀ  
SynchronousꢀSwitch-OnꢀDelayꢀTime  
C
ꢀ=ꢀ3300pFꢀEachꢀDriver  
30  
30  
95  
ns  
1D  
1D  
LOAD  
BG/TGꢀt  
BottomꢀGateꢀOffꢀtoꢀTopꢀGateꢀOnꢀDelayꢀ  
TopꢀSwitch-OnꢀDelayꢀTime  
C
ꢀ=ꢀ3300pFꢀEachꢀDriver  
ns  
LOAD  
t
MinimumꢀOn-Time  
(Noteꢀ7)  
ns  
ON(MIN)  
3868fb  
LTC3868  
elecTrical characTerisTics The l denotes the specifications which apply over the full operating  
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VRUN1,2 = 5V, EXTVCC = 0V unless otherwise noted.  
SYMBOL  
PARAMETER  
CONDITIONS  
MIN  
4.85  
4.85  
4.5  
TYP  
MAX  
UNITS  
INTV Linear Regulator  
CC  
V
V
V
V
V
V
InternalꢀV ꢀVoltage  
6Vꢀ<ꢀV ꢀ<ꢀ24V,ꢀV ꢀ=ꢀ0V  
EXTVCC  
5.1  
0.7  
5.1  
0.6  
4.7  
250  
5.35  
1.1  
V
%
V
INTVCCVIN  
LDOVIN  
CC  
IN  
INTV ꢀLoadꢀRegulation  
I
ꢀ=ꢀ0mAꢀtoꢀ50mA,ꢀV  
ꢀ=ꢀ0V  
CC  
CC  
EXTVCC  
InternalꢀV ꢀVoltage  
6Vꢀ<ꢀV ꢀ<ꢀ13V  
EXTVCC  
5.35  
1.1  
INTVCCEXT  
LDOEXT  
CC  
INTV ꢀLoadꢀRegulation  
I
ꢀ=ꢀ0mAꢀtoꢀ50mA,ꢀV  
CC  
ꢀ=ꢀ8.5V  
EXTVCC  
%
V
CC  
EXTV ꢀSwitchoverꢀVoltage  
EXTV ꢀRampingꢀPositive  
4.9  
EXTVCC  
CC  
CC  
EXTV ꢀHysteresis  
mV  
LDOHYS  
CC  
Oscillator and Phase-Locked Loop  
f
f
f
f
f
f
ProgrammableꢀFrequency  
ProgrammableꢀFrequency  
ProgrammableꢀFrequency  
LowꢀFixedꢀFrequency  
R
R
R
V
V
ꢀ=ꢀ25k,ꢀPLLIN/MODEꢀ=ꢀDCꢀVoltage  
ꢀ=ꢀ65k,ꢀPLLIN/MODEꢀ=ꢀDCꢀVoltage  
ꢀ=ꢀ105k,ꢀPLLIN/MODEꢀ=ꢀDCꢀVoltage  
ꢀ=ꢀ0V,ꢀPLLIN/MODEꢀ=ꢀDCꢀVoltage  
105  
440  
835  
350  
535  
kHz  
kHz  
kHz  
kHz  
kHz  
kHz  
25kΩ  
65kΩ  
105kΩ  
LOW  
FREQ  
FREQ  
FREQ  
FREQ  
FREQ  
375  
505  
320  
485  
75  
380  
585  
850  
HighꢀFixedꢀFrequency  
ꢀ=ꢀINTV ,ꢀPLLIN/MODEꢀ=ꢀDCꢀVoltage  
CC  
HIGH  
SYNC  
l
SynchronizableꢀFrequency  
PLLIN/MODEꢀ=ꢀExternalꢀClock  
PGOOD1 and PGOOD2 Outputs  
V
PGOODꢀVoltageꢀLow  
PGOODꢀLeakageꢀCurrent  
PGOODꢀTripꢀLevel  
I
ꢀ=ꢀ2mA  
PGOOD  
0.2  
0.4  
1
V
PGL  
I
V
V
ꢀ=ꢀ5V  
PGOOD  
µA  
PGOOD  
V
ꢀwithꢀRespectꢀtoꢀSetꢀRegulatedꢀVoltageꢀ  
FB  
FB  
–7  
PG  
ꢀꢀꢀV ꢀRampingꢀNegativeꢀ  
ꢀꢀꢀHysteresis  
–13  
–10ꢀ  
2.5  
%ꢀ  
%
V
ꢀwithꢀRespectꢀtoꢀSetꢀRegulatedꢀVoltageꢀ  
FB  
7
13  
FB  
ꢀꢀꢀV ꢀRampingꢀPositiveꢀ  
10ꢀ  
2.5  
%ꢀ  
%
ꢀꢀꢀHysteresis  
t
DelayꢀforꢀReportingꢀaꢀFaultꢀ(PGOODꢀLow)  
25  
µs  
PG  
Note 1:ꢀStressesꢀbeyondꢀthoseꢀlistedꢀunderꢀAbsoluteꢀMaximumꢀRatingsꢀ  
mayꢀcauseꢀpermanentꢀdamageꢀtoꢀtheꢀdevice.ꢀExposureꢀtoꢀanyꢀAbsoluteꢀ  
MaximumꢀRatingsꢀforꢀextendedꢀperiodsꢀmayꢀaffectꢀdeviceꢀreliabilityꢀandꢀ  
lifetime.ꢀ  
Note 4:ꢀTheꢀLTC3868ꢀisꢀtestedꢀinꢀaꢀfeedbackꢀloopꢀthatꢀservosꢀV  
ꢀtoꢀaꢀ  
ITH1,2  
specifiedꢀvoltageꢀandꢀmeasuresꢀtheꢀresultantꢀV  
.
FB1,2  
Note 5:ꢀDynamicꢀsupplyꢀcurrentꢀisꢀhigherꢀdueꢀtoꢀtheꢀgateꢀchargeꢀbeingꢀ  
deliveredꢀatꢀtheꢀswitchingꢀfrequency.ꢀSeeꢀApplicationsꢀinformation.  
Note 2:ꢀTheꢀLTC3868Eꢀisꢀguaranteedꢀtoꢀmeetꢀperformanceꢀspecificationsꢀ  
fromꢀ0°Cꢀtoꢀ85°C.ꢀSpecificationsꢀoverꢀtheꢀ–40°Cꢀtoꢀ85°Cꢀoperatingꢀ  
temperatureꢀrangeꢀareꢀassuredꢀbyꢀdesign,ꢀcharacterizationꢀandꢀcorrelationꢀ  
withꢀstatisticalꢀprocessꢀcontrols.ꢀTheꢀLTC3868Iꢀisꢀguaranteedꢀoverꢀtheꢀfullꢀ  
–40°Cꢀtoꢀ85°Cꢀoperatingꢀtemperatureꢀrange.  
Note 6:ꢀRiseꢀandꢀfallꢀtimesꢀareꢀmeasuredꢀusingꢀ10%ꢀandꢀ90%ꢀlevels.ꢀDelayꢀ  
timesꢀareꢀmeasuredꢀusingꢀ50%ꢀlevels.  
Note 7:ꢀTheꢀminimumꢀon-timeꢀconditionꢀisꢀspecifiedꢀforꢀanꢀinductorꢀ  
peak-to-peakꢀrippleꢀcurrentꢀ≥ꢀ40%ꢀofꢀI ꢀ(SeeꢀMinimumꢀOn-Timeꢀ  
MAX  
ConsiderationsꢀinꢀtheꢀApplicationsꢀInformationꢀsection).  
Note 3:ꢀT ꢀisꢀcalculatedꢀfromꢀtheꢀambientꢀtemperatureꢀT ꢀandꢀpowerꢀ  
J
A
dissipationꢀP ꢀaccordingꢀtoꢀtheꢀfollowingꢀformula:  
D
T ꢀ=ꢀT ꢀ+ꢀ(P •ꢀ34°C/W)  
J A Dꢀ  
3868fb  
LTC3868  
Typical perForMance characTerisTics  
Efficiency and Power Loss  
vs Output Current  
Efficiency vs Output Current  
100  
90  
100  
90  
10000  
1000  
100  
10  
FIGURE 12 CIRCUIT  
V
V
= 12V  
IN  
OUT  
V
IN  
= 5V  
= 3.3V  
80  
80  
70  
70  
V
IN  
= 12V  
60  
50  
60  
50  
40  
30  
20  
10  
0
40  
30  
20  
10  
0
Burst Mode  
OPERATION  
PULSE-  
SKIPPING  
FCM  
1
V
= 3.3V  
OUT  
FIGURE 12 CIRCUIT  
0.1  
0.0001 0.001  
0.01  
0.1 10  
1
0.0001 0.001  
0.01  
0.1  
1
10  
OUTPUT CURRENT (A)  
OUTPUT CURRENT (A)  
3868 G02  
3868 G01  
Load Step  
(Forced Continuous Mode)  
Efficiency vs Input Voltage  
Load Step (Burst Mode Operation)  
98  
96  
94  
92  
90  
88  
86  
84  
82  
80  
FIGURE 12 CIRCUIT  
V
= 3.3V  
OUT  
OUT  
V
V
OUT  
OUT  
I
= 4A  
100mV/DIV  
AC-  
100mV/DIV  
AC-  
COUPLED  
COUPLED  
I
L
I
L
2A/DIV  
2A/DIV  
3868 G05  
3868 G04  
V
= 3.3V  
20µs/DIV  
20  
INPUT VOLTAGE (V)  
25 28  
0
5
10  
15  
V
= 3.3V  
20µs/DIV  
OUT  
OUT  
FIGURE 12 CIRCUIT  
FIGURE 12 CIRCUIT  
3868 G03  
Load Step (Pulse-Skipping Mode)  
Inductor Current at Light Load  
Soft Start-Up  
V
OUT  
FORCED  
CONTINUOUS  
MODE  
V
OUT2  
100mV/DIV  
AC-  
2V/DIV  
COUPLED  
Burst Mode  
OPERATION  
2A/DIV  
V
OUT1  
2V/DIV  
I
L
2A/DIV  
PULSE-  
SKIPPING  
MODE  
3868 G07  
3868 G08  
3868 G06  
V
I
= 3.3V  
2µs/DIV  
20ms/DIV  
FIGURE 12 CIRCUIT  
V
= 3.3V  
20µs/DIV  
OUT  
LOAD  
OUT  
= 200µA  
FIGURE 12 CIRCUIT  
FIGURE 12 CIRCUIT  
3868fb  
LTC3868  
Typical perForMance characTerisTics  
Total Input Supply Current  
vs Input Voltage  
EXTVCC Switchover and INTVCC  
Voltages vs Temperature  
INTVCC Line Regulation  
400  
5.6  
5.2  
5.2  
5.1  
5.1  
FIGURE 12 CIRCUIT  
= 3.3V  
V
OUT  
350  
300  
5.4  
5.2  
ONE CHANNEL ON  
INTV  
CC  
300µA LOAD  
250  
200  
150  
100  
50  
5.0  
4.8  
4.6  
4.4  
4.2  
EXTV RISING  
CC  
NO LOAD  
EXTV FALLING  
CC  
5.0  
0
4.0  
10  
15  
INPUT VOLTAGE (V)  
25 28  
0
5
10  
15  
20  
25 28  
5
20  
–20  
5
55  
80 105 130  
–45  
30  
INPUT VOLTAGE (V)  
TEMPERATURE (°C)  
3868 G12  
3868 G10  
3868 G11  
Maximum Current Sense Voltage  
vs ITH Voltage  
Maximum Current Sense  
Threshold vs Duty Cycle  
SENSEPin Input Bias Current  
80  
60  
40  
20  
80  
60  
40  
20  
0
0
PULSE-SKIPPING  
FORCED CONTINUOUS  
Burst Mode OPERATION  
(FALLING)  
Burst Mode OPERATION  
(RISING)  
–50  
–100  
–150  
–200  
–250  
–300  
–350  
–400  
–450  
–500  
–550  
–600  
I
= INTV  
CC  
LIM  
I
= FLOAT  
LIM  
I
= GND  
LIM  
I
= GND  
LIM  
0
–20  
–40  
I
= FLOAT  
LIM  
LIM  
I
= INTV  
CC  
5% DUTY CYCLE  
1.2 1.4  
0.8  
10 20  
50  
60 70 80 90 100  
0
0.2 0.4 0.6  
1.0  
0
30 40  
0
10  
COMMON MODE VOLTAGE (V)  
SENSE  
15  
5
I
TH  
PIN VOLTAGE  
V
DUTY CYCLE (%)  
3868 G13  
3868 G15  
3868 G14  
Shutdown Current vs Temperature  
Foldback Current Limit  
Quiescent Current vs Temperature  
10  
9
90  
80  
70  
60  
50  
40  
30  
20  
10  
240  
230  
220  
210  
200  
190  
180  
170  
160  
150  
140  
130  
120  
110  
PLLIN/MODE = 0  
I
= INTV  
CC  
V
V
= 12V  
LIM  
IN  
OUT  
= 3.3V  
ONE CHANNEL ON  
8
I
= FLOAT  
= GND  
LIM  
7
I
LIM  
6
5
4
0
55  
TEMPERATURE (°C)  
105 130  
–45 –20  
5
30  
80  
–45 –20  
5
30  
55  
80 105 130  
0
0.1 0.2 0.3 0.4 0.5  
0.9  
0.6 0.7 0.8  
TEMPERATURE (°C)  
FEEDBACK VOLTAGE (V)  
3868 G17  
3868 G18  
3868 G16  
3868fb  
LTC3868  
Typical perForMance characTerisTics  
Regulated Feedback Voltage  
vs Temperature  
Soft-Start Pull-Up Current  
vs Temperature  
Shutdown (RUN) Threshold  
vs Temperature  
1.40  
1.35  
1.30  
1.25  
1.20  
1.15  
1.10  
1.05  
1.00  
0.95  
0.90  
808  
1.20  
806  
804  
1.15  
1.10  
802  
800  
798  
796  
794  
1.05  
1.00  
0.95  
0.90  
0.85  
792  
0.80  
–20  
5
55  
80 105 130  
–45  
5
30  
55  
80 105 130  
–45 –20  
5
30  
55  
80 105 130  
–45  
30  
–20  
TEMPERATURE (°C)  
TEMPERATURE (°C)  
TEMPERATURE (°C)  
3868 G20  
3868 G21  
3868 G19  
SENSEPin Input Current  
vs Temperature  
Shutdown Current  
vs Input Voltage  
Oscillator Frequency  
vs Temperature  
50  
0
–50  
14  
12  
800  
V
OUT  
= 3.3V  
700  
600  
–100  
–150  
–200  
–250  
–300  
–350  
–400  
–450  
–500  
–550  
–600  
FREQ = INTV  
CC  
10  
8
500  
400  
300  
200  
100  
FREQ = GND  
6
4
2
V
= 28V  
55  
OUT  
0
0
–45 –20  
5
30  
80 105 130  
25  
28  
–20  
5
55  
80 105 130  
5
10  
15  
20  
–45  
30  
TEMPERATURE (°C)  
INPUT VOLTAGE (V)  
TEMPERATURE (°C)  
3868 G22  
3868 G23  
3868 G24  
Oscillator Frequency  
vs Input Voltage  
Undervoltage Lockout Threshold  
vs Temperature  
4.4  
4.3  
4.2  
4.1  
4.0  
3.9  
3.8  
3.7  
3.6  
3.5  
3.4  
356  
354  
352  
350  
348  
346  
344  
–45  
5
30  
55  
80 105 130  
–20  
25  
28  
5
10  
15  
20  
TEMPERATURE (°C)  
INPUT VOLTAGE (V)  
3868 G25  
3868 G28  
3868fb  
LTC3868  
Typical perForMance characTerisTics  
Latchoff Thresholds  
vs Temperature  
INTVCC and EXTVCC vs Load Current  
2.3  
2.2  
2.1  
2.0  
1.9  
1.8  
1.7  
1.6  
1.5  
1.4  
1.3  
1.2  
5.20  
5.15  
5.10  
V
= 12V  
IN  
ARMING THRESHOLD  
EXTV = 0V  
CC  
5.05  
5.00  
4.95  
LATCH-OFF THRESHOLD  
EXTV = 8V  
CC  
0
20 40 60 80 100 120 140 160 180 200  
–45  
5
30  
55  
80  
130  
–20  
105  
TEMPERATURE (°C)  
LOAD CURRENT (mA)  
3868 G26  
3868 G27  
pin FuncTions  
CLKOUT (Pin 4):ꢀOutputꢀclockꢀsignalꢀavailableꢀtoꢀdaisy-  
SENSE1 , SENSE2 (Pin 1, Pin 9):ꢀTheꢀ(–)ꢀInputꢀtoꢀtheꢀ  
chainꢀotherꢀcontrollerꢀICsꢀforꢀadditionalꢀMOSFETꢀdriverꢀ  
Differentialꢀ Currentꢀ Comparators.ꢀ Whenꢀ greaterꢀ thanꢀ  
stages/phases.ꢀTheꢀoutputꢀlevelsꢀswingꢀfromꢀINTV ꢀtoꢀ  
INTV 0.5V,theSENSE pinsuppliescurrenttotheꢀ  
CC  
CC  
ground.ꢀ  
currentꢀcomparator.  
PLLIN/MODE (Pin 5):ꢀExternalꢀSynchronizationꢀInputꢀtoꢀ  
PhaseDetectorandForcedContinuousModeInput.Whenꢀ  
anꢀexternalꢀclockꢀisꢀappliedꢀtoꢀthisꢀpin,ꢀtheꢀphase-lockedꢀ  
loopꢀwillꢀforceꢀtheꢀrisingꢀTG1ꢀsignalꢀtoꢀbeꢀsynchronizedꢀ  
withꢀtheꢀrisingꢀedgeꢀofꢀtheꢀexternalꢀclock.ꢀWhenꢀnotꢀsyn-  
chronizingꢀtoꢀanꢀexternalꢀclock,ꢀthisꢀinput,ꢀwhichꢀactsꢀonꢀ  
bothcontrollers,determineshowtheLTC3868operatesatꢀ  
lightꢀloads.ꢀPullingꢀthisꢀpinꢀtoꢀgroundꢀselectsꢀBurstꢀModeꢀ  
operation.Aninternal100kresistortogroundalsoinvokesꢀ  
BurstModeoperationwhenthepinisoated.Tyingthispinꢀ  
FREQ (Pin 2):ꢀTheꢀFrequencyꢀControlꢀPinꢀforꢀtheꢀInternalꢀ  
VCO.ꢀConnectingꢀthisꢀpinꢀtoꢀGNDꢀforcesꢀtheꢀVCOꢀtoꢀaꢀfixedꢀ  
lowꢀfrequencyꢀofꢀ350kHz.ꢀConnectingꢀthisꢀpinꢀtoꢀINTV ꢀ  
CC  
forcesꢀ theꢀ VCOꢀ toꢀ aꢀ fixedꢀ highꢀ frequencyꢀ ofꢀ 535kHz.ꢀ  
Otherꢀ frequenciesꢀ betweenꢀ 50kHzꢀ andꢀ 900kHzꢀ canꢀ beꢀ  
programmedusingaresistorbetweenFREQandGND.ꢀ  
Anꢀinternalꢀ20µAꢀpull-upꢀcurrentꢀdevelopsꢀtheꢀvoltageꢀtoꢀ  
beꢀusedꢀbyꢀtheꢀVCOꢀtoꢀcontrolꢀtheꢀfrequency  
PHASMD (Pin 3):ꢀControlꢀinputꢀtoꢀphaseꢀselectorꢀwhichꢀ  
determinesꢀtheꢀphaseꢀrelationshipsꢀbetweenꢀcontrollerꢀ1,ꢀ  
controller2andtheCLKOUTsignal.Pullingthispintoꢀ  
groundꢀforcesꢀTG2ꢀandꢀCLKOUTꢀtoꢀbeꢀoutꢀofꢀphaseꢀ180°ꢀ  
toꢀINTV ꢀforcesꢀcontinuousꢀinductorꢀcurrentꢀoperation.ꢀ  
CC  
Tyingꢀthisꢀpinꢀtoꢀaꢀvoltageꢀgreaterꢀthanꢀ1.2Vꢀandꢀlessꢀthanꢀ  
INTV ꢀ–ꢀ1.3Vꢀselectsꢀpulse-skippingꢀoperation.ꢀ  
CC  
and6withrespecttoTG1.ConnectingthispintoINTV ꢀ  
CC  
SGND (Pin 6, Exposed Pad Pin 33):ꢀSmall-signalꢀgroundꢀ  
commonꢀtoꢀbothꢀcontrollers,ꢀmustꢀbeꢀroutedꢀseparatelyꢀ  
fromꢀhighꢀcurrentꢀgroundsꢀtoꢀtheꢀcommonꢀ(–)ꢀterminalsꢀ  
forcesꢀTG2ꢀandꢀCLKOUTꢀtoꢀbeꢀoutꢀofꢀphaseꢀ240°ꢀandꢀ120°ꢀ  
withꢀ respectꢀ toꢀ TG1.ꢀ Floatingꢀ thisꢀ pinꢀ forcesꢀ TG2ꢀ andꢀ  
CLKOUTꢀtoꢀbeꢀoutꢀofꢀphaseꢀ180°ꢀandꢀ90°ꢀwithꢀrespectꢀtoꢀ  
TG1.ꢀReferꢀtoꢀtheꢀTableꢀ1.ꢀ  
ofꢀtheꢀC ꢀcapacitors.ꢀTheꢀexposedꢀpadꢀmustꢀbeꢀsolderedꢀ  
IN  
toꢀtheꢀPCBꢀforꢀratedꢀthermalꢀperformance.  
3868fb  
LTC3868  
pin FuncTions  
RUN1, RUN2 (Pin 7, Pin 8):DigitalRunControlInputsforꢀ  
EachꢀController.ꢀForcingꢀeitherꢀofꢀtheseꢀpinsꢀbelowꢀ1.26Vꢀ  
shutsdownthatcontroller.Forcingbothofthesepinsbelowꢀ  
0.7VꢀshutsꢀdownꢀtheꢀentireꢀLTC3868,ꢀreducingꢀquiescentꢀ  
currentꢀtoꢀapproximatelyꢀ8µA.ꢀDoꢀnotꢀfloatꢀtheseꢀpins.  
SW1, SW2 (Pin 25, Pin 16):ꢀSwitchꢀNodeꢀConnectionsꢀ  
toꢀInductors.ꢀ  
TG1, TG2 (Pin 26, Pin 15):ꢀHighꢀCurrentꢀGateꢀDrivesꢀforꢀ  
TopꢀN-ChannelꢀMOSFETs.ꢀTheseꢀareꢀtheꢀoutputsꢀofꢀfloat-  
ingꢀdriversꢀwithꢀaꢀvoltageꢀswingꢀequalꢀtoꢀINTV ꢀ–ꢀ0.5Vꢀ  
CC  
I
(Pin 28):ꢀCurrentꢀComparatorꢀSenseꢀVoltageꢀRangeꢀ  
superimposedꢀonꢀtheꢀswitchꢀnodeꢀvoltageꢀSW.  
LIM  
Inputs.ꢀTyingꢀthisꢀpinꢀtoꢀSGND,ꢀFLOATꢀorꢀINTV ꢀsetsꢀtheꢀ  
CC  
PGOOD1, PGOOD2 (Pin 27, Pin 14):ꢀOpen-DrainꢀLogicꢀ  
maximumcurrentsensethresholdtooneofthreedifferentꢀ  
levelsꢀforꢀbothꢀcomparators.  
Output.ꢀPGOOD1,2ꢀisꢀpulledꢀtoꢀgroundꢀwhenꢀtheꢀvoltageꢀ  
onꢀtheꢀV ꢀpinꢀisꢀnotꢀwithinꢀ 10%ꢀofꢀitsꢀsetꢀpoint.  
FB1,2  
INTV (Pin19):OutputoftheInternalLinearLowDropoutꢀ  
CC  
SS1, SS2 (Pin 29, Pin 13):ꢀExternalꢀSoft-StartꢀInput.ꢀTheꢀ  
LTC3868regulatestheV ꢀvoltagetothesmallerof0.8Vꢀ  
Regulator.Thedriverandcontrolcircuitsarepoweredꢀ  
fromthisvoltagesource.Mustbedecoupledtopowerꢀ  
groundꢀwithꢀaꢀminimumꢀofꢀ4.7µFꢀceramicꢀorꢀotherꢀlowꢀ  
FB1,2  
orꢀtheꢀvoltageꢀonꢀtheꢀSS1,2ꢀpin.ꢀAnꢀinternalꢀ1µAꢀpull-upꢀ  
currentsourceisconnectedtothispin.Acapacitortoꢀ  
groundꢀatꢀthisꢀpinꢀsetsꢀtheꢀrampꢀtimeꢀtoꢀfinalꢀregulatedꢀ  
outputꢀvoltage.ꢀThisꢀpinꢀisꢀalsoꢀusedꢀasꢀtheꢀshort-circuitꢀ  
latchoffꢀtimer.  
ESRꢀcapacitor.ꢀDoꢀnotꢀuseꢀtheꢀINTV ꢀpinꢀforꢀanyꢀotherꢀ  
CC  
purpose.  
EXTV (Pin 20):ꢀExternalꢀPowerꢀInputꢀtoꢀanꢀInternalꢀLDOꢀ  
CC  
ConnectedꢀtoꢀINTV .ꢀThisꢀLDOꢀsuppliesꢀINTV ꢀpower,ꢀ  
CC  
CC  
I , I  
TH1 TH2  
(Pin 30, Pin 12):ꢀErrorꢀAmplifierꢀOutputsꢀandꢀ  
bypassingꢀtheꢀinternalꢀLDOꢀpoweredꢀfromꢀV ꢀwheneverꢀ  
IN  
SwitchingꢀRegulatorꢀCompensationꢀPoints.ꢀEachꢀassoci-  
atedchannel’scurrentcomparatortrippointincreasesꢀ  
withꢀthisꢀcontrolꢀvoltage.  
EXTV ishigherthan4.7V.SeeEXTV Connectioninꢀ  
CC  
CC  
theꢀApplicationsꢀInformationꢀsection.ꢀDoꢀnotꢀexceedꢀ14Vꢀ  
onꢀthisꢀpin.  
V
, V (Pin31, Pin11):Receivestheremotelysensedꢀ  
FB1 FB2  
PGND (Pin 21):ꢀDriverꢀPowerꢀGround.ꢀConnectsꢀtoꢀtheꢀ  
feedbackꢀ voltageꢀ forꢀ eachꢀ controllerꢀ fromꢀ anꢀ externalꢀ  
sourcesofbottom(synchronous)N-channelMOSFETsꢀ  
resistiveꢀdividerꢀacrossꢀtheꢀoutput.  
andꢀtheꢀ(–)ꢀterminal(s)ꢀofꢀC .  
IN  
+
+
SENSE1 , SENSE2 (Pin 32, Pin 10):ꢀTheꢀ(+)ꢀInputꢀtoꢀtheꢀ  
differentialꢀcurrentꢀcomparatorsꢀareꢀnormallyꢀconnectedꢀ  
toDCRsensingnetworksorcurrentsensingresistors.ꢀ  
V (Pin 22):ꢀMainꢀSupplyꢀPin.ꢀAꢀbypassꢀcapacitorꢀshouldꢀ  
IN  
beꢀtiedꢀbetweenꢀthisꢀpinꢀandꢀtheꢀsignalꢀgroundꢀpin.  
BG1, BG2 (Pin 23, Pin 18):HighCurrentGateDrivesꢀ  
TheI ꢀpinvoltageandcontrolledoffsetsbetweentheꢀ  
TH  
+
forꢀBottomꢀ(Synchronous)ꢀN-ChannelꢀMOSFETs.ꢀVoltageꢀ  
SENSE ꢀandꢀSENSE ꢀpinsꢀinꢀconjunctionꢀwithꢀR  
theꢀcurrentꢀtripꢀthreshold.  
ꢀsetꢀ  
SENSE  
swingꢀatꢀtheseꢀpinsꢀisꢀfromꢀgroundꢀtoꢀINTV .  
CC  
BOOST1,BOOST2(Pin24,Pin17):BootstrappedSuppliesꢀ  
toꢀtheꢀTopsideꢀFloatingꢀDrivers.ꢀCapacitorsꢀareꢀconnectedꢀ  
betweenꢀtheꢀBOOSTꢀandꢀSWꢀpinsꢀandꢀSchottkyꢀdiodesꢀareꢀ  
tiedꢀbetweenꢀtheꢀBOOSTꢀandꢀINTV ꢀpins.ꢀVoltageꢀswingꢀ  
CC  
atꢀtheꢀBOOSTꢀpinsꢀisꢀfromꢀINTV ꢀtoꢀ(V ꢀ+ꢀINTV ).  
CC  
IN  
CC  
3868fb  
LTC3868  
FuncTional DiagraM  
INTV  
CC  
V
IN  
DUPLICATE FOR SECOND  
CONTROLLER CHANNEL  
BOOST  
24, 17  
D
+
B
PHASMD  
3
CLKOUT  
4
PGOOD1  
27  
0.88V  
V
TG  
26, 15  
C
B
FB1  
+
DROP  
OUT  
TOP  
BOT  
C
IN  
D
0.72V  
0.88V  
DET  
BOT  
SW  
25, 16  
TOP ON  
+
S
R
Q
PGOOD2  
14  
INTV  
CC  
Q
BG  
23, 18  
SWITCH  
LOGIC  
V
FB2  
SHDN  
+
C
OUT  
0.72V  
PGND  
21  
20µA  
FREQ  
2
V
OUT  
VCO  
CLK2  
+
R
SENSE  
0.425V  
SLEEP  
CLK1  
L
ICMP  
IR  
+
+
PFD  
C
LP  
+
+
+
SENSE  
32, 10  
3mV  
SYNC  
DET  
2.7V  
0.55V  
PLLIN/MODE  
5
SENSE  
1, 9  
100k  
SLOPE COMP  
V
FB  
31, 11  
I
LIM  
R
B
CURRENT  
LIMIT  
+
28  
0.80V  
TRACK/SS  
EA  
V
R
A
IN  
22  
+
OV  
EXTV  
20  
CC  
I
TH  
C
C
0.88V  
30, 12  
5.1V  
LDO  
EN  
5.1V  
LDO  
EN  
SHDN  
RST  
FB  
C
C2  
R
C
0.5µA  
FOLDBACK  
2(V  
)
SS  
29, 13  
+
1µA  
4.7V  
11V  
SHORT CKT  
LATCH-OFF  
C
SHDN  
10µA  
SS  
RUN  
7, 8  
6
SGND  
19 INTV  
CC  
3868 FD  
3868fb  
ꢀ0  
LTC3868  
Shutdown and Start-Up (RUN1, RUN2  
and SS1, SS2 Pins)  
operaTion (Refer to the Functional Diagram)  
TheLTC3868usesaconstantfrequency,currentmodeꢀ  
step-downꢀarchitectureꢀwithꢀtheꢀtwoꢀcontrollerꢀchannelsꢀ  
operatingꢀ180ꢀdegreesꢀoutꢀofꢀphase.ꢀDuringꢀnormalꢀop-  
eration,ꢀeachꢀexternalꢀtopꢀMOSFETꢀisꢀturnedꢀonꢀwhenꢀtheꢀ  
clockꢀforꢀthatꢀchannelꢀsetsꢀtheꢀRSꢀlatch,ꢀandꢀisꢀturnedꢀoffꢀ  
whenꢀtheꢀmainꢀcurrentꢀcomparator,ꢀICMP,ꢀresetsꢀtheꢀRSꢀ  
latch.ꢀTheꢀpeakꢀinductorꢀcurrentꢀatꢀwhichꢀICMPꢀtripsꢀandꢀ  
TheꢀtwoꢀchannelsꢀofꢀtheꢀLTC3868ꢀcanꢀbeꢀindependentlyꢀ  
shutdownusingtheRUN1andRUN2pins.Pullingeitherofꢀ  
theseꢀpinsꢀbelowꢀ1.26Vꢀshutsꢀdownꢀtheꢀmainꢀcontrolꢀloopꢀ  
forꢀthatꢀcontroller.ꢀPullingꢀbothꢀpinsꢀbelowꢀ0.7Vꢀdisablesꢀ  
bothꢀcontrollersꢀandꢀmostꢀinternalꢀcircuits,ꢀincludingꢀtheꢀ  
INTV ꢀLDOs.ꢀInꢀthisꢀstate,ꢀtheꢀLTC3868ꢀdrawsꢀonlyꢀ8µAꢀ  
CC  
ofꢀquiescentꢀcurrent.  
resetsꢀtheꢀlatchꢀisꢀcontrolledꢀbyꢀtheꢀvoltageꢀonꢀtheꢀI ꢀpin,ꢀ  
TH  
whichꢀisꢀtheꢀoutputꢀofꢀtheꢀerrorꢀamplifier,ꢀEA.ꢀTheꢀerrorꢀ  
TheꢀRUNꢀpinꢀmayꢀbeꢀexternallyꢀpulledꢀupꢀorꢀdrivenꢀdirectlyꢀ  
byꢀlogic.ꢀWhenꢀdrivingꢀtheꢀRUNꢀpinꢀwithꢀaꢀlowꢀimpedanceꢀ  
source,ꢀdoꢀnotꢀexceedꢀtheꢀabsoluteꢀmaximumꢀratingꢀofꢀ  
8V.ꢀTheꢀRUNꢀpinꢀhasꢀanꢀinternalꢀ11Vꢀvoltageꢀclampꢀthatꢀ  
allowsꢀtheꢀRUNꢀpinꢀtoꢀbeꢀconnectedꢀthroughꢀaꢀresistorꢀtoꢀaꢀ  
amplifierꢀcomparesꢀtheꢀoutputꢀvoltageꢀfeedbackꢀsignalꢀatꢀ  
theꢀV ꢀpinꢀ(whichꢀisꢀgeneratedꢀwithꢀanꢀexternalꢀresistorꢀ  
FB  
dividerꢀ connectedꢀ acrossꢀ theꢀ outputꢀ voltage,ꢀ V ,ꢀ toꢀ  
OUTꢀ  
ground)totheinternal0.800Vreferencevoltage.Whentheꢀ  
loadꢀcurrentꢀincreases,ꢀitꢀcausesꢀaꢀslightꢀdecreaseꢀinꢀV ꢀ  
highervoltage(forexample,V ),solongasthemaximumꢀ  
FB  
IN  
relativeꢀtoꢀtheꢀreference,ꢀwhichꢀcausesꢀtheꢀEAꢀtoꢀincreaseꢀ  
currentꢀintoꢀtheꢀRUNꢀpinꢀdoesꢀnotꢀexceedꢀ100µA.  
theꢀI ꢀvoltageꢀuntilꢀtheꢀaverageꢀinductorꢀcurrentꢀmatchesꢀ  
TH  
Theꢀstart-upꢀofꢀeachꢀcontroller’sꢀoutputꢀvoltage,ꢀV ,ꢀisꢀ  
OUTꢀ  
theꢀnewꢀloadꢀcurrent.  
controlledꢀbyꢀtheꢀvoltageꢀonꢀtheꢀSSꢀpinꢀforꢀthatꢀchannel.ꢀ  
AfterꢀtheꢀtopꢀMOSFETꢀisꢀturnedꢀoffꢀeachꢀcycle,ꢀtheꢀbottomꢀ  
MOSFETisturnedonuntileithertheinductorcurrentstartsꢀ  
toꢀreverse,ꢀasꢀindicatedꢀbyꢀtheꢀcurrentꢀcomparatorꢀIR,ꢀorꢀ  
theꢀbeginningꢀofꢀtheꢀnextꢀclockꢀcycle.  
WhenthevoltageontheSSpinislessthanthe0.8Vinternalꢀ  
reference,theLTC3868regulatestheV ꢀvoltagetotheSSꢀ  
FB  
pinꢀvoltageꢀinsteadꢀofꢀtheꢀ0.8Vꢀreference.ꢀThisꢀallowsꢀtheꢀ  
SSꢀpinꢀtoꢀbeꢀusedꢀtoꢀprogramꢀaꢀsoft-startꢀbyꢀconnectingꢀ  
anexternalcapacitorfromtheSSpintoSGND.Aninternalꢀ  
1µAꢀpull-upꢀcurrentꢀchargesꢀthisꢀcapacitorꢀcreatingꢀaꢀvolt-  
ageꢀrampꢀonꢀtheꢀSSꢀpin.ꢀAsꢀtheꢀSSꢀvoltageꢀrisesꢀlinearlyꢀ  
fromꢀ0Vꢀtoꢀ0.8Vꢀ(andꢀbeyondꢀupꢀtoꢀtheꢀabsoluteꢀmaximumꢀ  
INTV /EXTV Power  
CC  
CC  
PowerꢀforꢀtheꢀtopꢀandꢀbottomꢀMOSFETꢀdriversꢀandꢀmostꢀ  
otherinternalcircuitryisderivedfromtheINTV pin.Whenꢀ  
CC  
ratingof6V),theoutputvoltageV ꢀrisessmoothlyfromꢀ  
OUT  
theꢀEXTV ꢀpinꢀisꢀleftꢀopenꢀorꢀtiedꢀtoꢀaꢀvoltageꢀlessꢀthanꢀ  
CC  
zeroꢀtoꢀitsꢀfinalꢀvalue.  
4.7V,ꢀtheꢀV ꢀLDOꢀ(lowꢀdropoutꢀlinearꢀregulator)ꢀsuppliesꢀ  
IN  
5.1VꢀfromꢀV ꢀtoꢀINTV .ꢀIfꢀEXTV ꢀisꢀtakenꢀaboveꢀ4.7V,ꢀ  
IN  
CC  
CC  
Short-Circuit Latchoff  
theꢀV ꢀLDOꢀisꢀturnedꢀoffꢀandꢀanꢀEXTV ꢀLDOꢀisꢀturnedꢀon.ꢀ  
IN  
CC  
Onceenabled,theEXTV LDOsupplies5.1VfromEXTV ꢀ  
Afterꢀtheꢀcontrollerꢀhasꢀbeenꢀstartedꢀandꢀbeenꢀgivenꢀad-  
equateꢀtimeꢀtoꢀrampꢀupꢀtheꢀoutputꢀvoltage,ꢀtheꢀSSꢀcapaci-  
torꢀisꢀusedꢀinꢀaꢀshort-circuitꢀtimeoutꢀcircuit.ꢀSpecifically,ꢀ  
onceꢀtheꢀvoltageꢀonꢀtheꢀSSꢀpinꢀrisesꢀaboveꢀ2Vꢀ(theꢀarmingꢀ  
threshold),theshort-circuittimeoutcircuitisenabled(seeꢀ  
Figure1).Iftheoutputvoltagefallsbelow70%ofitsnomi-  
nalꢀregulatedꢀvoltage,ꢀtheꢀSSꢀcapacitorꢀbeginsꢀdischarg-  
ingꢀwithꢀaꢀnetꢀ9µAꢀpulldownꢀcurrentꢀonꢀtheꢀassumptionꢀ  
thatꢀtheꢀoutputꢀisꢀinꢀanꢀovercurrentꢀand/orꢀshort-circuitꢀ  
condition.ꢀIfꢀtheꢀconditionꢀlastsꢀlongꢀenoughꢀtoꢀallowꢀtheꢀ  
SSꢀpinꢀvoltageꢀtoꢀfallꢀbelowꢀ1.5Vꢀ(theꢀlatchoffꢀthreshold),ꢀ  
theꢀcontrollerꢀwillꢀshutꢀdownꢀ(latchꢀoff)ꢀuntilꢀtheꢀRUNꢀpinꢀ  
CC  
CC  
toꢀINTV .ꢀUsingꢀtheꢀEXTV ꢀpinꢀallowsꢀtheꢀINTV ꢀpowerꢀ  
CC  
CC  
CC  
toꢀbeꢀderivedꢀfromꢀaꢀhighꢀefficiencyꢀexternalꢀsourceꢀsuchꢀ  
asꢀoneꢀofꢀtheꢀLTC3868ꢀswitchingꢀregulatorꢀoutputs.  
EachꢀtopꢀMOSFETꢀdriverꢀisꢀbiasedꢀfromꢀtheꢀfloatingꢀboot-  
strapcapacitor,C ,whichnormallyrechargesduringeachꢀ  
B
cyclethroughanexternaldiodewhenthetopMOSFETꢀ  
turnsꢀoff.ꢀIfꢀtheꢀinputꢀvoltage,ꢀV ,ꢀdecreasesꢀtoꢀaꢀvoltageꢀ  
IN  
closeꢀtoꢀV ,ꢀtheꢀloopꢀmayꢀenterꢀdropoutꢀandꢀattemptꢀ  
OUTꢀ  
toturnonthetopMOSFETcontinuously.Thedropoutꢀ  
detectorꢀdetectsꢀthisꢀandꢀforcesꢀtheꢀtopꢀMOSFETꢀoffꢀforꢀ  
aboutꢀone-twelfthꢀofꢀtheꢀclockꢀperiodꢀeveryꢀtenthꢀcycleꢀtoꢀ  
allowꢀC ꢀtoꢀrecharge.  
voltageꢀorꢀtheꢀV ꢀvoltageꢀisꢀrecycled.  
B
IN  
3868fb  
ꢀꢀ  
LTC3868  
operaTion (Refer to the Functional Diagram)  
INTV  
CC  
SSꢀvoltageꢀhasꢀnotꢀyetꢀreachedꢀ2V),ꢀaꢀsafe,ꢀlowꢀoutputꢀ  
currentꢀisꢀprovidedꢀdueꢀtoꢀinternalꢀcurrentꢀfoldbackꢀandꢀ  
actualꢀpowerꢀwastedꢀisꢀlowꢀdueꢀtoꢀtheꢀefficientꢀnatureꢀofꢀ  
thecurrentmodeswitchingregulator.Foldbackcurrentꢀ  
limitingꢀisꢀdisabledꢀduringꢀtheꢀsoft-startꢀintervalꢀ(asꢀlongꢀ  
SS VOLTAGE  
2V  
1.5V  
0.8V  
LATCHOFF  
COMMAND  
asꢀtheꢀV ꢀvoltageꢀisꢀkeepingꢀupꢀwithꢀtheꢀSSꢀvoltage).ꢀ  
FB  
0V  
SS PIN  
CURRENT  
Light Load Current Operation (Burst Mode Operation,  
Pulse-Skipping or Forced Continuous Mode)  
(PLLIN/MODE Pin)  
1µA  
1µA  
–9µA  
OUTPUT  
VOLTAGE  
TheLTC3868canbeenabledtoenterhighefficiencyꢀ  
BurstꢀModeꢀoperation,ꢀconstantꢀfrequencyꢀpulse-skip-  
pingmode,orforcedcontinuousconductionmodeatꢀ  
lowloadcurrents.ToꢀselectꢀBurstꢀModeꢀoperation,ꢀtieꢀtheꢀ  
PLLIN/MODEpintoground.Toꢀselectꢀforcedꢀcontinuousꢀ  
3868 F01  
LATCHOFF  
ENABLE  
ARMING  
SOFT-START INTERVAL  
t
LATCH  
Figure 1. Latchoff Timing Diagram  
operation,ꢀtieꢀtheꢀPLLIN/MODEꢀpinꢀtoꢀINTV .ꢀToꢀselectꢀ  
CC  
pulse-skippingꢀmode,ꢀtieꢀtheꢀPLLIN/MODEꢀpinꢀtoꢀaꢀDCꢀ  
Theꢀdelayꢀtimeꢀfromꢀwhenꢀaꢀshort-circuitꢀoccursꢀuntilꢀtheꢀ  
controllerꢀlatchesꢀoffꢀcanꢀbeꢀcalculatedꢀusingꢀtheꢀfollow-  
ingꢀequation:  
voltageꢀgreaterꢀthanꢀ1.2VꢀandꢀlessꢀthanꢀINTV ꢀ–ꢀ1.3V.  
CC  
WhenacontrollerisenabledforBurstModeoperation,theꢀ  
minimumꢀpeakꢀcurrentꢀinꢀtheꢀinductorꢀisꢀsetꢀtoꢀapproxi-  
matelyꢀ30%ꢀofꢀtheꢀmaximumꢀsenseꢀvoltageꢀevenꢀthoughꢀ  
ꢀ t  
ꢀ~ꢀC ꢀ(V ꢀ–ꢀ1.5V)/9µA  
SS SS  
LATCH  
whereꢀV ꢀisꢀtheꢀinitialꢀvoltageꢀ(mustꢀbeꢀgreaterꢀthanꢀ2V)ꢀ  
ontheSSpinatthetimetheshort-circuitoccurs.Normallyꢀ  
SS  
theꢀvoltageꢀonꢀtheꢀI ꢀpinꢀindicatesꢀaꢀlowerꢀvalue.ꢀIfꢀtheꢀ  
TH  
averageꢀinductorꢀcurrentꢀisꢀhigherꢀthanꢀtheꢀloadꢀcurrent,ꢀ  
theꢀSSꢀpinꢀvoltageꢀwillꢀhaveꢀbeenꢀpulledꢀupꢀtoꢀtheꢀINTV ꢀ  
CC  
theꢀerrorꢀamplifierꢀEAꢀwillꢀdecreaseꢀtheꢀvoltageꢀonꢀtheꢀI ꢀ  
TH  
voltageꢀ(5.1V)ꢀbyꢀtheꢀinternalꢀ1µAꢀpull-upꢀcurrent.  
pin.ꢀWhenꢀtheꢀI ꢀvoltageꢀdropsꢀbelowꢀ0.425V,ꢀtheꢀinternalꢀ  
TH  
sleepꢀsignalꢀgoesꢀhighꢀ(enablingꢀsleepꢀmode)ꢀandꢀbothꢀ  
NotethatthetwocontrollersontheLTC3868haveseparate,ꢀ  
independentshort-circuitlatchoffcircuits.Latchoffcanbeꢀ  
overridden/defeatedꢀbyꢀconnectingꢀaꢀresistorꢀ150kꢀorꢀlessꢀ  
fromꢀtheꢀSSꢀpinꢀtoꢀINTV .ꢀThisꢀresistorꢀprovidesꢀenoughꢀ  
pull-upꢀcurrentꢀtoꢀovercomeꢀtheꢀ9µAꢀpull-downꢀcurrentꢀ  
externalꢀMOSFETsꢀareꢀturnedꢀoff.ꢀ  
Inꢀsleepꢀmode,ꢀmuchꢀofꢀtheꢀinternalꢀcircuitryꢀisꢀturnedꢀoff,ꢀ  
reducingthequiescentcurrent.Ifonechannelisshutdownꢀ  
andtheotherchannelisinsleepmode,theLTC3868drawsꢀ  
CC  
presentꢀduringꢀaꢀshort-circuit.ꢀNoteꢀthatꢀthisꢀresistorꢀalsoꢀ onlyꢀ170µAꢀofꢀquiescentꢀcurrent.ꢀIfꢀbothꢀchannelsꢀareꢀinꢀ  
sleepꢀmode,ꢀtheꢀLTC3868ꢀdrawsꢀonlyꢀ300µAꢀofꢀquiescentꢀ  
current.ꢀInꢀsleepꢀmode,ꢀtheꢀloadꢀcurrentꢀisꢀsuppliedꢀbyꢀ  
theꢀoutputꢀcapacitor.ꢀAsꢀtheꢀoutputꢀvoltageꢀdecreases,ꢀtheꢀ  
EA’sꢀoutputꢀbeginsꢀtoꢀrise.ꢀWhenꢀtheꢀoutputꢀvoltageꢀdropsꢀ  
shortensꢀtheꢀsoft-startꢀperiod.  
Foldback Current  
Onꢀtheꢀotherꢀhand,ꢀwhenꢀtheꢀoutputꢀvoltageꢀfallsꢀtoꢀlessꢀ  
thanꢀ72%ꢀofꢀitsꢀnominalꢀlevel,ꢀfoldbackꢀcurrentꢀlimitingꢀ  
isꢀalsoꢀactivated,ꢀprogressivelyꢀloweringꢀtheꢀpeakꢀcurrentꢀ  
limitinproportiontotheseverityoftheovercurrentorꢀ  
short-circuitꢀcondition.ꢀEvenꢀifꢀaꢀshort-circuitꢀisꢀpresentꢀ  
andtheshort-circuitlatchoffisnotyetenabled(whenꢀ  
enough,ꢀtheꢀI ꢀpinꢀisꢀreconnectedꢀtoꢀtheꢀoutputꢀofꢀtheꢀ  
TH  
EA,ꢀtheꢀsleepꢀsignalꢀgoesꢀlow,ꢀandꢀtheꢀcontrollerꢀresumesꢀ  
normalꢀoperationꢀbyꢀturningꢀonꢀtheꢀtopꢀexternalꢀMOSFETꢀ  
onꢀtheꢀnextꢀcycleꢀofꢀtheꢀinternalꢀoscillator.  
3868fb  
ꢀꢁ  
LTC3868  
operaTion (Refer to the Functional Diagram)  
IfꢀtheꢀPLLIN/MODEꢀpinꢀisꢀnotꢀbeingꢀdrivenꢀbyꢀanꢀexternalꢀ  
clockꢀsource,ꢀtheꢀFREQꢀpinꢀcanꢀbeꢀtiedꢀtoꢀSGND,ꢀtiedꢀtoꢀ  
WhenꢀaꢀcontrollerꢀisꢀenabledꢀforꢀBurstꢀModeꢀoperation,ꢀ  
theꢀinductorꢀcurrentꢀisꢀnotꢀallowedꢀtoꢀreverse.ꢀTheꢀreverseꢀ  
currentꢀ comparator,ꢀ IR,ꢀ turnsꢀ offꢀ theꢀ bottomꢀ externalꢀ  
MOSFETjustbeforetheinductorcurrentreacheszero,ꢀ  
preventingitfromreversingandgoingnegative.Thus,ꢀ  
theꢀcontrollerꢀisꢀinꢀdiscontinuousꢀoperation.  
INTV orprogrammedthroughanexternalresistor.Tyingꢀ  
CC  
FREQtoSGNDselects350kHzwhiletyingFREQtoINTV ꢀ  
CC  
selects535kHz.PlacingaresistorbetweenFREQandSGNDꢀ  
allowsꢀtheꢀfrequencyꢀtoꢀbeꢀprogrammedꢀbetweenꢀ50kHzꢀ  
andꢀ900kHz,ꢀasꢀshownꢀinꢀFigureꢀ9.  
Inꢀforcedꢀcontinuousꢀoperationꢀorꢀwhenꢀclockedꢀbyꢀanꢀ  
externalꢀclockꢀsourceꢀtoꢀuseꢀtheꢀphase-lockedꢀloopꢀ(seeꢀ  
FrequencyꢀSelectionꢀandꢀPhase-LockedꢀLoopꢀsection),ꢀ  
theꢀinductorꢀcurrentꢀisꢀallowedꢀtoꢀreverseꢀatꢀlightꢀloadsꢀ  
orꢀunderꢀlargeꢀtransientꢀconditions.ꢀTheꢀpeakꢀinductorꢀ  
Aꢀphase-lockedꢀloopꢀ(PLL)ꢀisꢀavailableꢀonꢀtheꢀLTC3868ꢀ  
toꢀsynchronizeꢀtheꢀinternalꢀoscillatorꢀtoꢀanꢀexternalꢀclockꢀ  
sourcethatisconnectedtothePLLIN/MODEpin.Theꢀ  
phaseꢀdetectorꢀadjustsꢀtheꢀvoltageꢀ(throughꢀanꢀinternalꢀ  
lowpasslter)oftheVCOinputtoaligntheturn-onofꢀ  
controllerꢀ1’sꢀexternalꢀtopꢀMOSFETꢀtoꢀtheꢀrisingꢀedgeꢀofꢀ  
thesynchronizingsignal.Thus,theturn-onofcontrollerꢀ2’sꢀ  
externalꢀtopꢀMOSFETꢀisꢀ180ꢀdegreesꢀoutꢀofꢀphaseꢀtoꢀtheꢀ  
risingꢀedgeꢀofꢀtheꢀexternalꢀclockꢀsource.  
currentꢀisꢀdeterminedꢀbyꢀtheꢀvoltageꢀonꢀtheꢀI ꢀpin,ꢀjustꢀ  
TH  
asinnormaloperation.Inthismode,theefficiencyatlightꢀ  
loadsꢀisꢀlowerꢀthanꢀinꢀBurstꢀModeꢀoperation.ꢀHowever,ꢀ  
continuousoperationhastheadvantagesofloweroutputꢀ  
voltageꢀrippleꢀandꢀlessꢀinterferenceꢀtoꢀaudioꢀcircuitry.ꢀInꢀ  
forcedcontinuousmode,theoutputrippleisindependentꢀ  
ofꢀloadꢀcurrent.  
TheꢀVCOꢀinputꢀvoltageꢀisꢀprebiasedꢀtoꢀtheꢀoperatingꢀfre-  
quencyꢀsetꢀbyꢀtheꢀFREQꢀpinꢀbeforeꢀtheꢀexternalꢀclockꢀisꢀ  
applied.Ifprebiasedneartheexternalclockfrequency,ꢀ  
theꢀPLLꢀloopꢀonlyꢀneedsꢀtoꢀmakeꢀslightꢀchangesꢀtoꢀtheꢀ  
VCOꢀinputꢀinꢀorderꢀtoꢀsynchronizeꢀtheꢀrisingꢀedgeꢀofꢀtheꢀ  
externalꢀclock’sꢀtoꢀtheꢀrisingꢀedgeꢀofꢀTG1.ꢀTheꢀabilityꢀtoꢀ  
prebiasthelooplterallowsthePLLtolock-inrapidlyꢀ  
withoutꢀdeviatingꢀfarꢀfromꢀtheꢀdesiredꢀfrequency.  
WhenꢀtheꢀPLLIN/MODEꢀpinꢀisꢀconnectedꢀforꢀpulse-skip-  
pingꢀmode,ꢀtheꢀLTC3868ꢀoperatesꢀinꢀPWMꢀpulse-skippingꢀ  
modeatlightloads.Inthismode,constantfrequencyꢀ  
operationismaintaineddowntoapproximately1%ofꢀ  
designedmaximumoutputcurrent.Atverylightloads,theꢀ  
currentꢀcomparator,ꢀICMP,ꢀmayꢀremainꢀtrippedꢀforꢀseveralꢀ  
cyclesꢀandꢀforceꢀtheꢀexternalꢀtopꢀMOSFETꢀtoꢀstayꢀoffꢀforꢀ  
thesamenumberofcycles(i.e.,skippingpulses).Theꢀ  
inductorꢀcurrentꢀisꢀnotꢀallowedꢀtoꢀreverseꢀ(discontinuousꢀ  
operation).ꢀThisꢀmode,ꢀlikeꢀforcedꢀcontinuousꢀoperation,ꢀ  
exhibitsꢀlowꢀoutputꢀrippleꢀasꢀwellꢀasꢀlowꢀaudioꢀnoiseꢀandꢀ  
reducedꢀRFꢀinterferenceꢀwhenꢀcomparedꢀtoꢀBurstꢀModeꢀ  
operation.ꢀ Itꢀ providesꢀ higherꢀ lightꢀ loadꢀ efficiencyꢀ thanꢀ  
forcedꢀcontinuousꢀmode,ꢀbutꢀnotꢀnearlyꢀasꢀhighꢀasꢀBurstꢀ  
Modeꢀoperation.  
Theꢀtypicalꢀcaptureꢀrangeꢀofꢀtheꢀphase-lockedꢀloopꢀisꢀfromꢀ  
approximatelyꢀ50kHzꢀtoꢀ900kHz,ꢀwithꢀaꢀguaranteeꢀoverꢀallꢀ  
manufacturingvariationstobebetween75kHzand850kHz.ꢀ  
Inꢀotherꢀwords,ꢀtheꢀLTC3868’sꢀPLLꢀisꢀguaranteedꢀtoꢀlockꢀ  
toꢀanꢀexternalꢀclockꢀsourceꢀwhoseꢀfrequencyꢀisꢀbetweenꢀ  
75kHzꢀandꢀ850kHz.  
ThetypicalinputclockthresholdsonthePLLIN/MODEꢀ  
pinꢀareꢀ1.6Vꢀ(rising)ꢀandꢀ1.1Vꢀ(falling).  
PolyPhase® Applications (CLKOUT and PHASMD Pins)  
Frequency Selection and Phase-Locked Loop  
(FREQ and PLLIN/MODE Pins)  
TheꢀLTC3868ꢀfeaturesꢀtwoꢀpinsꢀ(CLKOUTꢀandꢀPHASMD)ꢀ  
thatꢀallowꢀotherꢀcontrollerꢀICsꢀtoꢀbeꢀdaisy-chainedꢀwithꢀ  
theꢀLTC3868ꢀinꢀPolyPhaseꢀapplications.ꢀTheꢀclockꢀoutputꢀ  
signalontheCLKOUTpincanbeusedtosynchronizeꢀ  
additionalpowerstagesinamultiphasepowersupplyꢀ  
solutionꢀfeedingꢀaꢀsingle,ꢀhighꢀcurrentꢀoutputꢀorꢀmultipleꢀ  
separateꢀoutputs.ꢀTheꢀPHASMDꢀpinꢀisꢀusedꢀtoꢀadjustꢀtheꢀ  
phaseꢀofꢀtheꢀCLKOUTꢀsignalꢀasꢀwellꢀasꢀtheꢀrelativeꢀphasesꢀ  
Theselectionofswitchingfrequencyisatradeoffbetweenꢀ  
efficiencyꢀ andꢀ componentꢀ size.ꢀ Lowꢀ frequencyꢀ opera-  
tionꢀincreasesꢀefficiencyꢀbyꢀreducingꢀMOSFETꢀswitchingꢀ  
losses,ꢀbutꢀrequiresꢀlargerꢀinductanceꢀand/orꢀcapacitanceꢀ  
toꢀmaintainꢀlowꢀoutputꢀrippleꢀvoltage.  
TheꢀswitchingꢀfrequencyꢀofꢀtheꢀLTC3868’sꢀcontrollersꢀcanꢀ  
beꢀselectedꢀusingꢀtheꢀFREQꢀpin.  
3868fb  
ꢀꢂ  
LTC3868  
operaTion (Refer to the Functional Diagram)  
Theory and Benefits of 2-Phase Operation  
betweenꢀtheꢀtwoꢀinternalꢀcontrollers,ꢀasꢀsummarizedꢀinꢀ  
Table1.Thephasesarecalculatedrelativetothezeroꢀ  
degreesꢀphaseꢀbeingꢀdefinedꢀasꢀtheꢀrisingꢀedgeꢀofꢀtheꢀtopꢀ  
gateꢀdriverꢀoutputꢀofꢀcontrollerꢀ1ꢀ(TG1).  
Whyꢀtheꢀneedꢀforꢀ2-phaseꢀoperation?ꢀUpꢀuntilꢀtheꢀ2-phaseꢀ  
family,ꢀ constantꢀ frequencyꢀ dualꢀ switchingꢀ regulatorsꢀ  
operatedꢀ bothꢀ channelsꢀ inꢀ phaseꢀ (i.e.,ꢀ singleꢀ phaseꢀ  
operation).ꢀThisꢀmeansꢀthatꢀbothꢀswitchesꢀturnedꢀonꢀatꢀ  
theꢀsameꢀtime,ꢀcausingꢀcurrentꢀpulsesꢀofꢀupꢀtoꢀtwiceꢀtheꢀ  
amplitudeꢀofꢀthoseꢀforꢀoneꢀregulatorꢀtoꢀbeꢀdrawnꢀfromꢀtheꢀ  
inputꢀcapacitorꢀandꢀbattery.ꢀTheseꢀlargeꢀamplitudeꢀcurrentꢀ  
pulsesꢀincreasedꢀtheꢀtotalꢀRMSꢀcurrentꢀflowingꢀfromꢀtheꢀ  
inputꢀcapacitor,ꢀrequiringꢀtheꢀuseꢀofꢀmoreꢀexpensiveꢀinputꢀ  
capacitorsandincreasingbothEMIandlossesintheinputꢀ  
capacitorꢀandꢀbattery.  
Table 1  
V
CONTROLLER 2 PHASE  
CLKOUT PHASE  
PHASMD  
GND  
180°  
180°  
240°  
60°  
90°  
Floating  
INTV  
120°  
CC  
Output Overvoltage Protection  
Anovervoltagecomparatorguardsagainsttransientover-  
shootsꢀasꢀwellꢀasꢀotherꢀmoreꢀseriousꢀconditionsꢀthatꢀmayꢀ  
Withꢀ 2-phaseꢀ operation,ꢀ theꢀ twoꢀ channelsꢀ ofꢀ theꢀ dualꢀ  
switchingregulatorareoperated180degreesoutofphase.ꢀ  
Thiseffectivelyinterleavesthecurrentpulsesdrawnbytheꢀ  
switches,greatlyreducingtheoverlaptimewheretheyaddꢀ  
together.ꢀTheꢀresultꢀisꢀaꢀsignificantꢀreductionꢀinꢀtotalꢀRMSꢀ  
inputꢀcurrent,ꢀwhichꢀinꢀturnꢀallowsꢀlessꢀexpensiveꢀinputꢀ  
capacitorsꢀtoꢀbeꢀused,ꢀreducesꢀshieldingꢀrequirementsꢀforꢀ  
EMIꢀandꢀimprovesꢀrealꢀworldꢀoperatingꢀefficiency.  
overvoltageꢀtheꢀoutput.ꢀWhenꢀtheꢀV ꢀpinꢀrisesꢀbyꢀmoreꢀ  
FB  
than10%aboveitsregulationpointof0.800V,thetopꢀ  
MOSFETꢀisꢀturnedꢀoffꢀandꢀtheꢀbottomꢀMOSFETꢀisꢀturnedꢀ  
onꢀuntilꢀtheꢀovervoltageꢀconditionꢀisꢀcleared.  
Power Good (PGOOD1 and PGOOD2) Pins  
EachPGOODpinisconnectedtoanopendrainofanꢀ  
internalꢀN-channelꢀMOSFET.ꢀTheꢀMOSFETꢀturnsꢀonꢀandꢀ  
Figureꢀ2ꢀcomparesꢀtheꢀinputꢀwaveformsꢀforꢀaꢀrepresenta-  
tiveꢀsingle-phaseꢀdualꢀswitchingꢀregulatorꢀtoꢀtheꢀLTC3868ꢀ  
2-phasedualswitchingregulator.Anactualmeasurementofꢀ  
theꢀRMSꢀinputꢀcurrentꢀunderꢀtheseꢀconditionsꢀshowsꢀthatꢀ  
pullsꢀtheꢀPGOODꢀpinꢀlowꢀwhenꢀtheꢀcorrespondingꢀV ꢀpinꢀ  
FB  
voltageꢀisꢀnotꢀwithinꢀ 10%ꢀofꢀtheꢀ0.8Vꢀreferenceꢀvoltage.ꢀ  
ThePGOODpinisalsopulledlowwhenthecorrespondingꢀ  
2-phaseoperationdroppedtheinputcurrentfrom2.53A  
RUNꢀpinꢀisꢀlowꢀ(shutꢀdown).ꢀWhenꢀtheꢀV ꢀpinꢀvoltageꢀ  
RMS  
FB  
to1.55A  
.Whilethisisanimpressivereductioninitself,ꢀ  
isꢀwithinꢀtheꢀ 10%ꢀrequirement,ꢀtheꢀMOSFETꢀisꢀturnedꢀ  
offꢀandꢀtheꢀpinꢀisꢀallowedꢀtoꢀbeꢀpulledꢀupꢀbyꢀanꢀexternalꢀ  
resistorꢀtoꢀaꢀsourceꢀnoꢀgreaterꢀthanꢀ6V.  
RMS  
2
rememberthatthepowerlossesareproportionaltoI  
,ꢀ  
RMS  
5V SWITCH  
20V/DIV  
3.3V SWITCH  
20V/DIV  
INPUT CURRENT  
5A/DIV  
INPUT VOLTAGE  
500mV/DIV  
3868 F02  
I
= 2.53A  
I
= 1.55A  
IN(MEAS) RMS  
IN(MEAS)  
RMS  
Figure 2. Input Waveforms Comparing Single-Phase (a) and 2-Phase (b) Operation for Dual Switching Regulators  
Converting 12V to 5V and 3.3V at 3A Each. The Reduced Input Ripple with the 2-Phase Regulator Allows  
Less Expensive Input Capacitors, Reduces Shielding Requirements for EMI and Improves Efficiency  
3868fb  
ꢀꢃ  
LTC3868  
operaTion (Refer to the Functional Diagram)  
meaningꢀthatꢀtheꢀactualꢀpowerꢀwastedꢀisꢀreducedꢀbyꢀaꢀfac-  
voltageꢀV ꢀ(DutyꢀCycleꢀ=ꢀV /V ).ꢀFigureꢀ3ꢀshowsꢀhowꢀ  
IN OUT IN  
torꢀofꢀ2.66.ꢀTheꢀreducedꢀinputꢀrippleꢀvoltageꢀalsoꢀmeansꢀ theRMSinputcurrentvariesforsinglephaseand2-phaseꢀ  
lessꢀpowerꢀisꢀlostꢀinꢀtheꢀinputꢀpowerꢀpath,ꢀwhichꢀcouldꢀ operationꢀforꢀ3.3Vꢀandꢀ5Vꢀregulatorsꢀoverꢀaꢀwideꢀinputꢀ  
includeꢀbatteries,ꢀswitches,ꢀtrace/connectorꢀresistancesꢀ voltageꢀrange.  
andprotectioncircuitry.Improvementsinbothconductedꢀ  
Itꢀcanꢀreadilyꢀbeꢀseenꢀthatꢀtheꢀadvantagesꢀofꢀ2-phaseꢀop-  
andꢀradiatedꢀEMIꢀalsoꢀdirectlyꢀaccrueꢀasꢀaꢀresultꢀofꢀtheꢀ  
erationꢀareꢀnotꢀjustꢀlimitedꢀtoꢀaꢀnarrowꢀoperatingꢀrange,ꢀ  
reducedꢀRMSꢀinputꢀcurrentꢀandꢀvoltage.  
formostapplicationsisthat2-phaseoperationwillreduceꢀ  
theinputcapacitorrequirementtothatforjustonechannelꢀ  
Ofꢀcourse,ꢀtheꢀimprovementꢀaffordedꢀbyꢀ2-phaseꢀopera-  
tionꢀisꢀaꢀfunctionꢀofꢀtheꢀdualꢀswitchingꢀregulator’sꢀrelativeꢀ operatingꢀatꢀmaximumꢀcurrentꢀandꢀ50%ꢀdutyꢀcycle.  
dutyꢀcyclesꢀwhich,ꢀinꢀturn,ꢀareꢀdependentꢀuponꢀtheꢀinputꢀ  
3.0  
SINGLE PHASE  
DUAL CONTROLLER  
2.5  
2.0  
1.5  
1.0  
0.5  
0
2-PHASE  
DUAL CONTROLLER  
V
O1  
V
O2  
= 5V/3A  
= 3.3V/3A  
0
10  
20  
30  
40  
INPUT VOLTAGE (V)  
3868 F03  
Figure 3. RMS Input Current Comparison  
3868fb  
ꢀꢄ  
LTC3868  
applicaTions inForMaTion  
andINTV +0.5V,thecurrenttransitionsfromthesmallerꢀ  
TheTypicalApplicationontherstpageisabasicLTC3868ꢀ  
applicationꢀ circuit.ꢀ LTC3868ꢀ canꢀ beꢀ configuredꢀ toꢀ useꢀ  
eitherꢀ DCRꢀ (inductorꢀ resistance)ꢀ sensingꢀ orꢀ lowꢀ valueꢀ  
resistorꢀ sensing.ꢀ Theꢀ choiceꢀ betweenꢀ theꢀ twoꢀ currentꢀ  
sensingschemesislargelyadesigntradeoffbetweenꢀ  
cost,ꢀ powerꢀ consumptionꢀ andꢀ accuracy.ꢀ DCRꢀ sensingꢀ  
isꢀbecomingꢀpopularꢀbecauseꢀitꢀsavesꢀexpensiveꢀcurrentꢀ  
sensingꢀresistorsꢀandꢀisꢀmoreꢀpowerꢀefficient,ꢀespeciallyꢀ  
inꢀ highꢀ currentꢀ applications.ꢀ However,ꢀ currentꢀ sensingꢀ  
resistorsꢀprovideꢀtheꢀmostꢀaccurateꢀcurrentꢀlimitsꢀforꢀtheꢀ  
controller.ꢀOtherꢀexternalꢀcomponentꢀselectionꢀisꢀdrivenꢀ  
byꢀtheꢀloadꢀrequirement,ꢀandꢀbeginsꢀwithꢀtheꢀselectionꢀofꢀ  
CC  
currentꢀtoꢀtheꢀhigherꢀcurrent.  
Filtercomponentsmutualtothesenselinesshouldbeꢀ  
placedꢀcloseꢀtoꢀtheꢀLTC3868,ꢀandꢀtheꢀsenseꢀlinesꢀshouldꢀ  
runꢀcloseꢀtogetherꢀtoꢀaꢀKelvinꢀconnectionꢀunderneathꢀtheꢀ  
currentꢀsenseꢀelementꢀ(shownꢀinꢀFigureꢀ4).ꢀSensingꢀcur-  
rentelsewherecaneffectivelyaddparasiticinductanceꢀ  
andꢀcapacitanceꢀtoꢀtheꢀcurrentꢀsenseꢀelement,ꢀdegradingꢀ  
theinformationatthesenseterminalsandmakingtheꢀ  
programmedꢀcurrentꢀlimitꢀunpredictable.ꢀIfꢀinductorꢀDCRꢀ  
sensingꢀisꢀusedꢀ(Figureꢀ5b),ꢀresistorꢀR1ꢀshouldꢀbeꢀplacedꢀ  
closetotheswitchingnode,topreventnoisefromcouplingꢀ  
intoꢀsensitiveꢀsmall-signalꢀnodes.  
R ꢀ(ifꢀR  
SENSE  
ꢀisꢀused)ꢀandꢀinductorꢀvalue.ꢀNext,ꢀtheꢀ  
SENSE  
powerMOSFETsandSchottkydiodesareselected.Finally,ꢀ  
inputꢀandꢀoutputꢀcapacitorsꢀareꢀselected.  
TO SENSE FILTER,  
NEXT TO THE CONTROLLER  
Current Limit Programming  
TheI ꢀpinisatri-levellogicinputwhichsetsthemaximumꢀ  
LIM  
C
OUT  
3868 F04  
currentꢀlimitꢀofꢀtheꢀconverter.ꢀWhenꢀI ꢀisꢀgrounded,ꢀtheꢀ  
LIM  
INDUCTOR OR R  
SENSE  
maximumꢀcurrentꢀlimitꢀthresholdꢀvoltageꢀofꢀtheꢀcurrentꢀ  
Figure 4. Sense Lines Placement with Inductor or Sense Resistor  
comparatorisprogrammedtobe30mV.WhenI ꢀisꢀ  
LIM  
floated,themaximumcurrentlimitthresholdis50mV.ꢀ  
Low Value Resistor Current Sensing  
WhenꢀI ꢀisꢀtiedꢀtoꢀINTV ,ꢀtheꢀmaximumꢀcurrentꢀlimitꢀ  
LIM  
CC  
Aꢀtypicalꢀsensingꢀcircuitꢀusingꢀaꢀdiscreteꢀresistorꢀisꢀshownꢀ  
thresholdꢀisꢀsetꢀtoꢀ75mV.  
inꢀ Figureꢀ 5a.ꢀ R  
outputꢀcurrent.  
ꢀ isꢀ chosenꢀ basedꢀ onꢀ theꢀ requiredꢀ  
SENSE  
+
SENSE and SENSE Pins  
+
TheꢀSENSE ꢀandꢀSENSE ꢀpinsꢀareꢀtheꢀinputsꢀtoꢀtheꢀcur-  
rentꢀcomparators.ꢀTheꢀcommonꢀmodeꢀvoltageꢀrangeꢀonꢀ  
Theꢀ currentꢀ comparatorꢀ hasꢀ aꢀ maximumꢀ thresholdꢀ  
ꢀdeterminedꢀbyꢀtheꢀI ꢀsetting.ꢀTheꢀcurrentꢀ  
V
SENSE(MAX)  
LIM  
thesepinsis0Vto16V(AbsoluteMaximum),enablingtheꢀ comparatorꢀthresholdꢀvoltageꢀsetsꢀtheꢀpeakꢀofꢀtheꢀinduc-  
LTC3868ꢀtoꢀregulateꢀoutputꢀvoltagesꢀupꢀtoꢀaꢀnominalꢀ14Vꢀ torꢀcurrent,ꢀyieldingꢀaꢀmaximumꢀaverageꢀoutputꢀcurrent,ꢀ  
(allowingꢀmarginꢀforꢀtolerancesꢀandꢀtransients).  
I
,ꢀequalꢀtoꢀtheꢀpeakꢀvalueꢀlessꢀhalfꢀtheꢀpeak-to-peakꢀ  
MAX  
rippleꢀcurrent,ꢀI .ꢀToꢀcalculateꢀtheꢀsenseꢀresistorꢀvalue,ꢀ  
+
L
TheꢀSENSE ꢀpinꢀisꢀhighꢀimpedanceꢀoverꢀtheꢀfullꢀcommonꢀ  
useꢀtheꢀequation:  
modeꢀrange,ꢀdrawingꢀatꢀmostꢀ 1µA.ꢀThisꢀhighꢀimpedanceꢀ  
allowsthecurrentcomparatorstobeusedininductorꢀ  
DCRꢀsensing.  
VSENSE(MAX)  
RSENSE  
=
IL  
IMAX  
+
TheꢀimpedanceꢀofꢀtheꢀSENSE ꢀpinꢀchangesꢀdependingꢀonꢀ  
2
thecommonmodevoltage.WhenSENSE islessthanꢀ  
Whenꢀusingꢀtheꢀcontrollerꢀinꢀveryꢀlowꢀdropoutꢀconditions,ꢀ  
theꢀmaximumꢀoutputꢀcurrentꢀlevelꢀwillꢀbeꢀreducedꢀdueꢀtoꢀ  
theꢀinternalꢀcompensationꢀrequiredꢀtoꢀmeetꢀstabilityꢀcri-  
terionꢀforꢀbuckꢀregulatorsꢀoperatingꢀatꢀgreaterꢀthanꢀ50%ꢀ  
INTV ꢀ–ꢀ0.5V,ꢀaꢀsmallꢀcurrentꢀofꢀlessꢀthanꢀ1µAꢀflowsꢀoutꢀ  
CC  
ofꢀtheꢀpin.ꢀWhenꢀSENSE ꢀisꢀaboveꢀINTV ꢀ+ꢀ0.5V,ꢀaꢀhigherꢀ  
current(~550µA)owsintothepin.BetweenINTV 0.5Vꢀ  
CC  
CC  
3868fb  
ꢀꢅ  
LTC3868  
applicaTions inForMaTion  
dutyfactor.AcurveisprovidedintheTypicalꢀPerformanceꢀ  
Characteristicsꢀ sectionꢀ toꢀ estimateꢀ thisꢀ reductionꢀ inꢀ  
peakꢀoutputꢀcurrentꢀdependingꢀuponꢀtheꢀoperatingꢀdutyꢀ  
factor.  
UsingꢀtheꢀinductorꢀrippleꢀcurrentꢀvalueꢀfromꢀtheꢀInductorꢀ  
ValueꢀCalculationꢀsection,ꢀtheꢀtargetꢀsenseꢀresistorꢀvalueꢀ  
is:  
VSENSE(MAX)  
RSENSE(EQUIV)  
=
Inductor DCR Sensing  
IL  
IMAX  
+
2
Forꢀapplicationsꢀrequiringꢀtheꢀhighestꢀpossibleꢀefficiencyꢀ  
atꢀhighꢀloadꢀcurrents,ꢀtheꢀLTC3850ꢀisꢀcapableꢀofꢀsensingꢀ  
theꢀvoltageꢀdropꢀacrossꢀtheꢀinductorꢀDCR,ꢀasꢀshownꢀinꢀ  
Figureꢀ5b.ꢀTheꢀDCRꢀofꢀtheꢀinductorꢀrepresentsꢀtheꢀsmallꢀ  
amountꢀofꢀDCꢀresistanceꢀofꢀtheꢀcopperꢀwire,ꢀwhichꢀcanꢀbeꢀ  
lessthan1fortoday’slowvalue,highcurrentinductors.ꢀ  
Inꢀaꢀhighꢀcurrentꢀapplicationꢀrequiringꢀsuchꢀanꢀinductor,ꢀ  
powerꢀlossꢀthroughꢀaꢀsenseꢀresistorꢀwouldꢀcostꢀseveralꢀ  
pointsꢀofꢀefficiencyꢀcomparedꢀtoꢀinductorꢀDCRꢀsensing.  
Toꢀensureꢀthatꢀtheꢀapplicationꢀwillꢀdeliverꢀfullꢀloadꢀcurrentꢀ  
overꢀ theꢀ fullꢀ operatingꢀ temperatureꢀ range,ꢀ chooseꢀ theꢀ  
minimumꢀvalueꢀforꢀtheꢀmaximumꢀcurrentꢀsenseꢀthresholdꢀ  
voltageꢀ (V  
)ꢀ inꢀ theꢀ Electricalꢀ Characteristicsꢀ  
SENSE(MAX)  
tableꢀ(30mV,ꢀ50mVꢀorꢀ75mVꢀdependingꢀonꢀtheꢀstateꢀofꢀ  
theꢀI ꢀpin).  
LIM  
Next,ꢀdetermineꢀtheꢀDCRꢀofꢀtheꢀinductor.ꢀWhenꢀprovided,ꢀ  
useꢀtheꢀmanufacturer’sꢀmaximumꢀvalue,ꢀusuallyꢀgivenꢀatꢀ  
20°C.ꢀIncreaseꢀthisꢀvalueꢀtoꢀaccountꢀforꢀtheꢀtemperatureꢀ  
coefficientꢀofꢀcopperꢀresistance,ꢀwhichꢀisꢀapproximatelyꢀ  
IfꢀtheꢀexternalꢀR1||R2ꢀ•ꢀC1ꢀtimeꢀconstantꢀisꢀchosenꢀtoꢀbeꢀ  
exactlyꢀequalꢀtoꢀtheꢀL/DCRꢀtimeꢀconstant,ꢀtheꢀvoltageꢀdropꢀ  
acrossꢀtheꢀexternalꢀcapacitorꢀisꢀequalꢀtoꢀtheꢀdropꢀacrossꢀ  
theinductorDCRmultipliedbyR2/(R1+R2).R2scalestheꢀ  
voltageꢀacrossꢀtheꢀsenseꢀterminalsꢀforꢀapplicationsꢀwhereꢀ  
theꢀDCRꢀisꢀgreaterꢀthanꢀtheꢀtargetꢀsenseꢀresistorꢀvalue.ꢀ  
Toꢀproperlyꢀdimensionꢀtheꢀexternalꢀfilterꢀcomponents,ꢀtheꢀ  
DCRꢀofꢀtheꢀinductorꢀmustꢀbeꢀknown.ꢀItꢀcanꢀbeꢀmeasuredꢀ  
usingagoodRLCmeter,buttheDCRtoleranceisnotꢀ  
alwaysꢀtheꢀsameꢀandꢀvariesꢀwithꢀtemperature;ꢀconsultꢀtheꢀ  
manufacturers’ꢀdataꢀsheetsꢀforꢀdetailedꢀinformation.  
0.4%/°C.ꢀAꢀconservativeꢀvalueꢀforꢀT  
ꢀisꢀ100°C.  
L(MAX)  
ToꢀscaleꢀtheꢀmaximumꢀinductorꢀDCRꢀtoꢀtheꢀdesiredꢀsenseꢀ  
resistorꢀ(R )ꢀvalue,ꢀuseꢀtheꢀdividerꢀratio:  
D
RSENSE(EQUIV)  
RD =  
DCRMAX atT  
L(MAX)  
C1ꢀisꢀusuallyꢀselectedꢀtoꢀbeꢀinꢀtheꢀrangeꢀofꢀ0.1µFꢀtoꢀ0.47µF.ꢀ  
ThisforcesR1||R2toaround2k,reducingerrorthatmightꢀ  
+
haveꢀbeenꢀcausedꢀbyꢀtheꢀSENSE ꢀpin’sꢀ 1µAꢀcurrent.  
V
V
IN  
V
IN  
V
IN  
IN  
INTV  
INTV  
CC  
CC  
INDUCTOR  
DCR  
BOOST  
TG  
BOOST  
TG  
R
SENSE  
L
SW  
V
OUT  
SW  
V
OUT  
LTC3868  
LTC3868  
BG  
BG  
R1  
C1* R2  
+
+
SENSE  
SENSE  
PLACE CAPACITOR NEAR  
SENSE PINS  
SENSE  
SENSE  
SGND  
SGND  
3868 F05b  
R2  
R1 + R2  
3868 F05a  
L
DCR  
||  
(R1 R2) C1 =  
*PLACE C1 NEAR  
SENSE PINS  
R
= DCR  
SENSE(EQ)  
(5a) Using a Resistor to Sense Current  
(5b) Using the Inductor DCR to Sense Current  
Figure 5. Current Sensing Methods  
3868fb  
ꢀꢆ  
LTC3868  
applicaTions inForMaTion  
TheꢀequivalentꢀresistanceꢀR1||R2ꢀisꢀscaledꢀtoꢀtheꢀroomꢀ  
Acceptingꢀ largerꢀ valuesꢀ ofꢀ I ꢀ allowsꢀ theꢀ useꢀ ofꢀ lowꢀ  
L
temperatureꢀinductanceꢀandꢀmaximumꢀDCR:  
inductances,ꢀbutꢀresultsꢀinꢀhigherꢀoutputꢀvoltageꢀrippleꢀ  
andꢀgreaterꢀcoreꢀlosses.ꢀAꢀreasonableꢀstartingꢀpointꢀforꢀ  
L
R1||R2 =  
settingꢀrippleꢀcurrentꢀisꢀI ꢀ=ꢀ0.3(I  
).ꢀTheꢀmaximumꢀ  
MAX  
L
DCR at 20°C C1  
(
)
I ꢀoccursꢀatꢀtheꢀmaximumꢀinputꢀvoltage.  
L
Theꢀinductorꢀvalueꢀalsoꢀhasꢀsecondaryꢀeffects.ꢀTheꢀtran-  
sitionꢀtoꢀBurstꢀModeꢀoperationꢀbeginsꢀwhenꢀtheꢀaverageꢀ  
inductorꢀcurrentꢀrequiredꢀresultsꢀinꢀaꢀpeakꢀcurrentꢀbelowꢀ  
Theꢀsenseꢀresistorꢀvaluesꢀare:  
R1RD  
1RD  
R1||R2  
RD  
R1=  
; R2 =  
30%ofthecurrentlimitdeterminedbyR  
.Lowerꢀ  
SENSE  
inductorvalues(higherI )willcausethistooccuratꢀ  
L
TheꢀmaximumꢀpowerꢀlossꢀinꢀR1ꢀisꢀrelatedꢀtoꢀdutyꢀcycle,ꢀ  
andꢀwillꢀoccurꢀinꢀcontinuousꢀmodeꢀatꢀtheꢀmaximumꢀinputꢀ  
voltage:  
lowerꢀloadꢀcurrents,ꢀwhichꢀcanꢀcauseꢀaꢀdipꢀinꢀefficiencyꢀinꢀ  
theꢀupperꢀrangeꢀofꢀlowꢀcurrentꢀoperation.ꢀInꢀBurstꢀModeꢀ  
operation,ꢀlowerꢀinductanceꢀvaluesꢀwillꢀcauseꢀtheꢀburstꢀ  
frequencyꢀtoꢀdecrease.  
V
IN(MAX) VOUT • V  
(
)
OUT  
P
R1=  
LOSS  
Inductor Core Selection  
R1  
OnceꢀtheꢀvalueꢀforꢀLꢀisꢀknown,ꢀtheꢀtypeꢀofꢀinductorꢀmustꢀ  
beꢀselected.ꢀHighꢀefficiencyꢀconvertersꢀgenerallyꢀcannotꢀ  
affordthecorelossfoundinlowcostpowderedironcores,ꢀ  
forcingtheuseofmoreexpensiveferriteormolypermalloyꢀ  
cores.ꢀActualꢀcoreꢀlossꢀisꢀindependentꢀofꢀcoreꢀsizeꢀforꢀaꢀ  
fixedinductorvalue,butitisverydependentoninductanceꢀ  
valueꢀselected.ꢀAsꢀinductanceꢀincreases,ꢀcoreꢀlossesꢀgoꢀ  
down.ꢀUnfortunately,ꢀincreasedꢀinductanceꢀrequiresꢀmoreꢀ  
turnsꢀofꢀwireꢀandꢀthereforeꢀcopperꢀlossesꢀwillꢀincrease.  
EnsureꢀthatꢀR1ꢀhasꢀaꢀpowerꢀratingꢀhigherꢀthanꢀthisꢀvalue.ꢀ  
Ifꢀhighꢀefficiencyꢀisꢀnecessaryꢀatꢀlightꢀloads,ꢀconsiderꢀthisꢀ  
powerꢀlossꢀwhenꢀdecidingꢀwhetherꢀtoꢀuseꢀDCRꢀsensingꢀorꢀ  
senseꢀresistors.ꢀLightꢀloadꢀpowerꢀlossꢀcanꢀbeꢀmodestlyꢀ  
higherꢀwithꢀaꢀDCRꢀnetworkꢀthanꢀwithꢀaꢀsenseꢀresistor,ꢀdueꢀ  
totheextraswitchinglossesincurredthroughR1.However,ꢀ  
DCRꢀsensingꢀeliminatesꢀaꢀsenseꢀresistor,ꢀreducesꢀconduc-  
tionꢀlossesꢀandꢀprovidesꢀhigherꢀefficiencyꢀatꢀheavyꢀloads.ꢀ  
Peakꢀefficiencyꢀisꢀaboutꢀtheꢀsameꢀwithꢀeitherꢀmethod.  
Ferriteꢀdesignsꢀhaveꢀveryꢀlowꢀcoreꢀlossꢀandꢀareꢀpreferredꢀ  
atꢀhighꢀswitchingꢀfrequencies,ꢀsoꢀdesignꢀgoalsꢀcanꢀcon-  
centrateꢀonꢀcopperꢀlossꢀandꢀpreventingꢀsaturation.ꢀFerriteꢀ  
corematerialsaturateshard,whichmeansthatinduc-  
tanceꢀcollapsesꢀabruptlyꢀwhenꢀtheꢀpeakꢀdesignꢀcurrentꢀisꢀ  
exceeded.ꢀThisꢀresultsꢀinꢀanꢀabruptꢀincreaseꢀinꢀinductorꢀ  
rippleꢀcurrentꢀandꢀconsequentꢀoutputꢀvoltageꢀripple.ꢀDoꢀ  
notꢀallowꢀtheꢀcoreꢀtoꢀsaturate!  
Inductor Value Calculation  
Theꢀoperatingꢀfrequencyꢀandꢀinductorꢀselectionꢀareꢀinter-  
relatedꢀinꢀthatꢀhigherꢀoperatingꢀfrequenciesꢀallowꢀtheꢀuseꢀ  
ofꢀsmallerꢀinductorꢀandꢀcapacitorꢀvalues.ꢀSoꢀwhyꢀwouldꢀ  
anyoneꢀeverꢀchooseꢀtoꢀoperateꢀatꢀlowerꢀfrequenciesꢀwithꢀ  
largercomponents?Theanswerisefficiency.Ahigherꢀ  
frequencygenerallyresultsinlowerefficiencybecauseꢀ  
ofꢀMOSFETꢀgateꢀchargeꢀlosses.ꢀInꢀadditionꢀtoꢀthisꢀbasicꢀ  
trade-off,ꢀtheꢀeffectꢀofꢀinductorꢀvalueꢀonꢀrippleꢀcurrentꢀandꢀ  
lowꢀcurrentꢀoperationꢀmustꢀalsoꢀbeꢀconsidered.  
Power MOSFET and Schottky Diode (Optional)  
Selection  
TwoꢀexternalꢀpowerꢀMOSFETsꢀmustꢀbeꢀselectedꢀforꢀeachꢀ  
controllerꢀinꢀtheꢀLTC3868:ꢀoneꢀN-channelꢀMOSFETꢀforꢀtheꢀ  
top(main)switch,andoneN-channelMOSFETfortheꢀ  
bottomꢀ(synchronous)ꢀswitch.  
Theinductorvaluehasadirecteffectonripplecurrent.Theꢀ  
inductorꢀrippleꢀcurrentꢀI ꢀdecreasesꢀwithꢀhigherꢀinduc-  
L
tanceꢀorꢀhigherꢀfrequencyꢀandꢀincreasesꢀwithꢀhigherꢀV :  
IN  
V
V
IN  
1
OUT   
1–  
OUT   
ΔIL =  
V
Thepeak-to-peakdrivelevelsaresetbytheINTV voltage.ꢀ  
CC  
f L  
( )( )  
Thisꢀvoltageꢀisꢀtypicallyꢀ5.2Vꢀduringꢀstart-upꢀ(seeꢀEXTV ꢀ  
CC  
3868fb  
ꢀꢇ  
LTC3868  
applicaTions inForMaTion  
2
BothMOSFETshaveI RlosseswhilethetopsideN-channelꢀ  
equationꢀincludesꢀanꢀadditionalꢀtermꢀforꢀtransitionꢀlosses,ꢀ  
Pinꢀ Connection).ꢀ Consequently,ꢀ logic-levelꢀ thresholdꢀ  
MOSFETsmustꢀbeꢀusedꢀinꢀmostꢀapplications.ꢀTheꢀonlyꢀ  
whichꢀareꢀhighestꢀatꢀhighꢀinputꢀvoltages.ꢀForꢀV ꢀ<ꢀ20Vꢀ  
exceptionꢀisꢀifꢀlowꢀinputꢀvoltageꢀisꢀexpectedꢀ(V ꢀ<ꢀ4V);ꢀ  
IN  
IN  
GS(TH)  
theꢀhighꢀcurrentꢀefficiencyꢀgenerallyꢀimprovesꢀwithꢀlargerꢀ  
then,ꢀsub-logicꢀlevelꢀthresholdꢀMOSFETsꢀ(V  
ꢀ<ꢀ3V)ꢀ  
MOSFETs,ꢀwhileꢀforꢀV ꢀ>ꢀ20Vꢀtheꢀtransitionꢀlossesꢀrapidlyꢀ  
shouldꢀbeꢀused.ꢀPayꢀcloseꢀattentionꢀtoꢀtheꢀBV ꢀspeci-  
IN  
DSS  
increasetothepointthattheuseofahigherR  
deviceꢀ  
DS(ON)  
ficationꢀforꢀtheꢀMOSFETsꢀasꢀwell;ꢀmanyꢀofꢀtheꢀlogic-levelꢀ  
withlowerC  
actuallyprovideshigherefficiency.Theꢀ  
MOSFETsꢀareꢀlimitedꢀtoꢀ30Vꢀorꢀless.  
MILLER  
synchronousꢀMOSFETꢀlossesꢀareꢀgreatestꢀatꢀhighꢀinputꢀ  
voltageꢀwhenꢀtheꢀtopꢀswitchꢀdutyꢀfactorꢀisꢀlowꢀorꢀduringꢀ  
aꢀshort-circuitꢀwhenꢀtheꢀsynchronousꢀswitchꢀisꢀonꢀcloseꢀ  
toꢀ100%ꢀofꢀtheꢀperiod.  
Selectionꢀ criteriaꢀ forꢀ theꢀ powerꢀ MOSFETsꢀ includeꢀ theꢀ  
on-resistance,ꢀR  
,ꢀMillerꢀcapacitance,ꢀC  
,ꢀinputꢀ  
DS(ON)  
MILLER  
voltageꢀandꢀmaximumꢀoutputꢀcurrent.ꢀMillerꢀcapacitance,ꢀ  
,ꢀcanꢀbeꢀapproximatedꢀfromꢀtheꢀgateꢀchargeꢀcurveꢀ  
C
MILLER  
Theꢀtermꢀ(1+ꢀδ)ꢀisꢀgenerallyꢀgivenꢀforꢀaꢀMOSFETꢀinꢀtheꢀ  
usuallyꢀ providedꢀ onꢀ theꢀ MOSFETꢀ manufacturers’ꢀ dataꢀ  
sheet.C ꢀisequaltotheincreaseingatechargeꢀ  
formꢀofꢀaꢀnormalizedꢀR  
ꢀvsꢀTemperatureꢀcurve,ꢀbutꢀ  
DS(ON)  
MILLER  
δ=0.005/°Ccanbeusedasanapproximationforlowꢀ  
alongꢀtheꢀhorizontalꢀaxisꢀwhileꢀtheꢀcurveꢀisꢀapproximatelyꢀ  
voltageꢀMOSFETs.  
flatꢀdividedꢀbyꢀtheꢀspecifiedꢀchangeꢀinꢀV .ꢀThisꢀresultꢀisꢀ  
DS  
thenꢀmultipliedꢀbyꢀtheꢀratioꢀofꢀtheꢀapplicationꢀappliedꢀV ꢀ  
DS  
TheoptionalSchottkydiodesD1andD2showninFigureꢀ10ꢀ  
conductꢀduringꢀtheꢀdead-timeꢀbetweenꢀtheꢀconductionꢀofꢀ  
theꢀtwoꢀpowerꢀMOSFETs.ꢀThisꢀpreventsꢀtheꢀbodyꢀdiodeꢀofꢀ  
thebottomMOSFETfromturningon,storingchargeduringꢀ  
thedead-timeandrequiringareverserecoveryperiodthatꢀ  
toꢀtheꢀgateꢀchargeꢀcurveꢀspecifiedꢀV .ꢀWhenꢀtheꢀICꢀisꢀ  
DS  
operatingꢀinꢀcontinuousꢀmodeꢀtheꢀdutyꢀcyclesꢀforꢀtheꢀtopꢀ  
andꢀbottomꢀMOSFETsꢀareꢀgivenꢀby:  
VOUT  
Main Switch Duty Cycle =  
couldꢀcostꢀasꢀmuchꢀasꢀ3%ꢀinꢀefficiencyꢀatꢀhighꢀV .ꢀAꢀ1Aꢀ  
IN  
V
IN  
toꢀ3AꢀSchottkyꢀisꢀgenerallyꢀaꢀgoodꢀcompromiseꢀforꢀbothꢀ  
regionsꢀofꢀoperationꢀdueꢀtoꢀtheꢀrelativelyꢀsmallꢀaverageꢀ  
current.Largerdiodesresultinadditionaltransitionlossesꢀ  
dueꢀtoꢀtheirꢀlargerꢀjunctionꢀcapacitance.  
V VOUT  
IN  
Synchronous Switch Duty Cycle =  
V
IN  
Theꢀ MOSFETꢀ powerꢀ dissipationsꢀ atꢀ maximumꢀ outputꢀ  
currentꢀareꢀgivenꢀby:  
C and C  
Selection  
IN  
OUT  
VOUT  
2
TheꢀselectionꢀofꢀC ꢀisꢀsimplifiedꢀbyꢀtheꢀ2-phaseꢀarchitec-  
IN  
PMAIN  
=
I
1+ δ R  
+
DS(ON)  
(
MAX) (  
)
tureꢀandꢀitsꢀimpactꢀonꢀtheꢀworst-caseꢀRMSꢀcurrentꢀdrawnꢀ  
throughtheinputnetwork(battery/fuse/capacitor).Itcanbeꢀ  
shownꢀthatꢀtheꢀworst-caseꢀcapacitorꢀRMSꢀcurrentꢀoccursꢀ  
whenꢀonlyꢀoneꢀcontrollerꢀisꢀoperating.ꢀTheꢀcontrollerꢀwithꢀ  
V
IN  
2   
IMAX  
2
V
R
C
f
(
)
(
DR)(  
)
IN  
MILLER  
theꢀhighestꢀ(V )(I )ꢀproductꢀneedsꢀtoꢀbeꢀusedꢀinꢀtheꢀ  
OUT OUT  
1
1
+
( )  
formulaꢀshownꢀinꢀEquationꢀ1ꢀtoꢀdetermineꢀtheꢀmaximumꢀ  
RMSꢀcapacitorꢀcurrentꢀrequirement.ꢀIncreasingꢀtheꢀout-  
putꢀcurrentꢀdrawnꢀfromꢀtheꢀotherꢀcontrollerꢀwillꢀactuallyꢀ  
decreaseꢀtheꢀinputꢀRMSꢀrippleꢀcurrentꢀfromꢀitsꢀmaximumꢀ  
value.ꢀTheꢀout-of-phaseꢀtechniqueꢀtypicallyꢀreducesꢀtheꢀ  
inputꢀcapacitor’sꢀRMSꢀrippleꢀcurrentꢀbyꢀaꢀfactorꢀofꢀ30%ꢀ  
toꢀ70%ꢀwhenꢀcomparedꢀtoꢀaꢀsingleꢀphaseꢀpowerꢀsupplyꢀ  
solution.  
VINTVCC – VTHMIN VTHMIN  
V – VOUT  
2
IN  
PSYNC  
=
I
1+ δ R  
(
MAX) (  
)
DS(ON)  
V
IN  
whereꢀδꢀisꢀtheꢀtemperatureꢀdependencyꢀofꢀR  
R ꢀ(approximatelyꢀ2Ω)ꢀisꢀtheꢀeffectiveꢀdriverꢀresistanceꢀ  
atꢀtheꢀMOSFET’sꢀMillerꢀthresholdꢀvoltage.ꢀV  
typicalꢀMOSFETꢀminimumꢀthresholdꢀvoltage.  
ꢀandꢀ  
DS(ON)  
DR  
ꢀisꢀtheꢀ  
THMIN  
Incontinuousmode,thesourcecurrentofthetopMOSFETꢀ  
isꢀaꢀsquareꢀwaveꢀofꢀdutyꢀcycleꢀ(V )/(V ).ꢀToꢀpreventꢀ  
OUT  
IN  
3868fb  
ꢀꢈ  
LTC3868  
applicaTions inForMaTion  
largeꢀvoltageꢀtransients,ꢀaꢀlowꢀESRꢀcapacitorꢀsizedꢀforꢀtheꢀ  
maximumꢀRMSꢀcurrentꢀofꢀoneꢀchannelꢀmustꢀbeꢀused.ꢀTheꢀ  
maximumꢀRMSꢀcapacitorꢀcurrentꢀisꢀgivenꢀby:  
TheselectionofC ꢀisdrivenbytheeffectiveseriesꢀ  
OUT  
resistance(ESR).Typically,oncetheESRrequirementꢀ  
isꢀsatisfied,ꢀtheꢀcapacitanceꢀisꢀadequateꢀforꢀfiltering.ꢀTheꢀ  
outputꢀrippleꢀ(V )ꢀisꢀapproximatedꢀby:  
OUT  
IMAX  
1/2  
CIN ꢀRequiredꢀIRMS  
V
V – V  
(1)  
(
OUT )(  
)
IN  
OUT  
1
V
IN  
ΔVOUT ≈ ΔI ESR+  
L   
8 • f • COUT  
Equationꢀ1ꢀhasꢀaꢀmaximumꢀatꢀV ꢀ=ꢀ2V ,ꢀwhereꢀI  
IN  
OUTꢀ  
RMS  
=ꢀI /2.ꢀThisꢀsimpleꢀworst-caseꢀconditionꢀisꢀcommonlyꢀ  
OUT  
whereꢀf ꢀisꢀtheꢀoperatingꢀfrequency,ꢀC ꢀisꢀtheꢀoutputꢀ  
O
OUT  
usedfordesignbecauseevensignificantdeviationsdonotꢀ  
offermuchrelief.Notethatcapacitormanufacturers’rippleꢀ  
currentꢀratingsꢀareꢀoftenꢀbasedꢀonꢀonlyꢀ2000ꢀhoursꢀofꢀlife.ꢀ  
Thisꢀmakesꢀitꢀadvisableꢀtoꢀfurtherꢀderateꢀtheꢀcapacitor,ꢀorꢀ  
toꢀchooseꢀaꢀcapacitorꢀratedꢀatꢀaꢀhigherꢀtemperatureꢀthanꢀ  
required.Severalcapacitorsmaybeparalleledtomeetꢀ  
sizeꢀorꢀheightꢀrequirementsꢀinꢀtheꢀdesign.ꢀDueꢀtoꢀtheꢀhighꢀ  
operatingꢀfrequencyꢀofꢀtheꢀLTC3868,ꢀceramicꢀcapacitorsꢀ  
capacitanceꢀandꢀI ꢀisꢀtheꢀrippleꢀcurrentꢀinꢀtheꢀinductor.ꢀ  
L
Theoutputrippleishighestatmaximuminputvoltageꢀ  
sinceꢀI ꢀincreasesꢀwithꢀinputꢀvoltage.  
L
Setting Output Voltage  
TheꢀLTC3868ꢀoutputꢀvoltagesꢀareꢀeachꢀsetꢀbyꢀanꢀexternalꢀ  
feedbackꢀresistorꢀdividerꢀcarefullyꢀplacedꢀacrossꢀtheꢀout-  
put,ꢀasꢀshownꢀinꢀFigureꢀ6.ꢀTheꢀregulatedꢀoutputꢀvoltageꢀ  
isꢀdeterminedꢀby:  
canꢀalsoꢀbeꢀusedꢀforꢀC .ꢀAlwaysꢀconsultꢀtheꢀmanufacturerꢀ  
IN  
ifꢀthereꢀisꢀanyꢀquestion.  
R
RA  
ThebenefitoftheLTC38682-phaseoperationcanbecalcu-  
latedbyusingtheEquation1forthehigherpowercontrollerꢀ  
andꢀthenꢀcalculatingꢀtheꢀlossꢀthatꢀwouldꢀhaveꢀresultedꢀifꢀ  
bothꢀcontrollerꢀchannelsꢀswitchedꢀonꢀatꢀtheꢀsameꢀtime.ꢀ  
TheꢀtotalꢀRMSꢀpowerꢀlostꢀisꢀlowerꢀwhenꢀbothꢀcontrollersꢀ  
areꢀoperatingꢀdueꢀtoꢀtheꢀreducedꢀoverlapꢀofꢀcurrentꢀpulsesꢀ  
requiredꢀthroughꢀtheꢀinputꢀcapacitor’sꢀESR.ꢀThisꢀisꢀwhyꢀ  
theꢀinputꢀcapacitor’sꢀrequirementꢀcalculatedꢀaboveꢀforꢀtheꢀ  
worst-caseꢀcontrollerꢀisꢀadequateꢀforꢀtheꢀdualꢀcontrollerꢀ  
design.ꢀAlso,ꢀtheꢀinputꢀprotectionꢀfuseꢀresistance,ꢀbatteryꢀ  
resistance,ꢀandꢀPCꢀboardꢀtraceꢀresistanceꢀlossesꢀareꢀalsoꢀ  
reducedꢀdueꢀtoꢀtheꢀreducedꢀpeakꢀcurrentsꢀinꢀaꢀ2-phaseꢀ  
system.Theoverallbenefitofamultiphasedesignwillꢀ  
onlyꢀbeꢀfullyꢀrealizedꢀwhenꢀtheꢀsourceꢀimpedanceꢀofꢀtheꢀ  
powerꢀsupply/batteryꢀisꢀincludedꢀinꢀtheꢀefficiencyꢀtesting.ꢀ  
TheꢀdrainsꢀofꢀtheꢀtopꢀMOSFETsꢀshouldꢀbeꢀplacedꢀwithinꢀ  
VOUT = 0.8V 1+  
B   
Toimprovethefrequencyresponse,afeedforwardca-  
pacitor,ꢀC ,ꢀmayꢀbeꢀused.ꢀGreatꢀcareꢀshouldꢀbeꢀtakenꢀtoꢀ  
FFꢀ  
routeꢀtheꢀV ꢀlineꢀawayꢀfromꢀnoiseꢀsources,ꢀsuchꢀasꢀtheꢀ  
FB  
inductorꢀorꢀtheꢀSWꢀline.  
V
OUT  
R
B
C
FF  
1/2 LTC3868  
V
FB  
R
A
3868 F06  
Figure 6. Setting Output Voltage  
Soft-Start (SS Pins)  
1cmofeachotherandshareacommonC ꢀ(s).Separatingꢀ  
IN  
theꢀsourcesꢀandꢀC ꢀmayꢀproduceꢀundesirableꢀvoltageꢀandꢀ  
IN  
Theꢀstart-upꢀofꢀeachꢀV ꢀisꢀcontrolledꢀbyꢀtheꢀvoltageꢀonꢀ  
OUT  
currentꢀresonancesꢀatꢀV .  
IN  
theꢀrespectiveꢀSSꢀpin.ꢀWhenꢀtheꢀvoltageꢀonꢀtheꢀSSꢀpinꢀisꢀ  
lessthantheinternal0.8Vreference,theLTC3868regulatesꢀ  
Aꢀsmallꢀ(0.1µFꢀtoꢀ1µF)ꢀbypassꢀcapacitorꢀbetweenꢀtheꢀchipꢀ  
theꢀV ꢀpinꢀvoltageꢀtoꢀtheꢀvoltageꢀonꢀtheꢀSSꢀpinꢀinsteadꢀ  
V ꢀpinꢀandꢀground,ꢀplacedꢀcloseꢀtoꢀtheꢀLTC3868,ꢀisꢀalsoꢀ  
FB  
IN  
ofꢀ0.8V.ꢀTheꢀSSꢀpinꢀcanꢀbeꢀusedꢀtoꢀprogramꢀanꢀexternalꢀ  
suggested.ꢀAꢀ10ΩꢀresistorꢀplacedꢀbetweenꢀC ꢀ(C1)ꢀandꢀ  
IN  
soft-startꢀfunction.  
theV ꢀpinprovidesfurtherisolationbetweenthetwoꢀ  
IN  
channels.  
3868fb  
ꢁ0  
LTC3868  
applicaTions inForMaTion  
Soft-startisenabledbysimplyconnectingacapacitorfromꢀ  
theSSpintoground,asshowninFigure7.Aninternal1µAꢀ  
currentꢀsourceꢀchargesꢀtheꢀcapacitor,ꢀprovidingꢀaꢀlinearꢀ  
rampingꢀvoltageꢀatꢀtheꢀSSꢀpin.ꢀTheꢀLTC3868ꢀwillꢀregulateꢀ  
theꢀV ꢀpinꢀ(andꢀhenceꢀV )ꢀaccordingꢀtoꢀtheꢀvoltageꢀonꢀ  
asꢀ discussedꢀ inꢀ theꢀ Efficiencyꢀ Considerationsꢀ section.ꢀ  
Theꢀjunctionꢀtemperatureꢀcanꢀbeꢀestimatedꢀbyꢀusingꢀtheꢀ  
equationsꢀgivenꢀinꢀNoteꢀ3ꢀofꢀtheꢀElectricalꢀCharacteristics.ꢀ  
Forexample,theLTC3868INTV ꢀcurrentꢀisꢀlimitedꢀtoꢀlessꢀ  
CC  
thanꢀ45mAꢀfromꢀaꢀ28VꢀsupplyꢀwhenꢀnotꢀusingꢀtheꢀEXTV ꢀ  
FB  
OUT  
CC  
theSSpin,allowingV ꢀtorisesmoothlyfrom0Vtoꢀ  
supplyꢀatꢀ70°Cꢀambientꢀtemperature:  
OUT  
itsꢀfinalꢀregulatedꢀvalue.ꢀTheꢀtotalꢀsoft-startꢀtimeꢀwillꢀbeꢀ  
approximately:  
ꢀ T ꢀ=ꢀ70°Cꢀ+ꢀ(45mA)(28V)(43°C/W)ꢀ=ꢀ125°C  
J
Toꢀpreventꢀtheꢀmaximumꢀjunctionꢀtemperatureꢀfromꢀbe-  
ingꢀexceeded,ꢀtheꢀinputꢀsupplyꢀcurrentꢀmustꢀbeꢀcheckedꢀ  
whileꢀoperatingꢀinꢀforcedꢀcontinuousꢀmodeꢀ(PLLIN/MODEꢀ  
0.8V  
1µA  
tSS = CSS  
=ꢀINTV )ꢀatꢀmaximumꢀV .  
CC  
IN  
1/2 LTC3868  
SS  
WhenꢀtheꢀvoltageꢀappliedꢀtoꢀEXTV ꢀrisesꢀaboveꢀ4.7V,ꢀtheꢀ  
CC  
V ꢀLDOꢀisꢀturnedꢀoffꢀandꢀtheꢀEXTV ꢀLDOꢀisꢀenabled.ꢀTheꢀ  
C
IN  
CC  
SS  
EXTV ꢀLDOꢀremainsꢀonꢀasꢀlongꢀasꢀtheꢀvoltageꢀappliedꢀtoꢀ  
SGND  
CC  
EXTV ꢀremainsꢀaboveꢀ4.5V.ꢀTheꢀEXTV ꢀLDOꢀattemptsꢀ  
3868 F07  
CC  
CC  
toꢀregulateꢀtheꢀINTV ꢀvoltageꢀtoꢀ5.1V,ꢀsoꢀwhileꢀEXTV ꢀ  
CC  
CC  
Figure 7. Using the TRACK/SS Pin to Program Soft-Start  
isꢀlessꢀthanꢀ5.1V,ꢀtheꢀLDOꢀisꢀinꢀdropoutꢀandꢀtheꢀINTV ꢀ  
CC  
voltageꢀisꢀapproximatelyꢀequalꢀtoꢀEXTV .ꢀWhenꢀEXTV ꢀ  
CC  
CC  
INTV Regulators  
CC  
isꢀgreaterꢀthanꢀ5.1V,ꢀupꢀtoꢀanꢀabsoluteꢀmaximumꢀofꢀ14V,ꢀ  
INTV ꢀisꢀregulatedꢀtoꢀ5.1V.  
CC  
Theꢀ LTC3868ꢀ featuresꢀ twoꢀ separateꢀ internalꢀ P-channelꢀ  
lowdropoutlinearregulators(LDO)thatsupplypowerꢀ  
atꢀtheꢀINTV ꢀpinꢀfromꢀeitherꢀtheꢀV ꢀsupplyꢀpinꢀorꢀtheꢀ  
UsingtheEXTVCCLDOallowstheMOSFETdriverandꢀ  
controlꢀpowerꢀtoꢀbeꢀderivedꢀfromꢀoneꢀofꢀtheꢀLTC3868’sꢀ  
switchingꢀregulatorꢀoutputsꢀ(4.7Vꢀ≤ꢀVOUTꢀ≤ꢀ14V)ꢀduringꢀ  
normalꢀoperationꢀandꢀfromꢀtheꢀVINꢀLDOꢀwhenꢀtheꢀout-  
putꢀisꢀoutꢀofꢀregulationꢀ(e.g.,ꢀstart-up,ꢀshort-circuit).ꢀIfꢀ  
moreꢀcurrentꢀisꢀrequiredꢀthroughꢀtheꢀEXTVCCꢀLDOꢀthanꢀ  
isspecified,anexternalSchottkydiodecanbeaddedꢀ  
betweenꢀtheꢀEXTVCCꢀandꢀINTVCCꢀpins.ꢀInꢀthisꢀcase,ꢀdoꢀ  
notꢀapplyꢀmoreꢀthanꢀ6VꢀtoꢀtheꢀEXTVCCꢀpinꢀandꢀmakeꢀsureꢀ  
thatꢀEXTVCCꢀ≤ꢀVIN.  
CC  
IN  
EXTV ꢀpinꢀdependingꢀonꢀtheꢀconnectionꢀofꢀtheꢀEXTV ꢀ  
CC  
CC  
pin.ꢀ INTV ꢀ powersꢀ theꢀ gateꢀ driversꢀ andꢀ muchꢀ ofꢀ theꢀ  
CC  
LTC3868’sꢀinternalꢀcircuitry.ꢀTheꢀV ꢀLDOꢀandꢀtheꢀEXTV ꢀ  
IN  
CC  
LDOꢀregulateꢀINTV ꢀtoꢀ5.1V.ꢀEachꢀofꢀtheseꢀcanꢀsupplyꢀaꢀ  
CC  
peakꢀcurrentꢀofꢀ50mAꢀandꢀmustꢀbeꢀbypassedꢀtoꢀgroundꢀ  
withꢀaꢀminimumꢀofꢀ4.7µFꢀlowꢀESRꢀcapacitor.ꢀNoꢀmatterꢀ  
whattypeofbulkcapacitorisused,anadditional1µFꢀ  
ceramicꢀcapacitorꢀplacedꢀdirectlyꢀadjacentꢀtoꢀtheꢀINTV ꢀ  
CC  
andꢀPGNDꢀpinsꢀisꢀhighlyꢀrecommended.ꢀGoodꢀbypassingꢀ  
isꢀneededꢀtoꢀsupplyꢀtheꢀhighꢀtransientꢀcurrentsꢀrequiredꢀ  
bytheMOSFETgatedriversandtopreventinteractionꢀ  
betweenꢀtheꢀchannels.  
Significantꢀefficiencyꢀandꢀthermalꢀgainsꢀcanꢀbeꢀrealizedꢀ  
byꢀpoweringꢀINTV ꢀfromꢀtheꢀoutput,ꢀsinceꢀtheꢀV ꢀcur-  
CC  
IN  
rentꢀresultingꢀfromꢀtheꢀdriverꢀandꢀcontrolꢀcurrentsꢀwillꢀbeꢀ  
scaledꢀbyꢀaꢀfactorꢀofꢀ(DutyꢀCycle)/(SwitcherꢀEfficiency).ꢀ  
Forꢀ5Vꢀtoꢀ14Vꢀregulatorꢀoutputs,ꢀthisꢀmeansꢀconnectingꢀ  
theꢀEXTV ꢀpinꢀdirectlyꢀtoꢀV .ꢀTyingꢀtheꢀEXTV ꢀpinꢀtoꢀ  
HighinputvoltageapplicationsinwhichlargeMOSFETsareꢀ  
beingꢀdrivenꢀatꢀhighꢀfrequenciesꢀmayꢀcauseꢀtheꢀmaximumꢀ  
junctiontemperatureratingfortheLTC3868tobeexceeded.ꢀ  
CC  
OUTꢀ  
CC  
anꢀ8.5Vꢀsupplyꢀreducesꢀtheꢀjunctionꢀtemperatureꢀinꢀtheꢀ  
previousꢀexampleꢀfromꢀ125°Cꢀto:  
TheINTV current,whichisdominatedbythegatechargeꢀ  
CC  
current,maybesuppliedbyeithertheV ꢀLDOortheꢀ  
IN  
ꢀ T ꢀ=ꢀ70°Cꢀ+ꢀ(45mA)(8.5V)(43°C/W)ꢀ=ꢀ86°C  
J
EXTV ꢀLDO.ꢀWhenꢀtheꢀvoltageꢀonꢀtheꢀEXTV ꢀpinꢀisꢀlessꢀ  
CC  
CC  
However,ꢀforꢀ3.3Vꢀandꢀotherꢀlowꢀvoltageꢀoutputs,ꢀaddi-  
than4.7V,theV ꢀLDOisenabled.Powerdissipationfortheꢀ  
IN  
tionalꢀcircuitryꢀisꢀrequiredꢀtoꢀderiveꢀINTV ꢀpowerꢀfromꢀ  
CC  
ICꢀinꢀthisꢀcaseꢀisꢀhighestꢀandꢀisꢀequalꢀtoꢀV ꢀ•ꢀI  
gateꢀchargeꢀcurrentꢀisꢀdependentꢀonꢀoperatingꢀfrequencyꢀ  
.ꢀTheꢀ  
IN INTVCC  
theꢀoutput.  
3868fb  
ꢁꢀ  
LTC3868  
applicaTions inForMaTion  
Theꢀfollowingꢀlistꢀsummarizesꢀtheꢀfourꢀpossibleꢀconnec-  
desiredꢀMOSFET.ꢀThisꢀenhancesꢀtheꢀtopꢀMOSFETꢀswitchꢀ  
tionsꢀforꢀEXTV :  
andꢀturnsꢀitꢀon.ꢀTheꢀswitchꢀnodeꢀvoltage,ꢀSW,ꢀrisesꢀtoꢀV ꢀ  
CC  
IN  
andtheBOOSTpinfollows.WiththetopsideMOSFETꢀ  
1.ꢀEXTV LeftOpen(orGrounded).ThiswillcauseINTV ꢀ  
CC  
CC  
on,ꢀtheꢀboostꢀvoltageꢀisꢀaboveꢀtheꢀinputꢀsupply:ꢀV  
ꢀ=ꢀ  
BOOST  
toꢀbeꢀpoweredꢀfromꢀtheꢀinternalꢀ5.1Vꢀregulatorꢀresult-  
ingꢀinꢀanꢀefficiencyꢀpenaltyꢀofꢀupꢀtoꢀ10%ꢀatꢀhighꢀinputꢀ  
voltages.  
V ꢀ+ꢀV  
.ꢀTheꢀvalueꢀofꢀtheꢀboostꢀcapacitor,ꢀC ,ꢀneedsꢀ  
IN  
INTVCC  
B
toꢀbeꢀ100ꢀtimesꢀthatꢀofꢀtheꢀtotalꢀinputꢀcapacitanceꢀofꢀtheꢀ  
topsideMOSFET(s).Thereversebreakdownoftheexternalꢀ  
2.ꢀEXTV ꢀConnectedꢀDirectlyꢀtoꢀV .ꢀThisꢀisꢀtheꢀnormalꢀ  
CC  
OUTꢀ  
SchottkyꢀdiodeꢀmustꢀbeꢀgreaterꢀthanꢀV  
.ꢀ  
IN(MAX)  
connectionꢀforꢀaꢀ5Vꢀtoꢀ14Vꢀregulatorꢀandꢀprovidesꢀtheꢀ  
Whenꢀadjustingꢀtheꢀgateꢀdriveꢀlevel,ꢀtheꢀfinalꢀarbiterꢀisꢀtheꢀ  
totalꢀinputꢀcurrentꢀforꢀtheꢀregulator.ꢀIfꢀaꢀchangeꢀisꢀmadeꢀ  
andꢀtheꢀinputꢀcurrentꢀdecreases,ꢀthenꢀtheꢀefficiencyꢀhasꢀ  
improved.Ifthereisnochangeininputcurrent,thenthereꢀ  
isꢀnoꢀchangeꢀinꢀefficiency.  
highestꢀefficiency.  
3.ꢀEXTV ConnectedtoanExternalSupply.Ifanexternalꢀ  
CC  
supplyꢀisꢀavailableꢀinꢀtheꢀ5Vꢀtoꢀ14Vꢀrange,ꢀitꢀmayꢀbeꢀ  
usedꢀtoꢀpowerꢀEXTV .ꢀEnsureꢀthatꢀEXTV ꢀ<ꢀV .  
CC  
CC  
IN  
4.ꢀEXTV ConnectedtoanOutput-DerivedBoostNetwork.ꢀ  
CC  
Fault Conditions: Current Limit and Current Foldback  
Forꢀ3.3Vꢀandꢀotherꢀlowꢀvoltageꢀregulators,ꢀefficiencyꢀ  
gainsꢀcanꢀstillꢀbeꢀrealizedꢀbyꢀconnectingꢀEXTV ꢀtoꢀanꢀ  
Whenꢀtheꢀoutputꢀcurrentꢀhitsꢀtheꢀcurrentꢀlimit,ꢀtheꢀoutputꢀ  
voltageꢀbeginsꢀtoꢀdrop.ꢀIfꢀtheꢀoutputꢀvoltageꢀfallsꢀbelowꢀ  
70%ꢀofꢀitsꢀnominalꢀoutputꢀlevel,ꢀthenꢀtheꢀmaximumꢀsenseꢀ  
voltageꢀisꢀprogressivelyꢀloweredꢀfromꢀaboutꢀone-halfꢀofꢀ  
itsꢀmaximumꢀselectedꢀvalue.ꢀUnderꢀshort-circuitꢀcondi-  
tionsꢀwithꢀveryꢀlowꢀdutyꢀcycles,ꢀtheꢀLTC3868ꢀwillꢀbeginꢀ  
cycleꢀskippingꢀinꢀorderꢀtoꢀlimitꢀtheꢀshort-circuitꢀcurrent.ꢀ  
InꢀthisꢀsituationꢀtheꢀbottomꢀMOSFETꢀwillꢀbeꢀdissipatingꢀ  
mostꢀofꢀtheꢀpowerꢀbutꢀlessꢀthanꢀinꢀnormalꢀoperation.ꢀTheꢀ  
short-circuitꢀrippleꢀcurrentꢀisꢀdeterminedꢀbyꢀtheꢀminimumꢀ  
CC  
output-derivedvoltagethathasbeenboostedtogreaterꢀ  
thanꢀ4.7V.ꢀThisꢀcanꢀbeꢀdoneꢀwithꢀtheꢀcapacitiveꢀchargeꢀ  
pumpꢀshownꢀinꢀFigureꢀ8.ꢀEnsureꢀthatꢀEXTV ꢀ<ꢀV .  
CC  
IN  
C
IN  
BAT85  
BAT85  
BAT85  
V
IN  
MTOP  
MBOT  
VN2222LL  
on-time,ꢀt  
,ꢀofꢀtheꢀLTC3868ꢀ(≈95ns),ꢀtheꢀinputꢀvolt-  
TG1  
1/2 LTC3868  
ON(MIN)  
ageꢀandꢀinductorꢀvalue:  
L
R
SENSE  
V
OUT  
EXTV  
SW  
CC  
V
L
ON(MIN) IN   
ΔIL(SC) = t  
C
OUT  
D
BG1  
3868 F08  
Theꢀresultingꢀaverageꢀshort-circuitꢀcurrentꢀis:  
PGND  
50% •I  
1
2
ISC =  
LIM(MAX) IL(SC)  
Figure 8. Capacitive Charge Pump for EXTVCC  
RSENSE  
Topside MOSFET Driver Supply (C , D )  
Fault Conditions: Overvoltage Protection (Crowbar)  
B
B
Externalbootstrapcapacitors,C ,connectedtotheBOOSTꢀ  
Theꢀovervoltageꢀcrowbarꢀisꢀdesignedꢀtoꢀblowꢀaꢀsystemꢀ  
inputꢀfuseꢀwhenꢀtheꢀoutputꢀvoltageꢀofꢀtheꢀregulatorꢀrisesꢀ  
muchhigherthannominallevels.Thecrowbarcauseshugeꢀ  
currentsꢀtoꢀflow,ꢀthatꢀblowꢀtheꢀfuseꢀtoꢀprotectꢀagainstꢀaꢀ  
shortedꢀtopꢀMOSFETꢀifꢀtheꢀshortꢀoccursꢀwhileꢀtheꢀcontrol-  
lerꢀisꢀoperating.  
B
pinssupplythegatedrivevoltagesforthetopsideMOSFETs.ꢀ  
CapacitorꢀC ꢀinꢀtheꢀFunctionalꢀDiagramꢀisꢀchargedꢀthoughꢀ  
B
externalꢀdiodeꢀD ꢀfromꢀINTV ꢀwhenꢀtheꢀSWꢀpinꢀisꢀlow.ꢀ  
B
CC  
WhenꢀoneꢀofꢀtheꢀtopsideꢀMOSFETsꢀisꢀtoꢀbeꢀturnedꢀon,ꢀtheꢀ  
driverꢀplacesꢀtheꢀC ꢀvoltageꢀacrossꢀtheꢀgate-sourceꢀofꢀtheꢀ  
B
3868fb  
ꢁꢁ  
LTC3868  
applicaTions inForMaTion  
1000  
900  
800  
700  
600  
500  
400  
300  
200  
100  
0
Aꢀcomparatorꢀmonitorsꢀtheꢀoutputꢀforꢀovervoltageꢀcondi-  
tions.Thecomparatordetectsfaultsgreaterthan10%ꢀ  
aboveꢀtheꢀnominalꢀoutputꢀvoltage.ꢀWhenꢀthisꢀconditionꢀ  
isꢀsensed,ꢀtheꢀtopꢀMOSFETꢀisꢀturnedꢀoffꢀandꢀtheꢀbottomꢀ  
MOSFETꢀisꢀturnedꢀonꢀuntilꢀtheꢀovervoltageꢀconditionꢀisꢀ  
cleared.ThebottomMOSFETremainsoncontinuouslyꢀ  
forꢀasꢀlongꢀasꢀtheꢀovervoltageꢀconditionꢀpersists;ꢀifꢀV  
returnstoasafelevel,normaloperationautomaticallyꢀ  
resumes.ꢀ  
OUT  
AshortedtopMOSFETwillresultinahighcurrentconditionꢀ  
whichꢀwillꢀopenꢀtheꢀsystemꢀfuse.ꢀTheꢀswitchingꢀregulatorꢀ  
willꢀregulateꢀproperlyꢀwithꢀaꢀleakyꢀtopꢀMOSFETꢀbyꢀalteringꢀ  
theꢀdutyꢀcycleꢀtoꢀaccommodateꢀtheꢀleakage.  
15 25 35 45 55 65 75 85 95 105 115 125  
FREQ PIN RESISTOR (kΩ)  
3868 F09  
Figure 9. Relationship Between Oscillator Frequency  
and Resistor Value at the FREQ Pin  
Phase-Locked Loop and Frequency Synchronization  
TheLTC3868hasaninternalphase-lockedloop(PLL)ꢀ  
comprisedꢀofꢀaꢀphaseꢀfrequencyꢀdetector,ꢀaꢀlowpassꢀfilter,ꢀ  
andꢀaꢀvoltage-controlledꢀoscillatorꢀ(VCO).ꢀThisꢀallowsꢀtheꢀ  
turn-onꢀofꢀtheꢀtopꢀMOSFETꢀofꢀcontrollerꢀ1ꢀtoꢀbeꢀlockedꢀtoꢀ  
theꢀrisingꢀedgeꢀofꢀanꢀexternalꢀclockꢀsignalꢀappliedꢀtoꢀtheꢀ  
PLLIN/MODEpin.Theturn-onofcontroller2’stopMOSFETꢀ  
isꢀthusꢀ180ꢀdegreesꢀoutꢀofꢀphaseꢀwithꢀtheꢀexternalꢀclock.ꢀ  
Theꢀphaseꢀdetectorꢀisꢀanꢀedgeꢀsensitiveꢀdigitalꢀtypeꢀthatꢀ  
providesꢀzeroꢀdegreesꢀphaseꢀshiftꢀbetweenꢀtheꢀexternalꢀ  
andꢀinternalꢀoscillators.ꢀThisꢀtypeꢀofꢀphaseꢀdetectorꢀdoesꢀ  
notꢀexhibitꢀfalseꢀlockꢀtoꢀharmonicsꢀofꢀtheꢀexternalꢀclock.ꢀ  
LTC3868’sinternalVCO,whichisnominally55kHzto1MHz.ꢀ  
Thisꢀisꢀguaranteedꢀtoꢀbeꢀbetweenꢀ75kHzꢀandꢀ850kHz.  
Typically,ꢀ theꢀ externalꢀ clockꢀ (onꢀ theꢀ PLLIN/MODEꢀ pin)ꢀ  
inputꢀhighꢀthresholdꢀisꢀ1.6V,ꢀwhileꢀtheꢀinputꢀlowꢀthresholdꢀ  
isꢀ1.1V.  
RapidꢀphaseꢀlockingꢀcanꢀbeꢀachievedꢀbyꢀusingꢀtheꢀFREQꢀ  
pinꢀ toꢀ setꢀ aꢀ free-runningꢀ frequencyꢀ nearꢀ theꢀ desiredꢀ  
synchronizationfrequency.TheVCO’sinputvoltageisꢀ  
prebiasedꢀatꢀaꢀfrequencyꢀcorrespondingꢀtoꢀtheꢀfrequencyꢀ  
setꢀbyꢀtheꢀFREQꢀpin.ꢀOnceꢀprebiased,ꢀtheꢀPLLꢀonlyꢀneedsꢀ  
toadjustthefrequencyslightlytoachievephaselockꢀ  
andꢀsynchronization.ꢀAlthoughꢀitꢀisꢀnotꢀrequiredꢀthatꢀtheꢀ  
free-runningꢀfrequencyꢀbeꢀnearꢀexternalꢀclockꢀfrequency,ꢀ  
doingsowillpreventtheoperatingfrequencyfrompassingꢀ  
throughꢀaꢀlargeꢀrangeꢀofꢀfrequenciesꢀasꢀtheꢀPLLꢀlocks.  
Ifꢀtheꢀexternalꢀclockꢀfrequencyꢀisꢀgreaterꢀthanꢀtheꢀinternalꢀ  
oscillator’sfrequency,f ,thencurrentissourcedcontinu-  
OSC  
ouslyꢀfromꢀtheꢀphaseꢀdetectorꢀoutput,ꢀpullingꢀupꢀtheꢀVCOꢀ  
input.ꢀWhenꢀtheꢀexternalꢀclockꢀfrequencyꢀisꢀlessꢀthanꢀf ,ꢀ  
OSC  
currentꢀisꢀsunkꢀcontinuously,ꢀpullingꢀdownꢀtheꢀVCOꢀinput.ꢀ  
Ifꢀtheꢀexternalꢀandꢀinternalꢀfrequenciesꢀareꢀtheꢀsameꢀbutꢀ  
exhibitꢀaꢀphaseꢀdifference,ꢀtheꢀcurrentꢀsourcesꢀturnꢀonꢀforꢀ  
anꢀamountꢀofꢀtimeꢀcorrespondingꢀtoꢀtheꢀphaseꢀdifference.ꢀ  
TheꢀvoltageꢀatꢀtheꢀVCOꢀinputꢀisꢀadjustedꢀuntilꢀtheꢀphaseꢀ  
andꢀfrequencyꢀofꢀtheꢀinternalꢀandꢀexternalꢀoscillatorsꢀareꢀ  
identical.ꢀAtꢀtheꢀstableꢀoperatingꢀpoint,ꢀtheꢀphaseꢀdetectorꢀ  
outputꢀisꢀhighꢀimpedanceꢀandꢀtheꢀinternalꢀfilterꢀcapacitor,ꢀ  
Tableꢀ2ꢀsummarizesꢀtheꢀdifferentꢀstatesꢀinꢀwhichꢀtheꢀFREQꢀ  
pinꢀcanꢀbeꢀused.  
Table 2  
FREQ PIN  
PLLIN/MODE PIN  
DCꢀVoltage  
FREQUENCY  
350kHz  
0V  
INTV  
DCꢀVoltage  
535kHz  
CC  
Resistor  
DCꢀVoltage  
50kHz–900kHz  
C ,ꢀholdsꢀtheꢀvoltageꢀatꢀtheꢀVCOꢀinput.  
LPꢀ  
AnyꢀofꢀtheꢀAbove  
ExternalꢀClock  
Phase–Lockedꢀtoꢀ  
ExternalꢀClock  
NotethattheLTC3868canonlybesynchronizedtoanꢀ  
externalꢀ clockꢀ whoseꢀ frequencyꢀ isꢀ withinꢀ rangeꢀ ofꢀ theꢀ  
3868fb  
ꢁꢂ  
LTC3868  
applicaTions inForMaTion  
Minimum On-Time Considerations  
1.ꢀTheꢀV ꢀcurrentꢀisꢀtheꢀDCꢀinputꢀsupplyꢀcurrentꢀgivenꢀ  
IN  
inꢀtheꢀElectricalꢀCharacteristicsꢀtable,ꢀwhichꢀexcludesꢀ  
Minimumꢀon-time,ꢀt  
,ꢀisꢀtheꢀsmallestꢀtimeꢀdurationꢀ  
ON(MIN)  
MOSFETꢀdriverꢀandꢀcontrolꢀcurrents.ꢀV ꢀcurrentꢀtypi-  
IN  
thatꢀtheꢀLTC3868ꢀisꢀcapableꢀofꢀturningꢀonꢀtheꢀtopꢀMOSFET.ꢀ  
callyꢀresultsꢀinꢀaꢀsmallꢀ(<0.1%)ꢀloss.  
Itisdeterminedbyinternaltimingdelaysandthegateꢀ  
chargerequiredtoturnonthetopMOSFET.Lowdutyꢀ 2.ꢀINTV ꢀcurrentꢀisꢀtheꢀsumꢀofꢀtheꢀMOSFETꢀdriverꢀandꢀ  
CC  
cycleꢀapplicationsꢀmayꢀapproachꢀthisꢀminimumꢀon-timeꢀ  
limitꢀandꢀcareꢀshouldꢀbeꢀtakenꢀtoꢀensureꢀthat:  
controlꢀcurrents.ꢀTheꢀMOSFETꢀdriverꢀcurrentꢀresultsꢀ  
fromꢀ switchingꢀ theꢀ gateꢀ capacitanceꢀ ofꢀ theꢀ powerꢀ  
MOSFETs.ꢀ Eachꢀ timeꢀ aꢀ MOSFETꢀ gateꢀ isꢀ switchedꢀ  
fromꢀlowꢀtoꢀhighꢀtoꢀlowꢀagain,ꢀaꢀpacketꢀofꢀcharge,ꢀdQ,ꢀ  
VOUT  
tON(MIN)  
<
V
f
IN
( )  
movesꢀfromꢀINTV ꢀtoꢀground.ꢀTheꢀresultingꢀdQ/dtꢀisꢀ  
CC  
aꢀcurrentꢀoutꢀofꢀINTV ꢀthatꢀisꢀtypicallyꢀmuchꢀlargerꢀ  
CC  
Ifꢀtheꢀdutyꢀcycleꢀfallsꢀbelowꢀwhatꢀcanꢀbeꢀaccommodatedꢀ  
byꢀtheꢀminimumꢀon-time,ꢀtheꢀcontrollerꢀwillꢀbeginꢀtoꢀskipꢀ  
cycles.ꢀTheꢀoutputꢀvoltageꢀwillꢀcontinueꢀtoꢀbeꢀregulated,ꢀ  
butꢀtheꢀrippleꢀvoltageꢀandꢀcurrentꢀwillꢀincrease.  
thanꢀtheꢀcontrolꢀcircuitꢀcurrent.ꢀInꢀcontinuousꢀmode,ꢀ  
I
ꢀ=ꢀf(Q ꢀ+ꢀQ ),ꢀwhereꢀQ ꢀandꢀQ ꢀareꢀtheꢀgateꢀ  
GATECHG  
T B T B  
chargesꢀofꢀtheꢀtopsideꢀandꢀbottomꢀsideꢀMOSFETs.  
ꢀ SupplyingINTV fromanoutput-derivedpowersourceꢀ  
CC  
Theꢀminimumꢀon-timeꢀforꢀtheꢀLTC3868ꢀisꢀapproximatelyꢀ  
95ns.ꢀHowever,ꢀasꢀtheꢀpeakꢀsenseꢀvoltageꢀdecreasesꢀtheꢀ  
minimumꢀon-timeꢀgraduallyꢀincreasesꢀupꢀtoꢀaboutꢀ130ns.ꢀ  
Thisꢀisꢀofꢀparticularꢀconcernꢀinꢀforcedꢀcontinuousꢀapplica-  
tionswithlowripplecurrentatlightloads.Ifthedutycycleꢀ  
dropsꢀbelowꢀtheꢀminimumꢀon-timeꢀlimitꢀinꢀthisꢀsituation,ꢀ  
aꢀsignificantꢀamountꢀofꢀcycleꢀskippingꢀcanꢀoccurꢀwithꢀcor-  
respondinglyꢀlargerꢀcurrentꢀandꢀvoltageꢀripple.  
throughꢀ EXTV ꢀ willꢀ scaleꢀ theꢀ V ꢀ currentꢀ requiredꢀ  
CC  
IN  
forꢀtheꢀdriverꢀandꢀcontrolꢀcircuitsꢀbyꢀaꢀfactorꢀofꢀ(Dutyꢀ  
Cycle)/(Efficiency).Forexample,ina20Vto5Vapplica-  
tion,ꢀ10mAꢀofꢀINTV ꢀcurrentꢀresultsꢀinꢀapproximatelyꢀ  
CC  
2.5mAꢀofꢀV ꢀcurrent.ꢀThisꢀreducesꢀtheꢀmidcurrentꢀlossꢀ  
IN  
fromꢀ10%ꢀorꢀmoreꢀ(ifꢀtheꢀdriverꢀwasꢀpoweredꢀdirectlyꢀ  
fromꢀV )ꢀtoꢀonlyꢀaꢀfewꢀpercent.  
IN  
2
3.ꢀI RꢀlossesꢀareꢀpredictedꢀfromꢀtheꢀDCꢀresistancesꢀofꢀtheꢀ  
fuseꢀ(ifꢀused),ꢀMOSFET,ꢀinductor,ꢀcurrentꢀsenseꢀresis-  
tor,ꢀandꢀinputꢀandꢀoutputꢀcapacitorꢀESR.ꢀInꢀcontinuousꢀ  
modeꢀtheꢀaverageꢀoutputꢀcurrentꢀflowsꢀthroughꢀLꢀandꢀ  
Efficiency Considerations  
Theꢀpercentꢀefficiencyꢀofꢀaꢀswitchingꢀregulatorꢀisꢀequalꢀtoꢀ  
theꢀoutputꢀpowerꢀdividedꢀbyꢀtheꢀinputꢀpowerꢀtimesꢀ100%.ꢀ  
Itꢀisꢀoftenꢀusefulꢀtoꢀanalyzeꢀindividualꢀlossesꢀtoꢀdetermineꢀ  
whatꢀisꢀlimitingꢀtheꢀefficiencyꢀandꢀwhichꢀchangeꢀwouldꢀ  
producethemostimprovement.Percentefficiencycanꢀ  
beꢀexpressedꢀas:  
R
,ꢀbutꢀisꢀchoppedꢀbetweenꢀtheꢀtopsideꢀMOSFETꢀ  
SENSE  
andꢀ theꢀ synchronousꢀ MOSFET.ꢀ Ifꢀ theꢀ twoꢀ MOSFETsꢀ  
haveꢀapproximatelyꢀtheꢀsameꢀR  
,ꢀthenꢀtheꢀresis-  
DS(ON)  
tanceꢀofꢀoneꢀMOSFETꢀcanꢀsimplyꢀbeꢀsummedꢀwithꢀtheꢀ  
2
resistancesꢀofꢀL,ꢀR  
ꢀandꢀESRꢀtoꢀobtainꢀI Rꢀlosses.ꢀ  
=30mΩ,R ꢀ=50mΩ,ꢀ  
SENSE  
Forexample,ifeachR  
ꢀ %Efficiencyꢀ=ꢀ100%ꢀ–ꢀ(L1ꢀ+ꢀL2ꢀ+ꢀL3ꢀ+ꢀ...)  
DS(ON)  
L
R
ꢀ=ꢀ10mΩꢀandꢀR ꢀ=ꢀ40mΩꢀ(sumꢀofꢀbothꢀinputꢀ  
SENSE  
ESR  
whereꢀL1,ꢀL2,ꢀetc.ꢀareꢀtheꢀindividualꢀlossesꢀasꢀaꢀpercent-  
ageꢀofꢀinputꢀpower.  
andoutputcapacitancelosses),thenthetotalresistanceꢀ  
isꢀ130mΩ.ꢀThisꢀresultsꢀinꢀlossesꢀrangingꢀfromꢀ3%ꢀtoꢀ  
13%ꢀasꢀtheꢀoutputꢀcurrentꢀincreasesꢀfromꢀ1Aꢀtoꢀ5Aꢀforꢀ  
aꢀ5Vꢀoutput,ꢀorꢀaꢀ4%ꢀtoꢀ20%ꢀlossꢀforꢀaꢀ3.3Vꢀoutput.ꢀ  
Althoughꢀallꢀdissipativeꢀelementsꢀinꢀtheꢀcircuitꢀproduceꢀ  
losses,ꢀfourꢀmainꢀsourcesꢀusuallyꢀaccountꢀforꢀmostꢀofꢀtheꢀ  
lossesꢀinꢀLTC3868ꢀcircuits:ꢀ1)ꢀICꢀV ꢀcurrent,ꢀ2)ꢀINTV ꢀ  
EfficiencyꢀvariesꢀasꢀtheꢀinverseꢀsquareꢀofꢀV ꢀforꢀtheꢀ  
IN  
CC  
OUT  
2
regulatorꢀ current,ꢀ 3)ꢀ I Rꢀ losses,ꢀ 4)ꢀ topsideꢀ MOSFETꢀ  
sameꢀexternalꢀcomponentsꢀandꢀoutputꢀpowerꢀlevel.ꢀTheꢀ  
combinedꢀeffectsꢀofꢀincreasinglyꢀlowerꢀoutputꢀvoltagesꢀ  
andhighercurrentsrequiredbyhighperformancedigitalꢀ  
systemsisnotdoublingbutquadruplingtheimportanceꢀ  
ofꢀlossꢀtermsꢀinꢀtheꢀswitchingꢀregulatorꢀsystem!  
transitionꢀlosses.  
3868fb  
ꢁꢃ  
LTC3868  
applicaTions inForMaTion  
canꢀalsoꢀbeꢀestimatedꢀbyꢀexaminingꢀtheꢀriseꢀtimeꢀatꢀtheꢀ  
pin.TheITHexternalcomponentsshowninFigure12ꢀ  
circuitꢀwillꢀprovideꢀanꢀadequateꢀstartingꢀpointꢀforꢀmostꢀ  
applications.  
4.ꢀTransitionꢀlossesꢀapplyꢀonlyꢀtoꢀtheꢀtopsideꢀMOSFET(s),ꢀ  
andbecomesignificantonlywhenoperatingathighꢀ  
inputꢀ voltagesꢀ (typicallyꢀ 15Vꢀ orꢀ greater).ꢀ Transitionꢀ  
lossesꢀcanꢀbeꢀestimatedꢀfrom:  
TheI ꢀseriesR -C ltersetsthedominantpole-zeroꢀ  
ꢀ ꢀ TransitionꢀLossꢀ=ꢀ(1.7)ꢀ•ꢀV ꢀ•ꢀ2ꢀ•ꢀI  
ꢀ•ꢀC ꢀ•ꢀf  
O(MAX) RSS  
TH  
C
C
IN  
loopꢀcompensation.ꢀTheꢀvaluesꢀcanꢀbeꢀmodifiedꢀslightlyꢀ  
(fromꢀ0.5ꢀtoꢀ2ꢀtimesꢀtheirꢀsuggestedꢀvalues)ꢀtoꢀoptimizeꢀ  
transientꢀresponseꢀonceꢀtheꢀfinalꢀPCꢀlayoutꢀisꢀdoneꢀandꢀ  
theꢀparticularꢀoutputꢀcapacitorꢀtypeꢀandꢀvalueꢀhaveꢀbeenꢀ  
determined.Theoutputcapacitorsneedtobeselectedꢀ  
becauseꢀtheꢀvariousꢀtypesꢀandꢀvaluesꢀdetermineꢀtheꢀloopꢀ  
gainꢀandꢀphase.ꢀAnꢀoutputꢀcurrentꢀpulseꢀofꢀ20%ꢀtoꢀ80%ꢀ  
ofꢀfull-loadꢀcurrentꢀhavingꢀaꢀriseꢀtimeꢀofꢀ1µsꢀtoꢀ10µsꢀwillꢀ  
ꢀ Otherꢀhiddenꢀlossesꢀsuchꢀasꢀcopperꢀtraceꢀandꢀinternalꢀ  
batteryꢀresistancesꢀcanꢀaccountꢀforꢀanꢀadditionalꢀ5%ꢀ  
toꢀ10%ꢀefficiencyꢀdegradationꢀinꢀportableꢀsystems.ꢀItꢀ  
isꢀveryꢀimportantꢀtoꢀincludeꢀtheseꢀsystemꢀlevelꢀlossesꢀ  
duringꢀtheꢀdesignꢀphase.ꢀTheꢀinternalꢀbatteryꢀandꢀfuseꢀ  
resistancelossescanbeminimizedbymakingsurethatꢀ  
C ꢀhasꢀadequateꢀchargeꢀstorageꢀandꢀveryꢀlowꢀESRꢀatꢀ  
IN  
theswitchingfrequency.A25Wsupplywilltypicallyꢀ  
requireꢀ aꢀ minimumꢀ ofꢀ 20µFꢀ toꢀ 40µFꢀ ofꢀ capacitanceꢀ  
havingamaximumof20mΩto50mΩofESR.Theꢀ  
LTC38682-phasearchitecturetypicallyhalvesthisinputꢀ  
capacitanceꢀ requirementꢀ overꢀ competingꢀ solutions.ꢀ  
Otherꢀ lossesꢀ includingꢀ Schottkyꢀ conductionꢀ lossesꢀ  
duringdead-timeandinductorcorelossesgenerallyꢀ  
accountꢀforꢀlessꢀthanꢀ2%ꢀtotalꢀadditionalꢀloss.  
produceꢀoutputꢀvoltageꢀandꢀI ꢀpinꢀwaveformsꢀthatꢀwillꢀ  
TH  
giveꢀaꢀsenseꢀofꢀtheꢀoverallꢀloopꢀstabilityꢀwithoutꢀbreakingꢀ  
theꢀfeedbackꢀloop.ꢀ  
Placingꢀ aꢀ resistiveꢀ loadꢀ andꢀ aꢀ powerꢀ MOSFETꢀ directlyꢀ  
acrossꢀtheꢀoutputꢀcapacitorꢀandꢀdrivingꢀtheꢀgateꢀwithꢀanꢀ  
appropriateꢀsignalꢀgeneratorꢀisꢀaꢀpracticalꢀwayꢀtoꢀproduceꢀ  
aꢀrealisticꢀloadꢀstepꢀcondition.ꢀTheꢀinitialꢀoutputꢀvoltageꢀ  
stepꢀresultingꢀfromꢀtheꢀstepꢀchangeꢀinꢀoutputꢀcurrentꢀmayꢀ  
notꢀbeꢀwithinꢀtheꢀbandwidthꢀofꢀtheꢀfeedbackꢀloop,ꢀsoꢀthisꢀ  
signalꢀcannotꢀbeꢀusedꢀtoꢀdetermineꢀphaseꢀmargin.ꢀThisꢀ  
Checking Transient Response  
Theꢀregulatorꢀloopꢀresponseꢀcanꢀbeꢀcheckedꢀbyꢀlookingꢀatꢀ  
theꢀloadꢀcurrentꢀtransientꢀresponse.ꢀSwitchingꢀregulatorsꢀ  
takeꢀseveralꢀcyclesꢀtoꢀrespondꢀtoꢀaꢀstepꢀinꢀDCꢀ(resistive)ꢀ  
loadcurrent.Whenaloadstepoccurs,VOUTshiftsbyꢀ  
anꢀamountꢀequalꢀtoꢀILOADꢀ(ESR),ꢀwhereꢀESRꢀisꢀtheꢀef-  
fectiveꢀseriesꢀresistanceꢀofꢀCOUT.ꢀILOADꢀalsoꢀbeginsꢀtoꢀ  
chargeꢀorꢀdischargeꢀCOUTꢀgeneratingꢀtheꢀfeedbackꢀerrorꢀ  
signalthatforcestheregulatortoadapttothecurrentꢀ  
changeꢀandꢀreturnꢀVOUTꢀtoꢀitsꢀsteady-stateꢀvalue.ꢀDuringꢀ  
thisꢀrecoveryꢀtimeꢀVOUTꢀcanꢀbeꢀmonitoredꢀforꢀexcessiveꢀ  
overshootorꢀ ringing,ꢀ whichꢀ wouldindicateꢀ aꢀ stabilityꢀ  
problem.ꢀOPTI-LOOPꢀcompensationꢀallowsꢀtheꢀtransientꢀ  
responsetobeoptimizedoverawiderangeofoutputꢀ  
capacitanceꢀandꢀESRꢀvalues.ꢀThe availability of the ITH pin  
not only allows optimization of control loop behavior, but  
it also provides a DC coupled and AC filtered closed-loop  
response test point. The DC step, rise time and settling  
at this test point truly reflects the closed-loop response.ꢀ  
Assumingꢀaꢀpredominantlyꢀsecondꢀorderꢀsystem,ꢀphaseꢀ  
marginand/ordampingfactorcanbeestimatedusingtheꢀ  
percentageꢀofꢀovershootꢀseenꢀatꢀthisꢀpin.ꢀTheꢀbandwidthꢀ  
isꢀwhyꢀitꢀisꢀbetterꢀtoꢀlookꢀatꢀtheꢀI ꢀpinꢀsignalꢀwhichꢀisꢀinꢀ  
TH  
thefeedbackloopandisthelteredandcompensatedꢀ  
controlꢀloopꢀresponse.ꢀ  
TheꢀgainꢀofꢀtheꢀloopꢀwillꢀbeꢀincreasedꢀbyꢀincreasingꢀR ꢀ  
C
andꢀtheꢀbandwidthꢀofꢀtheꢀloopꢀwillꢀbeꢀincreasedꢀbyꢀde-  
creasingꢀC .ꢀIfꢀR ꢀisꢀincreasedꢀbyꢀtheꢀsameꢀfactorꢀthatꢀC ꢀ  
C
C
C
isꢀdecreased,ꢀtheꢀzeroꢀfrequencyꢀwillꢀbeꢀkeptꢀtheꢀsame,ꢀ  
therebykeepingthephaseshiftthesameinthemostꢀ  
criticalꢀfrequencyꢀrangeꢀofꢀtheꢀfeedbackꢀloop.ꢀTheꢀoutputꢀ  
voltageꢀsettlingꢀbehaviorꢀisꢀrelatedꢀtoꢀtheꢀstabilityꢀofꢀtheꢀ  
closed-loopsystemandwilldemonstratetheactualoverallꢀ  
supplyꢀperformance.  
Aꢀsecond,ꢀmoreꢀsevereꢀtransientꢀisꢀcausedꢀbyꢀswitchingꢀ  
inꢀloadsꢀwithꢀlargeꢀ(>1µF)ꢀsupplyꢀbypassꢀcapacitors.ꢀTheꢀ  
dischargedbypasscapacitorsareeffectivelyputinparallelꢀ  
withꢀC ,ꢀcausingꢀaꢀrapidꢀdropꢀinꢀV .ꢀNoꢀregulatorꢀcanꢀ  
OUTꢀ  
OUTꢀ  
alterꢀitsꢀdeliveryꢀofꢀcurrentꢀquicklyꢀenoughꢀtoꢀpreventꢀthisꢀ  
suddenꢀstepꢀchangeꢀinꢀoutputꢀvoltageꢀifꢀtheꢀloadꢀswitchꢀ  
resistanceꢀisꢀlowꢀandꢀitꢀisꢀdrivenꢀquickly.ꢀIfꢀtheꢀratioꢀofꢀ  
3868fb  
ꢁꢄ  
LTC3868  
applicaTions inForMaTion  
C
ꢀtoꢀC ꢀisꢀgreaterꢀthanꢀ1:50,ꢀtheꢀswitchꢀriseꢀtimeꢀ  
ThepowerdissipationonthetopsideMOSFETcanbeeasilyꢀ  
estimated.ꢀChoosingꢀaꢀFairchildꢀFDS6982SꢀdualꢀMOSFETꢀ  
LOAD  
OUT  
shouldꢀbeꢀcontrolledꢀsoꢀthatꢀtheꢀloadꢀriseꢀtimeꢀisꢀlimitedꢀ  
toꢀapproximatelyꢀ25ꢀ•ꢀC .ꢀThusꢀaꢀ10µFꢀcapacitorꢀwouldꢀ  
resultsꢀin:ꢀR  
ꢀ=ꢀ0.035Ω/0.022Ω,ꢀC  
ꢀ=ꢀ215pF.ꢀAtꢀ  
LOAD  
DS(ON)  
MILLER  
requireꢀaꢀ250µsꢀriseꢀtime,ꢀlimitingꢀtheꢀchargingꢀcurrentꢀ  
toꢀaboutꢀ200mA.  
maximumꢀinputꢀvoltageꢀwithꢀT(estimated)ꢀ=ꢀ50°C:  
2   
3.3V  
22V  
PMAIN  
=
5A 1+ 0.005 50°C – 25°C  
(
)
(
)(  
)
Design Example  
2 5A  
Asꢀ aꢀ designꢀ exampleꢀ forꢀ oneꢀ channel,ꢀ assumeꢀ V ꢀ =ꢀ  
0.035Ω + 22V  
2.5215pF •  
IN  
(
) (  
)
1
(
)(  
)
2
12V(nominal),ꢀV ꢀ=ꢀ22Vꢀ(max),ꢀV ꢀ=ꢀ3.3V,ꢀI ꢀ=ꢀ5A,ꢀ  
IN  
OUT  
MAX  
1
V
ꢀ=ꢀ75mVꢀandꢀfꢀ=ꢀ350kHz.  
SENSE(MAX)  
+
350kHz = 331mW  
(
)
5V – 2.3V 2.3V  
Theinductancevalueischosenrstbasedona30%rippleꢀ  
currentꢀassumption.ꢀTheꢀhighestꢀvalueꢀofꢀrippleꢀcurrentꢀ  
occursꢀatꢀtheꢀmaximumꢀinputꢀvoltage.ꢀTieꢀtheꢀFREQꢀpinꢀ  
toꢀ GND,ꢀ generatingꢀ 350kHzꢀ operation.ꢀ Theꢀ minimumꢀ  
inductanceꢀforꢀ30%ꢀrippleꢀcurrentꢀis:  
Aꢀshort-circuitꢀtoꢀgroundꢀwillꢀresultꢀinꢀaꢀfoldedꢀbackꢀcur-  
rentꢀof:  
95ns 22V  
(
)
32mV  
0.012  
1
ISC =  
= 2.98A  
4.7µH  
VOUT  
f L  
( )( )  
VOUT  
IL(NOM)  
=
1–  
IN(NOM)   
V
withꢀaꢀtypicalꢀvalueꢀofꢀR  
ꢀandꢀδꢀ=ꢀ(0.005/°C)(25°C)ꢀ  
DS(ON)  
=0.125.Theresultingpowerdissipatedinthebottomꢀ  
A4.7µHinductorwillproduce29%ripplecurrent.Theꢀ  
peakꢀinductorꢀcurrentꢀwillꢀbeꢀtheꢀmaximumꢀDCꢀvalueꢀplusꢀ  
oneꢀhalfꢀtheꢀrippleꢀcurrent,ꢀorꢀ5.73A.ꢀIncreasingꢀtheꢀrippleꢀ  
currentꢀwillꢀalsoꢀhelpꢀensureꢀthatꢀtheꢀminimumꢀon-timeꢀ  
ofꢀ95nsꢀisꢀnotꢀviolated.ꢀTheꢀminimumꢀon-timeꢀoccursꢀatꢀ  
MOSFETꢀis:  
2
P
= 2.98A 1.125 0.022Ω = 220mW  
(
) (  
)(  
)
SYNC  
whichꢀisꢀlessꢀthanꢀunderꢀfull-loadꢀconditions.  
maximumꢀV :  
IN  
C ꢀisꢀchosenꢀforꢀanꢀRMSꢀcurrentꢀratingꢀofꢀatꢀleastꢀ3Aꢀatꢀ  
IN  
VOUT  
3.3V  
temperatureassumingonlythischannelison.C ꢀisꢀ  
OUT  
tON(MIN)  
=
=
= 429ns  
V
f
22V 350kHz  
chosenꢀwithꢀanꢀESRꢀofꢀ0.02Ωꢀforꢀlowꢀoutputꢀripple.ꢀTheꢀ  
outputꢀrippleꢀinꢀcontinuousꢀmodeꢀwillꢀbeꢀhighestꢀatꢀtheꢀ  
maximumꢀinputꢀvoltage.ꢀTheꢀoutputꢀvoltageꢀrippleꢀdueꢀtoꢀ  
ESRꢀisꢀapproximately:  
IN(MAX)
( )  
(
)
TheꢀequivalentꢀR  
ꢀresistorꢀvalueꢀcanꢀbeꢀcalculatedꢀbyꢀ  
SENSE  
usingꢀtheꢀminimumꢀvalueꢀforꢀtheꢀmaximumꢀcurrentꢀsenseꢀ  
thresholdꢀ(64mV):  
ꢀ V ꢀ=ꢀR ꢀ(I )ꢀ=ꢀ0.02Ω(1.45A)ꢀ=ꢀ29mV  
ORIPPLE ESR L P-P  
64mV  
5.73A  
RSENSE  
= 0.01Ω  
Choosingꢀ1%ꢀresistors:ꢀR ꢀ=ꢀ25kꢀandꢀR ꢀ=ꢀ78.1kꢀyieldsꢀ  
A
B
anꢀoutputꢀvoltageꢀofꢀ3.299V.  
3868fb  
ꢁꢅ  
LTC3868  
applicaTions inForMaTion  
PC Board Layout Checklist  
6.ꢀKeepꢀtheꢀswitchingꢀnodesꢀ(SW1,ꢀSW2),ꢀtopꢀgateꢀnodesꢀ  
(TG1,TG2),andboostnodes(BOOST1,BOOST2)awayꢀ  
fromꢀ sensitiveꢀ small-signalꢀ nodes,ꢀ especiallyꢀ fromꢀ  
theoppositeschannel’svoltageandcurrentsensingꢀ  
feedbackꢀpins.ꢀAllꢀofꢀtheseꢀnodesꢀhaveꢀveryꢀlargeꢀandꢀ  
fastꢀmovingꢀsignalsꢀandꢀthereforeꢀshouldꢀbeꢀkeptꢀonꢀ  
theꢀoutput sideꢀofꢀtheꢀLTC3868ꢀandꢀoccupyꢀminimumꢀ  
PCꢀtraceꢀarea.  
Whenꢀlayingꢀoutꢀtheꢀprintedꢀcircuitꢀboard,ꢀtheꢀfollowingꢀ  
checklistꢀshouldꢀbeꢀusedꢀtoꢀensureꢀproperꢀoperationꢀofꢀ  
theꢀIC.ꢀTheseꢀitemsꢀareꢀalsoꢀillustratedꢀgraphicallyꢀinꢀtheꢀ  
layoutdiagramofFigure10.Figure11illustratesthecurrentꢀ  
waveformspresentinthevariousbranchesofthe2-phaseꢀ  
synchronousregulatorsoperatinginthecontinuousmode.ꢀ  
Checkꢀtheꢀfollowingꢀinꢀyourꢀlayout:  
7.Useamodifiedstargroundtechnique:alowimpedance,ꢀ  
largeꢀcopperꢀareaꢀcentralꢀgroundingꢀpointꢀonꢀtheꢀsameꢀ  
sideꢀofꢀtheꢀPCꢀboardꢀasꢀtheꢀinputꢀandꢀoutputꢀcapacitorsꢀ  
1.ꢀAreꢀtheꢀtopꢀN-channelꢀMOSFETsꢀMTOP1ꢀandꢀMTOP2ꢀ  
locatedꢀwithinꢀ1cmꢀofꢀeachꢀotherꢀwithꢀaꢀcommonꢀdrainꢀ  
connectionatC ?Donotattempttosplittheinputꢀ  
IN  
withꢀtie-insꢀforꢀtheꢀbottomꢀofꢀtheꢀINTV ꢀdecouplingꢀ  
CC  
decouplingꢀforꢀtheꢀtwoꢀchannelsꢀasꢀitꢀcanꢀcauseꢀaꢀlargeꢀ  
resonantꢀloop.  
capacitor,ꢀtheꢀbottomꢀofꢀtheꢀvoltageꢀfeedbackꢀresistiveꢀ  
dividerꢀandꢀtheꢀSGNDꢀpinꢀofꢀtheꢀIC.  
2.ꢀAreꢀtheꢀsignalꢀandꢀpowerꢀgroundsꢀkeptꢀseparate?ꢀTheꢀ  
combinedꢀICꢀsignalꢀgroundꢀpinꢀandꢀtheꢀgroundꢀreturnꢀ  
PC Board Layout Debugging  
ofꢀC  
ꢀmustꢀreturnꢀtoꢀtheꢀcombinedꢀC ꢀ(–)ꢀter-  
INTVCC  
OUT  
Startꢀwithꢀoneꢀcontrollerꢀonꢀatꢀaꢀtime.ꢀItꢀisꢀhelpfulꢀtoꢀuseꢀ  
aꢀDC-50MHzꢀcurrentꢀprobeꢀtoꢀmonitorꢀtheꢀcurrentꢀinꢀtheꢀ  
inductorꢀ whileꢀ testingꢀ theꢀ circuit.ꢀ Monitorꢀ theꢀ outputꢀ  
switchingnode(SWpin)tosynchronizetheoscilloscopeꢀ  
toꢀ theꢀ internalꢀ oscillatorꢀ andꢀ probeꢀ theꢀ actualꢀ outputꢀ  
voltageꢀasꢀwell.ꢀCheckꢀforꢀproperꢀperformanceꢀoverꢀtheꢀ  
operatingꢀvoltageꢀandꢀcurrentꢀrangeꢀexpectedꢀinꢀtheꢀap-  
plication.Thefrequencyofoperationshouldbemaintainedꢀ  
overꢀtheꢀinputꢀvoltageꢀrangeꢀdownꢀtoꢀdropoutꢀandꢀuntilꢀ  
theꢀoutputꢀloadꢀdropsꢀbelowꢀtheꢀlowꢀcurrentꢀoperationꢀ  
threshold—typicallyꢀ 10%ꢀ ofꢀ theꢀ maximumꢀ designedꢀ  
currentꢀlevelꢀinꢀBurstꢀModeꢀoperation.  
minals.ꢀTheꢀpathꢀformedꢀbyꢀtheꢀtopꢀN-channelꢀMOSFET,ꢀ  
SchottkyꢀdiodeꢀandꢀtheꢀC ꢀcapacitorꢀshouldꢀhaveꢀshortꢀ  
IN  
leadsꢀandꢀPCꢀtraceꢀlengths.ꢀTheꢀoutputꢀcapacitorꢀ(–)ꢀ  
terminalsshouldbeconnectedascloseaspossibleꢀ  
toꢀtheꢀ(–)ꢀterminalsꢀofꢀtheꢀinputꢀcapacitorꢀbyꢀplacingꢀ  
theꢀcapacitorsꢀnextꢀtoꢀeachꢀotherꢀandꢀawayꢀfromꢀtheꢀ  
Schottkyꢀloopꢀdescribedꢀabove.  
3.ꢀDoꢀtheꢀLTC3868ꢀV ꢀpins’ꢀresistiveꢀdividersꢀconnectꢀtoꢀ  
FB  
theꢀ(+)ꢀterminalsꢀofꢀC ?ꢀTheꢀresistiveꢀdividerꢀmustꢀbeꢀ  
OUT  
connectedꢀbetweenꢀtheꢀ(+)ꢀterminalꢀofꢀC ꢀandꢀsignalꢀ  
OUT  
ground.ꢀTheꢀfeedbackꢀresistorꢀconnectionsꢀshouldꢀnotꢀ  
beꢀalongꢀtheꢀhighꢀcurrentꢀinputꢀfeedsꢀfromꢀtheꢀinputꢀ  
capacitor(s).  
Thedutycyclepercentageshouldbemaintainedfromcycleꢀ  
tocycleinawell-designed,lownoisePCBimplementation.ꢀ  
Variationꢀinꢀtheꢀdutyꢀcycleꢀatꢀaꢀsubharmonicꢀrateꢀcanꢀsug-  
gestꢀnoiseꢀpickupꢀatꢀtheꢀcurrentꢀorꢀvoltageꢀsensingꢀinputsꢀ  
orꢀinadequateꢀloopꢀcompensation.ꢀOvercompensationꢀofꢀ  
theꢀloopꢀcanꢀbeꢀusedꢀtoꢀtameꢀaꢀpoorꢀPCꢀlayoutꢀifꢀregula-  
torꢀ bandwidthꢀ optimizationꢀ isꢀ notꢀ required.ꢀ Onlyꢀ afterꢀ  
eachꢀcontrollerꢀisꢀcheckedꢀforꢀitsꢀindividualꢀperformanceꢀ  
shouldꢀbothꢀcontrollersꢀbeꢀturnedꢀonꢀatꢀtheꢀsameꢀtime.ꢀ  
Aparticularlydifficultregionofoperationiswhenoneꢀ  
controllerꢀchannelꢀisꢀnearingꢀitsꢀcurrentꢀcomparatorꢀtripꢀ  
pointwhentheotherchannelisturningonitstopMOSFET.ꢀ  
Thisꢀoccursꢀaroundꢀ50%ꢀdutyꢀcycleꢀonꢀeitherꢀchannelꢀdueꢀ  
toꢀtheꢀphasingꢀofꢀtheꢀinternalꢀclocksꢀandꢀmayꢀcauseꢀminorꢀ  
dutyꢀcycleꢀjitter.  
+
4.ꢀAretheSENSE andSENSE leadsroutedtogetherwithꢀ  
minimumPCtracespacing?Theltercapacitorbetweenꢀ  
+
SENSE ꢀandꢀSENSE ꢀshouldꢀbeꢀasꢀcloseꢀasꢀpossibleꢀ  
toꢀtheꢀIC.ꢀEnsureꢀaccurateꢀcurrentꢀsensingꢀwithꢀKelvinꢀ  
connectionsꢀatꢀtheꢀSENSEꢀresistor.  
5.ꢀIstheINTV decouplingcapacitorconnectedcloseꢀ  
CC  
toꢀtheꢀIC,ꢀbetweenꢀtheꢀINTV ꢀandꢀtheꢀpowerꢀgroundꢀ  
CC  
pins?ꢀThisꢀcapacitorꢀcarriesꢀtheꢀMOSFETꢀdrivers’ꢀcur-  
rentꢀpeaks.ꢀAnꢀadditionalꢀ1µFꢀceramicꢀcapacitorꢀplacedꢀ  
immediatelynexttotheINTV andPGNDpinscanhelpꢀ  
CC  
improveꢀnoiseꢀperformanceꢀsubstantially.  
3868fb  
ꢁꢆ  
LTC3868  
applicaTions inForMaTion  
R
SS1  
PU2  
V
PULL-UP  
LTC3868  
(<6V)  
PGOOD2  
I
PGOOD2  
PGOOD1  
TG1  
R
TH1  
PU1  
V
PULL-UP  
(<6V)  
V
PGOOD1  
FB1  
L1  
R
SENSE  
+
V
SENSE1  
SENSE1  
FREQ  
OUT1  
SW1  
C
B1  
M1  
M2  
D1  
BOOST1  
BG1  
PHASMD  
CLKOUT  
PLLIN/MODE  
RUN1  
C
C
OUT1  
V
IN  
f
1µF  
CERAMIC  
IN  
R
C
VIN  
IN  
PGND  
GND  
RUN2  
EXTV  
CC  
V
OUT1  
C
IN  
C
SGND  
INTVCC  
V
IN  
INTV  
CC  
SENSE2  
OUT2  
D2  
1µF  
CERAMIC  
+
BG2  
SENSE2  
M4  
M3  
BOOST2  
V
FB2  
TH2  
C
B2  
SW2  
TG2  
I
R
SENSE  
V
OUT2  
SS2  
L2  
3868 F10  
Figure 10. Recommended Printed Circuit Layout Diagram  
3868fb  
ꢁꢇ  
LTC3868  
applicaTions inForMaTion  
SW1  
L1  
R
SENSE1  
V
OUT1  
D1  
C
R
L1  
OUT1  
V
IN  
R
IN  
C
IN  
SW2  
L2  
R
SENSE2  
V
OUT2  
D2  
C
R
L2  
OUT2  
BOLD LINES INDICATE  
HIGH SWITCHING  
CURRENT. KEEP LINES  
TO A MINIMUM LENGTH.  
3868 F11  
Figure 11. Branch Current Waveforms  
3868fb  
ꢁꢈ  
LTC3868  
applicaTions inForMaTion  
Reduceꢀ V ꢀ fromꢀ itsꢀ nominalꢀ levelꢀ toꢀ verifyꢀ operationꢀ  
forꢀinductiveꢀcouplingꢀbetweenꢀC ,ꢀSchottkyꢀandꢀtheꢀtopꢀ  
IN  
IN  
oftheregulatorindropout.Checktheoperationoftheꢀ  
MOSFETꢀcomponentsꢀtoꢀtheꢀsensitiveꢀcurrentꢀandꢀvoltageꢀ  
sensingꢀtraces.ꢀInꢀaddition,ꢀinvestigateꢀcommonꢀgroundꢀ  
pathꢀvoltageꢀpickupꢀbetweenꢀtheseꢀcomponentsꢀandꢀtheꢀ  
SGNDꢀpinꢀofꢀtheꢀIC.  
undervoltageꢀlockoutꢀcircuitꢀbyꢀfurtherꢀloweringꢀV ꢀwhileꢀ  
IN  
monitoringꢀtheꢀoutputsꢀtoꢀverifyꢀoperation.  
Investigatewhetheranyproblemsexistonlyathigherout-  
putꢀcurrentsꢀorꢀonlyꢀatꢀhigherꢀinputꢀvoltages.ꢀIfꢀproblemsꢀ  
coincidewithhighinputvoltagesandlowoutputcurrents,ꢀ  
lookꢀforꢀcapacitiveꢀcouplingꢀbetweenꢀtheꢀBOOST,ꢀSW,ꢀTG,ꢀ  
andpossiblyBGconnectionsandthesensitivevoltageꢀ  
andꢀcurrentꢀpins.ꢀTheꢀcapacitorꢀplacedꢀacrossꢀtheꢀcurrentꢀ  
sensingꢀpinsꢀneedsꢀtoꢀbeꢀplacedꢀimmediatelyꢀadjacentꢀtoꢀ  
theꢀpinsꢀofꢀtheꢀIC.ꢀThisꢀcapacitorꢀhelpsꢀtoꢀminimizeꢀtheꢀ  
effectsꢀofꢀdifferentialꢀnoiseꢀinjectionꢀdueꢀtoꢀhighꢀfrequencyꢀ  
capacitiveꢀ coupling.ꢀ Ifꢀ problemsꢀ areꢀ encounteredꢀ withꢀ  
highꢀcurrentꢀoutputꢀloadingꢀatꢀlowerꢀinputꢀvoltages,ꢀlookꢀ  
Anembarrassingproblem,whichcanbemissedinanꢀ  
otherwiseꢀproperlyꢀworkingꢀswitchingꢀregulator,ꢀresultsꢀ  
whenthecurrentsensingleadsarehookedupbackwards.ꢀ  
Theꢀoutputꢀvoltageꢀunderꢀthisꢀimproperꢀhookupꢀwillꢀstillꢀ  
beꢀmaintainedꢀbutꢀtheꢀadvantagesꢀofꢀcurrentꢀmodeꢀcontrolꢀ  
willꢀnotꢀbeꢀrealized.ꢀCompensationꢀofꢀtheꢀvoltageꢀloopꢀwillꢀ  
beꢀ muchꢀ moreꢀ sensitiveꢀ toꢀ componentꢀ selection.ꢀ Thisꢀ  
behaviorꢀcanꢀbeꢀinvestigatedꢀbyꢀtemporarilyꢀshortingꢀoutꢀ  
theꢀcurrentꢀsensingꢀresistor—don’tꢀworry,ꢀtheꢀregulatorꢀ  
willꢀstillꢀmaintainꢀcontrolꢀofꢀtheꢀoutputꢀvoltage.  
3868fb  
ꢂ0  
LTC3868  
Typical applicaTions  
R
B1  
INTV  
215k  
CC  
LTC3868  
+
100k  
C
15pF  
SENSE1  
F1  
PGOOD2  
C1  
1nF  
100k  
R
A1  
68.1k  
SENSE1  
PGOOD1  
BG1  
L1  
3.3µH  
MBOT1  
MTOP1  
V
FB1  
V
3.3V  
5A  
OUT1  
C
ITH1A  
150pF  
SW1  
R
C
C
SENSE1  
6mΩ  
OUT1  
B1  
0.47µF  
BOOST1  
TG1  
150µF  
R
ITH1  
15k  
I
TH1  
D1  
D2  
C
ITH1  
820pF  
C
SS1  
0.1µF  
V
IN  
V
IN  
9V TO 24V  
C
IN  
22µF  
SS1  
I
INTV  
CC  
LIM  
PHSMD  
C
INT  
4.7µF  
CLKOUT  
PLLIN/MODE  
SGND  
PGND  
MTOP2  
MBOT2  
EXTV  
TG2  
CC  
RUN1  
RUN2  
FREQ  
C
B2  
0.47µF  
BOOST2  
L2  
7.2µH  
R
SENSE2  
8mΩ  
C
0.1µF  
SS2  
V
8.5V  
3A  
OUT2  
SW2  
BG2  
SS2  
C
C
680pF  
OUT2  
ITH2  
R
27k  
150µF  
ITH2  
I
TH2  
C
100pF  
C2  
ITH2A  
V
FB2  
R
A2  
44.2k  
+
SENSE2  
C
1nF  
F2  
39pF  
SENSE2  
R
B2  
422k  
3868 F12  
C
, C : SANYO 10TPD150M  
OUT1 OUT2  
L1: SUMIDA CDEP105-3R2M  
L2: SUMIDA CDEP105-7R2M  
MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP  
Efficiency vs Output Current  
Start-Up  
SW Node Waveforms  
100  
90  
V
80  
OUT2  
V
OUT  
= 8.5V  
V
OUT  
= 3.3V  
2V/DIV  
70  
SW1  
5V/DIV  
60  
50  
V
OUT1  
2V/DIV  
40  
30  
20  
10  
0
SW2  
5V/DIV  
V
= 12V  
IN  
Burst Mode OPERATION  
0.1 10  
OUTPUT CURRENT (A)  
3868 F12c  
3868 F12d  
20ms/DIV  
1µs/DIV  
0.000010.0001 0.001 0.01  
1
3868 F12b  
Figure 12. High Efficiency Dual 8.5V/3.3V Step-Down Converter  
3868fb  
ꢂꢀ  
LTC3868  
Typical applicaTions  
High Efficiency Dual 2.5V/3.3V Step-Down Converter  
R
B1  
INTV  
143k  
CC  
LTC3868  
+
100k  
C
22pF  
SENSE1  
F1  
PGOOD2  
C1  
1nF  
100k  
R
A1  
68.1k  
SENSE1  
PGOOD1  
BG1  
L1  
2.4µH  
MBOT1  
MTOP1  
V
FB1  
V
2.5V  
5A  
OUT1  
C
100pF  
ITH1A  
SW1  
R
C
C
SENSE1  
6mΩ  
OUT1  
B1  
0.47µF  
BOOST1  
TG1  
150µF  
R
ITH1  
22k  
I
TH1  
D1  
D2  
C
ITH1  
820pF  
C
SS1  
0.01µF  
V
IN  
V
IN  
4V TO 24V  
C
IN  
22µF  
SS1  
I
INTV  
CC  
LIM  
PHSMD  
C
INT  
4.7µF  
CLKOUT  
PLLIN/MODE  
SGND  
PGND  
MTOP2  
MBOT2  
EXTV  
TG2  
CC  
RUN1  
RUN2  
FREQ  
C
B2  
0.47µF  
BOOST2  
L2  
3.2µH  
R
SENSE2  
6mΩ  
C
0.01µF  
SS2  
V
3.3V  
5A  
OUT2  
SW2  
BG2  
SS2  
C
C
820pF  
OUT2  
ITH2  
R
15k  
150µF  
ITH2  
I
TH2  
C
150pF  
C2  
ITH2A  
V
FB2  
R
A2  
68.1k  
+
SENSE2  
C
1nF  
F2  
15pF  
SENSE2  
R
B2  
215k  
3868 F13  
C
, C : SANYO 10TPD150M  
OUT1 OUT2  
L1: SUMIDA CDEP105-2R5  
L2: SUMIDA CDEP105-3R2M  
MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP  
3868fb  
ꢂꢁ  
LTC3868  
Typical applicaTions  
High Efficiency Dual 12V/5V Step-Down Converter  
R
B1  
475k  
INTV  
CC  
100k  
100k  
+
C
SENSE1  
SENSE1  
F1  
PGOOD2  
C1  
1nF  
33pF  
R
A1  
PGOOD1  
BG1  
34k  
L1  
8.8µH  
MBOT1  
MTOP1  
V
FB1  
V
12V  
3A  
OUT1  
C
ITH1A  
100pF  
SW1  
R
C
C
SENSE1  
9mΩ  
OUT1  
B1  
BOOST1  
TG1  
47µF  
0.47µF  
R
ITH1  
10k  
I
TH1  
D1  
D2  
LTC3868  
C
SS1  
0.01µF  
C
ITH1  
680pF  
V
IN  
V
IN  
12.5V TO 24V  
C
IN  
SS1  
22µF  
I
INTV  
CC  
LIM  
C
INT  
PHSMD  
4.7µF  
CLKOUT  
PLLIN/MODE  
SGND  
PGND  
MTOP2  
MBOT2  
EXTV  
TG2  
CC  
RUN1  
RUN2  
FREQ  
R
C
FREQ  
B2  
BOOST2  
60k  
0.47µF  
L2  
4.3µH  
R
SENSE2  
6mΩ  
C
0.01µF  
SS2  
V
5V  
5A  
OUT2  
SW2  
BG2  
SS2  
C
C
680pF  
OUT2  
ITH2  
R
17k  
150µF  
ITH2  
I
TH2  
C
100pF  
C2  
ITH2A  
V
FB2  
R
A2  
C
: KEMET T525D476M016E035  
: SANYO 10TPD150M  
OUT1  
OUT2  
+
SENSE2  
75k  
C
L1: SUMIDA CDR7D43MN  
L2: SUMIDA CDEP105-4R3M  
MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP  
C
1nF  
F2  
15pF  
SENSE2  
R
B2  
393k  
3858 TA02a  
3868fb  
ꢂꢂ  
LTC3868  
Typical applicaTions  
High Efficiency Dual 1V/1.2V Step-Down Converter  
R
B1  
28.7k  
INTV  
CC  
100k  
100k  
+
C
SENSE1  
SENSE1  
F1  
PGOOD2  
C1  
1nF  
56pF  
R
A1  
PGOOD1  
BG1  
115k  
L1  
0.47µH  
MBOT1  
MTOP1  
V
FB1  
V
1V  
8A  
OUT1  
C
ITH1A  
200pF  
SW1  
C
R
OUT1  
C
SENSE1  
3mΩ  
B1  
BOOST1  
TG1  
220µF  
0.47µF  
R
ITH1  
3.93k  
×2  
I
TH1  
D1  
D2  
LTC3868  
C
1000pF  
ITH1  
C
SS1  
0.01µF  
V
IN  
V
IN  
12V  
C
IN  
SS1  
22µF  
I
INTV  
CC  
LIM  
C
INT  
PHSMD  
4.7µF  
CLKOUT  
PLLIN/MODE  
SGND  
PGND  
MTOP2  
MBOT2  
EXTV  
TG2  
CC  
RUN1  
RUN2  
FREQ  
R
C
FREQ  
B2  
BOOST2  
60k  
0.47µF  
L2  
0.47µH  
R
SENSE2  
3mΩ  
C
0.01µF  
SS2  
V
1.2V  
8A  
OUT2  
SW2  
BG2  
SS2  
C
OUT2  
C
1000pF  
ITH2  
220µF  
R
3.93k  
ITH2  
×2  
I
TH2  
C
200pF  
C2  
ITH2A  
V
FB2  
R
C
, C  
: SANYO 2R5TPE220M  
A2  
OUT1 OUT2  
+
SENSE2  
115k  
L1: SUMIDA CDEP105-0R4  
L2: SUMIDA CDEP105-0R4  
MTOP1, MTOP2: RENESAS RJK0305  
MBOT1, MBOT2: RENESAS RJK0328  
C
1nF  
F2  
56pF  
SENSE2  
R
B2  
57.6k  
3868 TA03a  
3868fb  
ꢂꢃ  
LTC3868  
Typical applicaTions  
High Efficiency Dual 1V/1.2V Step-Down Converter with Inductor DCR Current Sensing  
R
B1  
R
S1  
1.18k  
28.7k  
+
C
SENSE1  
SENSE1  
F1  
INTV  
CC  
C1  
0.1µF  
56pF  
100k  
R
A1  
PGOOD1  
115k  
L1  
0.47µH  
MBOT1  
MTOP1  
V
BG1  
SW1  
FB1  
V
OUT1  
C
ITH1A  
200pF  
1V  
C
OUT1 8A  
C
B1  
BOOST1  
TG1  
220µF  
0.47µF  
R
ITH1  
3.93k  
×2  
I
TH1  
D1  
D2  
LTC3868  
C
1000pF  
ITH1  
C
SS1  
0.01µF  
V
IN  
V
IN  
12V  
C
IN  
SS1  
22µF  
INTV  
CC  
C
INT  
4.7µF  
PGND  
PLLIN/MODE  
SGND  
MTOP2  
MBOT2  
EXTV  
TG2  
CC  
RUN1  
RUN2  
FREQ  
R
C
FREQ  
B2  
BOOST2  
65k  
0.47µF  
L2  
0.47µH  
C
0.01µF  
SS2  
V
OUT2  
1.2V  
SW2  
BG2  
SS2  
C
OUT2 8A  
C
1000pF  
ITH2  
220µF  
R
3.93k  
ITH2  
×2  
I
TH2  
C
220pF  
C2  
ITH2A  
V
FB2  
R
A2  
+
SENSE2  
115k  
C
, C  
: SANYO 2R5TPE220M  
OUT1 OUT2  
L1, L2: VISHAY IHL P2525CZERR47M06  
MTOP1, MTOP2: RENESAS RJK0305  
MBOT1, MBOT2: RENESAS RJK0328  
C
0.1µF  
F2  
56pF  
SENSE2  
R
S2  
1.18k  
R
B2  
57.6k  
3868 TA05  
3868fb  
ꢂꢄ  
LTC3868  
package DescripTion  
UH Package  
32-Lead Plastic QFN (5mm × 5mm)  
(Reference LTC DWG # 05-08-1693 Rev D)  
0.70 p0.05  
5.50 p0.05  
4.10 p0.05  
3.45 p 0.05  
3.50 REF  
(4 SIDES)  
3.45 p 0.05  
PACKAGE OUTLINE  
0.25 p 0.05  
0.50 BSC  
RECOMMENDED SOLDER PAD LAYOUT  
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED  
BOTTOM VIEW—EXPOSED PAD  
PIN 1 NOTCH R = 0.30 TYP  
OR 0.35 s 45o CHAMFER  
R = 0.05  
TYP  
0.00 – 0.05  
R = 0.115  
TYP  
0.75 p 0.05  
5.00 p 0.10  
(4 SIDES)  
31 32  
0.40 p 0.10  
PIN 1  
TOP MARK  
(NOTE 6)  
1
2
3.45 p 0.10  
3.50 REF  
(4-SIDES)  
3.45 p 0.10  
(UH32) QFN 0406 REV D  
0.200 REF  
0.25 p 0.05  
0.50 BSC  
NOTE:  
1. DRAWING PROPOSED TO BE A JEDEC PACKAGE OUTLINE  
M0-220 VARIATION WHHD-(X) (TO BE APPROVED)  
2. DRAWING NOT TO SCALE  
3. ALL DIMENSIONS ARE IN MILLIMETERS  
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE  
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm ON ANY SIDE  
5. EXPOSED PAD SHALL BE SOLDER PLATED  
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION  
ON THE TOP AND BOTTOM OF PACKAGE  
3868fb  
ꢂꢅ  
LTC3868  
revision hisTory (Revision history begins at Rev B)  
REV  
DATE DESCRIPTION  
PAGE NUMBER  
B
12/09 ChangeꢀtoꢀAbsoluteꢀMaximumꢀRatings  
ChangeꢀtoꢀElectricalꢀCharacteristics  
ChangeꢀtoꢀTypicalꢀPerformanceꢀCharacteristics  
ChangeꢀtoꢀPinꢀFunctions  
2
3
6
8,ꢀ9  
TextꢀChangesꢀtoꢀOperationꢀSection  
TextꢀChangesꢀtoꢀApplicationsꢀInformationꢀSection  
ChangeꢀtoꢀTableꢀ2  
11,ꢀ12,ꢀ13  
21,ꢀ22,ꢀ23,ꢀ26  
23  
28  
38  
ChangeꢀtoꢀFigureꢀ10  
ChangesꢀtoꢀRelatedꢀParts  
3868fb  
InformationfurnishedbyLinearTechnologyCorporationisbelievedtobeaccurateandreliable.ꢀ  
However,ꢀnoꢀresponsibilityꢀisꢀassumedꢀforꢀitsꢀuse.ꢀLinearꢀTechnologyꢀCorporationꢀmakesꢀnoꢀrepresenta-  
tionꢀthatꢀtheꢀinterconnectionꢀofꢀitsꢀcircuitsꢀasꢀdescribedꢀhereinꢀwillꢀnotꢀinfringeꢀonꢀexistingꢀpatentꢀrights.  
ꢂꢆ  
LTC3868  
relaTeD parTs  
PART NUMBER  
DESCRIPTION  
COMMENTS  
LTC3857/LTC3857-1 LowꢀI ,ꢀDualꢀOutputꢀ2-PhaseꢀSynchronousꢀStep-Downꢀ  
Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ50kHzꢀtoꢀ900kHz,ꢀ  
4Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ24V,ꢀI ꢀ=ꢀ50µA,  
IN OUT Q  
Q
DC/DCꢀControllersꢀwithꢀ99%ꢀDutyꢀCycle  
LTC3858/LTC3858-1 LowꢀI ,ꢀDualꢀOutputꢀ2-PhaseꢀSynchronousꢀStep-Downꢀ  
Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ50kHzꢀtoꢀ900kHz,ꢀ  
4Vꢀ≤ꢀV ꢀ≤ꢀ24V,ꢀꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ14V,ꢀI ꢀ=ꢀ170µA,  
Q
DC/DCꢀControllersꢀwithꢀ99%ꢀDutyꢀCycle  
IN  
OUT  
Q
LTC3834/LTC3834-1 LowꢀI ,ꢀSynchronousꢀStep-DownꢀDC/DCꢀControllers  
Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ140kHzꢀtoꢀ650kHz,ꢀ  
4Vꢀ≤ꢀV ꢀ≤ꢀ36V,ꢀꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ10V,ꢀI ꢀ=ꢀ30µA,  
Q
IN  
OUT  
Q
LTC3835/LTC3835-1 LowꢀI ,ꢀSynchronousꢀStep-DownꢀDC/DCꢀControllers  
Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ140kHzꢀtoꢀ650kHz,ꢀ  
4Vꢀ≤ꢀV ꢀ≤ꢀ36V,ꢀꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ10V,ꢀI ꢀ=ꢀ80µA,  
Q
IN  
OUT  
Q
LT3845  
LowꢀI ,ꢀHighꢀVoltageꢀSynchronousꢀStep-Downꢀꢀ  
AdjustableꢀFixedꢀOperatingꢀFrequencyꢀ100kHzꢀtoꢀ500kHz,ꢀ  
4Vꢀ≤ꢀV ꢀ≤ꢀ60V,ꢀ1.23Vꢀ≤ꢀV ꢀ≤ꢀ36V,ꢀI ꢀ=ꢀ120µA,ꢀTSSOP-16  
Q
DC/DCꢀController  
IN  
OUT  
Q
LT3800  
LowꢀI ,ꢀHighꢀVoltageꢀSynchronousꢀStep-Downꢀꢀ  
Fixedꢀ200kHzꢀOperatingꢀFrequency,ꢀ4Vꢀ≤ꢀV ꢀ≤ꢀ60V,ꢀꢀ  
IN  
1.23Vꢀ≤ꢀV ꢀ≤ꢀ36V,ꢀI ꢀ=ꢀ100µA,ꢀTSSOP-16  
OUT Q  
Q
DC/DCꢀController  
LTC3824  
LowꢀI ,ꢀHighꢀVoltageꢀDC/DCꢀController,ꢀ100%ꢀDutyꢀCycle SelectableꢀFixedꢀ200kHzꢀtoꢀ600kHzꢀOperatingꢀFrequency,ꢀ  
Q
4Vꢀ≤ꢀV ꢀ≤ꢀ60V,ꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀV ,ꢀI ꢀ=ꢀ40µA,ꢀMSOP-10E  
IN  
OUT  
IN Q  
LTC3850/LTC3850-1ꢀ Dualꢀ2-Phase,ꢀHighꢀEfficiencyꢀSynchronousꢀStep-Downꢀ  
Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ250kHzꢀtoꢀ780kHz,ꢀ  
LTC3850-2  
DC/DCꢀControllers,ꢀR  
Tracking  
ꢀorꢀDCRꢀCurrentꢀSensingꢀandꢀ 4Vꢀ≤ꢀV ꢀ≤ꢀ30V,ꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ5.25V  
SENSE IN OUT  
LTC3855  
Dual,ꢀMultiphase,ꢀSynchronousꢀDC/DCꢀStep-Downꢀ  
ControllerꢀwithꢀDiffampꢀandꢀDCRꢀTemperatureꢀ  
Compensation  
Phase-LockableꢀFixedꢀFrequencyꢀ250kHzꢀtoꢀ770kHz,ꢀꢀ  
4.5Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ12.5V  
IN  
OUT  
LTC3853  
LTC3854  
LTC3775  
TripleꢀOutput,ꢀMultiphaseꢀSynchronousꢀStep-DownꢀDC/DCꢀ Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ250kHzꢀtoꢀ750kHz,ꢀ  
Controller,ꢀR ꢀorꢀDCRꢀCurrentꢀSensingꢀandꢀTracking 4Vꢀ≤ꢀV ꢀ≤ꢀ24V,ꢀV ꢀUpꢀtoꢀ13.5V  
SENSE  
IN  
OUT  
SmallꢀFootprintꢀWideꢀV ꢀRangeꢀSynchronousꢀStep-Downꢀ Fixedꢀ400kHzꢀOperatingꢀFrequency,ꢀ4.5Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀꢀ  
IN  
IN  
DC/DCꢀController  
0.8Vꢀ≤ꢀV ꢀ≤ꢀ5.25V,ꢀ2mmꢀ×ꢀ3mmꢀQFN-12,ꢀMSOP-12  
OUT  
HighꢀFrequencyꢀSynchronousꢀVoltageꢀModeꢀStep-Downꢀ FastꢀTransientꢀResponse,ꢀt  
ꢀ=ꢀ30ns,ꢀ4Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀ  
ON(MIN)  
IN  
DC/DCꢀController  
NoꢀR ™ꢀWideꢀV ꢀRangeꢀSynchronousꢀStep-Downꢀ  
0.6Vꢀ≤ꢀV ꢀ≤ꢀ0.8V ,ꢀMSOP-16E,ꢀ3mmꢀ×ꢀ3mmꢀQFN-16  
OUT IN  
LTC3851A/ꢀ  
LTC3851A-1  
Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ250kHzꢀtoꢀ750kHz,ꢀ  
4Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ5.25V,ꢀMSOP-16E,ꢀ3mmꢀ×ꢀ3mmꢀ  
SENSE  
IN  
DC/DCꢀControllers  
IN  
OUT  
QFN-16,ꢀSSOP-16  
LTC3878/LTC3879  
LTM4600HV  
NoꢀR  
ꢀConstantꢀOn-TimeꢀSynchronousꢀStep-Downꢀ  
VeryꢀFastꢀTransientꢀResponse,ꢀt  
ꢀ=ꢀ43ns,ꢀ4Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀ  
ON(MIN) IN  
SENSE  
DC/DCꢀControllers  
V
ꢀUpꢀ90%ꢀofꢀV ,ꢀMSOP-16E,ꢀ3mmꢀ×ꢀ3mmꢀQFN-16,ꢀSSOP-16  
OUT IN  
10AꢀDC/DCꢀµModule®ꢀCompleteꢀPowerꢀSupply  
HighꢀEfficiency,ꢀCompactꢀSize,ꢀUltraFast™ꢀTransientꢀResponse,ꢀ  
4.5Vꢀ≤ꢀV ꢀ≤ꢀ28V,ꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ5V,ꢀ15mmꢀ×ꢀ15mmꢀ×ꢀ2.8mm  
IN  
OUT  
LTM4601AHV  
12AꢀDC/DCꢀµModuleꢀCompleteꢀPowerꢀSupply  
HighꢀEfficiency,ꢀCompactꢀSize,ꢀUltraFastꢀTransientꢀResponse,ꢀ  
4.5Vꢀ≤ꢀV ꢀ≤ꢀ28V,ꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ5V,ꢀ15mmꢀ×ꢀ15mmꢀ×ꢀ2.8mm  
IN  
OUT  
3868fb  
LT 0110 REV B • PRINTED IN USA  
Linear Technology Corporation  
1630ꢀ McCarthyꢀ Blvd.,ꢀ Milpitas,ꢀ CAꢀ 95035-7417  
ꢂꢇ  
ꢀ  
LINEAR TECHNOLOGY CORPORATION 2009  
(408)ꢀ432-1900ꢀ ꢀFAX:ꢀ(408)ꢀ434-0507ꢀ ꢀwww.linear.com  

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