LTC3857EUHPBF [Linear]
Low IQ, Dual, 2-Phase Synchronous Step-Down Controller; 低IQ ，双通道，两相同步降压型控制器
| 型号： | LTC3857EUHPBF |
| 厂家： | 
Linear |
| 描述： | Low IQ, Dual, 2-Phase Synchronous Step-Down Controller |
| 文件： | 总38页 (文件大小：524K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC3857
Low I , Dual, 2-Phase
Q
Synchronous Step-Down
Controller
FeaTures
DescripTion
Theꢀ LTC®3857ꢀ isꢀ aꢀ highꢀ performanceꢀ dualꢀ step-downꢀ
switchingꢀregulatorꢀcontrollerꢀthatꢀdrivesꢀallꢀN-channelꢀ
synchronousꢀpowerꢀMOSFETꢀstages.ꢀAꢀconstantꢀfrequencyꢀ
currentꢀmodeꢀarchitectureꢀallowsꢀaꢀphase-lockableꢀfre-
quencyꢀofꢀupꢀtoꢀ850kHz.ꢀPowerꢀlossꢀandꢀnoiseꢀdueꢀtoꢀtheꢀ
ESRꢀofꢀtheꢀinputꢀcapacitorꢀESRꢀareꢀminimizedꢀbyꢀoperatingꢀ
theꢀtwoꢀcontrollerꢀoutputꢀstagesꢀoutꢀofꢀphase.
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ꢀ Low Operating I : 50µA (One Channel On)
Q
n
n
n
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ꢀ Wide Output Voltage Range: 0.8V ≤ V
≤ 24V
OUT
ꢀ Wide V Range: 4V to 38V (40V Abs Max)
IN
ꢀ R
or DCR Current Sensing
SENSE
ꢀ Out-of-PhaseꢀControllersꢀReduceꢀRequiredꢀInputꢀ
CapacitanceꢀandꢀPowerꢀSupplyꢀInducedꢀNoise
®
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ꢀ OPTI-LOOP ꢀCompensationꢀMinimizesꢀC
OUT
ꢀ Phase-LockableꢀFrequencyꢀ(75kHz-850kHz)
ꢀ ProgrammableꢀFixedꢀFrequencyꢀ(50kHz-900kHz)
ꢀ SelectableꢀContinuous,ꢀPulse-SkippingꢀorꢀLowꢀRippleꢀ
BurstꢀMode®ꢀOperationꢀatꢀLightꢀLoads
Theꢀ50μAꢀno-loadꢀquiescentꢀcurrentꢀextendsꢀoperatingꢀrunꢀ
timeꢀinꢀbattery-poweredꢀsystems.ꢀTheꢀLTC3857ꢀfeaturesꢀaꢀ
precisionꢀ0.8Vꢀreferenceꢀandꢀpowerꢀgoodꢀoutputꢀindicators.ꢀAꢀ
wideꢀ4Vꢀtoꢀ38Vꢀinputꢀsupplyꢀrangeꢀencompassesꢀaꢀwideꢀrangeꢀ
ofꢀintermediateꢀbusꢀvoltagesꢀandꢀbatteryꢀchemistries.
n
n
n
n
n
n
n
n
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ꢀ SelectableꢀCurrentꢀLimit
ꢀ VeryꢀLowꢀDropoutꢀOperation:ꢀ99%ꢀDutyꢀCycle
ꢀ AdjustableꢀOutputꢀVoltageꢀSoft-StartꢀorꢀTracking
ꢀ PowerꢀGoodꢀOutputꢀVoltageꢀMonitors
ꢀ OutputꢀOvervoltageꢀProtection
IndependentꢀTRACK/SSꢀpinsꢀforꢀeachꢀcontrollerꢀrampꢀtheꢀ
outputꢀvoltagesꢀduringꢀstart-up.ꢀCurrentꢀfoldbackꢀlimitsꢀ
MOSFETꢀheatꢀdissipationꢀduringꢀshort-circuitꢀconditions.ꢀ
TheꢀPLLIN/MODEꢀpinꢀselectsꢀamongꢀBurstꢀModeꢀopera-
tion,ꢀpulse-skippingꢀmode,ꢀorꢀcontinuousꢀinductorꢀcurrentꢀ
modeꢀatꢀlightꢀloads.ꢀ
ꢀ LowꢀShutdownꢀI ꢀ:ꢀ<8µA
Q
ꢀ InternalꢀLDOꢀPowersꢀGateꢀDriveꢀfromꢀV ꢀorꢀEXTV
IN
CC
ꢀ NoꢀCurrentꢀFoldbackꢀDuringꢀStart-up
ꢀ SmallꢀLowꢀProfileꢀ(0.75mm)ꢀ5mmꢀ×ꢀ5mmꢀQFNꢀPackage
Forꢀaꢀleadedꢀ28-leadꢀSSOPꢀpackageꢀwithꢀaꢀfixedꢀcurrentꢀ
limitꢀandꢀoneꢀPGOODꢀoutput,ꢀwithoutꢀphaseꢀmodulationꢀ
orꢀaꢀclockꢀoutput,ꢀseeꢀtheꢀLTC3857-1ꢀdataꢀsheet.
applicaTions
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ꢀ AutomotiveꢀAlways-OnꢀSystems
L,ꢀLT,ꢀLTC,ꢀLTM,ꢀBurstꢀMode,ꢀOPTI-LOOP,ꢀPolyPhase,ꢀµModule,ꢀLinearꢀTechnologyꢀandꢀtheꢀ
LinearꢀlogoꢀareꢀregisteredꢀtrademarksꢀandꢀNoꢀR
ꢀandꢀUltraFastꢀareꢀtrademarksꢀofꢀLinearꢀ
SENSE
n
ꢀ BatteryꢀOperatedꢀDigitalꢀDevices
TechnologyꢀCorporation.ꢀAllꢀotherꢀtrademarksꢀareꢀtheꢀpropertyꢀofꢀtheirꢀrespectiveꢀowners.ꢀ
ProtectedꢀbyꢀU.S.ꢀPatents,ꢀincludingꢀ5481178,ꢀ5929620,ꢀ6177787,ꢀ6144194,ꢀ5408150,ꢀ
6580258,ꢀ5705919,ꢀ6100678.
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ꢀ DistributedꢀDCꢀPowerꢀSystems
Typical applicaTion
Efficiency and Power Loss
High Efficiency Dual 3.3V/8.5V Step-Down Converter
V
IN
vs Output Current
9V TO 38V
22µF
50V
100
90
10000
1000
100
10
4.7µF
V
V
= 12V
IN
OUT
V
INTV
CC
IN
= 3.3V
TG1
TG2
FIGURE 13 CIRCUIT
80
0.1µF
0.1µF
BOOST1
SW1
BOOST2
SW2
3.3µH
7.2µH
70
60
50
BG1
BG2
LTC3857
PGND
40
30
20
10
0
+
+
SENSE1
SENSE2
0.010Ω
193k
0.007Ω
1
–
–
V
SENSE2
OUT2
SENSE1
FB1
V
OUT1
3.3V
5A
8.5V
3.5A
V
V
I
FB2
0.1
62.5k
I
0.000010.0001 0.001 0.01
0.1
1
10
TH1
TH2
150µF
680pF
15k
680pF
15k
150µF
OUTPUT CURRENT (A)
TRACK/SS1 SGND TRACK/SS2
0.1µF
20k
3857 TA01b
20k
0.1µF
3857 TA01
3857fa
ꢀ
RUN1,ꢀRUN2................................................ –0.3Vꢀtoꢀ8V
SENSE2 ꢀVoltages
...................................... –0.3Vꢀtoꢀ28V
EXTV ꢀ...................................................... –0.3Vꢀtoꢀ14V
ꢀ BOOST1,ꢀBOOST2ꢀꢀ................................ –0.3Vꢀtoꢀ46V
LTC3857
absoluTe MaxiMuM raTings
pin conFiguraTion
(Note 1)
TOP VIEW
InputꢀSupplyꢀVoltageꢀ(V )ꢀ......................... –0.3Vꢀtoꢀ40V
IN
TopsideꢀDriverꢀVoltagesꢀ
SwitchꢀVoltageꢀ(SW1,ꢀSW2)ꢀꢀ........................ –5Vꢀtoꢀ40V
(BOOST1-SW1),ꢀ(BOOST2-SW2),ꢀINTV ꢀ... –0.3Vꢀtoꢀ6V
32 31 30 29 28 27 26 25
–
SENSE1
FREQ
1
2
3
4
5
6
7
8
24 BOOST1
23 BG1
CC
PHASMD
CLKOUT
PLLIN/MODE
SGND
V
IN
22
21
ꢀ MaximumꢀCurrentꢀSourcedꢀintoꢀPinꢀ
PGND
33
SGND
ꢀ fromꢀSourceꢀ>8V...............................................100µA
20 EXTV
CC
CC
+
–
+
–
SENSE1 ,ꢀSENSE2 ,ꢀSENSE1
INTV
19
18 BG2
17 BOOST2
RUN1
PLLIN/MODE,ꢀFREQꢀVoltagesꢀꢀ.............. –0.3VꢀtoꢀINTV
RUN2
CC
CC
9
10 11 12 13 14 15 16
I
,ꢀPHASMDꢀVoltagesꢀꢀ....................... –0.3VꢀtoꢀINTV
LIM
CC
I
,ꢀI ,V ,ꢀV ꢀVoltagesꢀ...................... –0.3Vꢀtoꢀ6V
TH1 TH2 FB1 FB2
PGOOD1,ꢀPGOOD2ꢀVoltagesꢀꢀ....................... –0.3Vꢀtoꢀ6V
TRACK/SS1,ꢀTRACK/SS2ꢀVoltagesꢀ.............. –0.3Vꢀtoꢀ6V
OperatingꢀJunctionꢀTemperatureꢀRangeꢀ
(Noteꢀ2................................................... –40°Cꢀtoꢀ125°C
MaximumꢀJunctionꢀTemperatureꢀ(Noteꢀ3)ꢀ............ 125°C
StorageꢀTemperatureꢀRange................... –65°Cꢀtoꢀ150°C
UH PACKAGE
32-LEAD (5mm s 5mm) PLASTIC QFN
ꢀ
T
ꢀ=ꢀ125°C,ꢀθ ꢀ=ꢀ34°C/W
JMAX JA
EXPOSEDꢀPADꢀ(PINꢀ33)ꢀISꢀSGND,ꢀMUSTꢀBEꢀSOLDEREDꢀTOꢀPCB
orDer inForMaTion
LEAD FREE FINISH
LTC3857EUH#PBF
LTC3857IUH#PBF
TAPE AND REEL
PART MARKING*
3857
PACKAGE DESCRIPTION
TEMPERATURE RANGE
–40°Cꢀtoꢀ125°C
LTC3857EUH#TRPBF
LTC3857IUH#TRPBF
32-Leadꢀ(5mmꢀ×ꢀ5mm)ꢀPlasticꢀQFN
32-Leadꢀ(5mmꢀ×ꢀ5mm)ꢀPlasticꢀQFN
3857
–40°Cꢀtoꢀ125°C
ConsultꢀLTCꢀMarketingꢀforꢀpartsꢀspecifiedꢀwithꢀwiderꢀoperatingꢀtemperatureꢀranges.ꢀꢀ*Theꢀtemperatureꢀgradeꢀisꢀidentifiedꢀbyꢀaꢀlabelꢀonꢀtheꢀshippingꢀcontainer.
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
junction 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
V
V
InputꢀSupplyꢀOperatingꢀVoltageꢀRange
RegulatedꢀFeedbackꢀVoltage
4
38
V
IN
(Noteꢀ4)ꢀI
ꢀVoltageꢀ=ꢀ1.2Vꢀ
TH1,2
ꢀ
ꢀ
ꢀ
ꢀ
FB1,2
l
–40°Cꢀtoꢀ125°Cꢀ
–40°Cꢀtoꢀ85°C
0.788ꢀ
0.792
0.800ꢀ
0.800
0.812ꢀ
0.808
Vꢀ
V
I
FeedbackꢀCurrent
(Noteꢀ4)
5
50
nA
FB1,2
V
ReferenceꢀVoltageꢀLineꢀRegulation
(Noteꢀ4)ꢀV ꢀ=ꢀ4.5Vꢀtoꢀ38V
0.002
0.02
%/V
REFLNREG
IN
3857fa
ꢁ
LTC3857
elecTrical characTerisTics The l denotes the specifications which apply over the full operating
junction 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
V
OutputꢀVoltageꢀLoadꢀRegulation
(Note4)ꢀ
ꢀ
ꢀ
ꢀ
LOADREG
l
l
MeasuredꢀinꢀServoꢀLoop,ꢀꢀ
0.01
0.1
%
∆I ꢀVoltageꢀ=ꢀ1.2Vꢀtoꢀ0.7V
TH
(Note4)ꢀ
ꢀ
ꢀ
ꢀ
%
MeasuredꢀinꢀServoꢀLoop,ꢀꢀ
–0.01
–0.1
∆I ꢀVoltageꢀ=ꢀ1.2Vꢀtoꢀ2V
TH
g
ꢀ
TransconductanceꢀAmplifierꢀg
InputꢀDCꢀSupplyꢀCurrent
(Noteꢀ4)ꢀI
ꢀ=ꢀ1.2V,ꢀSink/Sourceꢀ=ꢀ5µA
TH1,2
2
mmho
mA
m1,2
m
I
Q
(Noteꢀ5)
Pulse-SkippingꢀorꢀForcedꢀContinuousꢀ
Modeꢀ(OneꢀChannelꢀOn)
RUN1ꢀ=ꢀ5VꢀandꢀRUN2ꢀ=ꢀ0Vꢀorꢀꢀ
RUN1ꢀ=ꢀ0VꢀandꢀRUN2ꢀ=ꢀ5V,ꢀꢀ
1.3
V
ꢀ=ꢀ0.83Vꢀ(NoꢀLoad)
FB1
Pulse-SkippingꢀorꢀForcedꢀContinuousꢀ
Modeꢀ(BothꢀChannelsꢀOn)
RUN1,2ꢀ=ꢀ5V,ꢀV
ꢀ=ꢀ0.83Vꢀ(NoꢀLoad)
2
mA
µA
FB1,2
SleepꢀModeꢀ(OneꢀChannelꢀOn)
RUN1ꢀ=ꢀ5VꢀandꢀRUN2ꢀ=ꢀ0Vꢀorꢀꢀ
RUN1ꢀ=ꢀ0VꢀandꢀRUN2ꢀ=ꢀ5V,ꢀꢀ
50
75
V
ꢀ=ꢀ0.83Vꢀ(NoꢀLoad)
FB1
SleepꢀModeꢀ(BothꢀChannelsꢀOn)
Shutdown
RUN1,2ꢀ=ꢀ5V,ꢀV
RUN1,2ꢀ=ꢀ0V
ꢀ=ꢀ0.83Vꢀ(NoꢀLoad)
FB1,2
65
8
120
20
µA
µA
l
l
UVLO
UndervoltageꢀLockout
INTV ꢀRampingꢀUpꢀ
ꢀ
4.0ꢀ
3.8
4.2ꢀ
4
Vꢀ
V
CC
INTV ꢀRampingꢀDown
3.6
CC
V
FeedbackꢀOvervoltageꢀProtection
MeasuredꢀatꢀV
EachꢀChannel
,ꢀRelativeꢀtoꢀRegulatedꢀV
FB1,2
7
10
13
1
%
OVL
FB1,2
+
–
+
I
I
SENSE ꢀPinꢀCurrent
µA
SENSE
SENSE
–
SENSE ꢀPinsꢀCurrent
EachꢀChannelꢀ
ꢀ
ꢀ
ꢀ
ꢀ
µAꢀ
µA
–
V
V
ꢀ<ꢀINTV ꢀ–ꢀ0.5Vꢀ
1ꢀ
SENSE
SENSE
CC
CC
–
ꢀ>ꢀINTV ꢀ+ꢀ0.5V
550
700
DF
MaximumꢀDutyꢀFactor
Soft-StartꢀChargeꢀCurrent
RUNꢀPinꢀOnꢀThreshold
InꢀDropout,ꢀFREQꢀ=ꢀ0V
ꢀ=ꢀ0V
98
0.7
99.4
1
%
µA
V
MAX
I
V
1.4
TRACK/SS1,2
TRACK1,2
l
V
V
V
ꢀOn
V
,ꢀV ꢀRising
RUN1 RUN2
1.21
1.26
50
1.31
RUN1,2
RUN1,2
ꢀHyst RUNꢀPinꢀHysteresis
mV
l
l
l
MaximumꢀCurrentꢀSenseꢀThreshold
V
FB1,2
V
FB1,2
V
FB1,2
ꢀ=ꢀ0.7V,ꢀV
ꢀ=ꢀ0.7V,ꢀV
ꢀ=ꢀ0.7V,ꢀV
–, –ꢀ=ꢀ3.3V,ꢀI ꢀ=ꢀ0ꢀ
22ꢀ
43ꢀ
64
30ꢀ
50ꢀ
75
36ꢀ
57ꢀ
85
mVꢀ
mVꢀ
mV
SENSE(MAX)
SENSE1
SENSE1
SENSE1
2
2
2
LIM
LIM
LIM
–, –ꢀ=ꢀ3.3V,ꢀI ꢀ=ꢀFLOATꢀ
–, –ꢀ=ꢀ3.3V,ꢀI ꢀ=ꢀINTV
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
ꢀ
TGꢀTransistionꢀTime:ꢀ
ꢀꢀꢀRiseꢀTimeꢀ
ꢀꢀꢀFallꢀTime
(Noteꢀ6)ꢀ
ꢀ
ꢀ
nsꢀ
ns
TG1,2ꢀt ꢀ
C
C
ꢀ=ꢀ3300pFꢀ
25ꢀ
16
r
LOAD
LOAD
TG1,2ꢀt
ꢀ=ꢀ3300pF
f
ꢀ
BGꢀTransistionꢀTime:ꢀ
ꢀꢀꢀ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
ns
1D
1D
LOAD
BG/TGꢀt
BottomꢀGateꢀOffꢀtoꢀTopꢀGateꢀOnꢀDelayꢀ
TopꢀSwitch-OnꢀDelayꢀTime
C
LOAD
ꢀ=ꢀ3300pFꢀEachꢀDriver
30
ns
3857fa
ꢂ
LTC3857
elecTrical characTerisTics The l denotes the specifications which apply over the full operating
junction 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
t
MinimumꢀOn-Time
(Noteꢀ7)
95
ns
ON(MIN)
INTV Linear Regulator
CC
V
V
V
V
V
V
InternalꢀV ꢀVoltage
6Vꢀ<ꢀV ꢀ<ꢀ38V,ꢀV ꢀ=ꢀ0V
EXTVCC
4.85
4.85
4.5
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
CC
ꢀ=ꢀ0V
CC
EXTVCC
InternalꢀV ꢀVoltage
6Vꢀ<ꢀV ꢀ<ꢀ13V
EXTVCC
5.35
1.1
INTVCCEXT
LDOEXT
CC
INTV ꢀLoadꢀRegulation
I ꢀ=ꢀ0mAꢀtoꢀ50mA,ꢀV
CC
ꢀ=ꢀ8.5V
%
V
CC
EXTVCC
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
ꢀ=ꢀ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
V
V
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
ꢀ=ꢀ5V
PGOOD
µA
PGOOD
V
V
ꢀwithꢀRespectꢀtoꢀSetꢀRegulatedꢀVoltageꢀ
FB
FB
ꢀ
ꢀ
ꢀ
ꢀ
PG
ꢀꢀꢀV ꢀRampingꢀNegativeꢀ
–13
–10ꢀ
2.5
–7
%ꢀ
%
ꢀꢀꢀHysteresis
V
ꢀwithꢀRespectꢀtoꢀSetꢀRegulatedꢀVoltageꢀ
FB
ꢀ
7
ꢀ
ꢀ
ꢀ
FB
ꢀꢀꢀV ꢀRampingꢀPositiveꢀ
10ꢀ
2.5
13
%ꢀ
%
ꢀꢀꢀHysteresis
t
PG
DelayꢀforꢀReportingꢀaꢀFault
25
µs
Note 1:ꢀStressesꢀbeyondꢀthoseꢀlistedꢀunderꢀAbsoluteꢀMaximumꢀRatingsꢀ
mayꢀcauseꢀpermanentꢀdamageꢀtoꢀtheꢀdevice.ꢀExposureꢀtoꢀanyꢀAbsoluteꢀ
MaximumꢀRatingsꢀforꢀextendedꢀperiodsꢀmayꢀaffectꢀdeviceꢀreliabilityꢀandꢀ
lifetime.ꢀ
Note 4:ꢀTheꢀLTC3857ꢀisꢀtestedꢀinꢀaꢀfeedbackꢀloopꢀthatꢀservosꢀV
ꢀtoꢀaꢀ
ITH1,2
specifiedꢀvoltageꢀandꢀmeasuresꢀtheꢀresultantꢀV .ꢀTheꢀspecificationꢀatꢀ
FB1,2
85°Cꢀisꢀnotꢀtestedꢀinꢀproduction.ꢀThisꢀspecificationꢀisꢀassuredꢀbyꢀdesign,ꢀ
characterizationꢀandꢀcorrelationꢀtoꢀproductionꢀtestingꢀatꢀ125°C.
Note 2:ꢀTheꢀLTC3857Eꢀisꢀguaranteedꢀtoꢀmeetꢀperformanceꢀspecificationsꢀ
fromꢀ0°Cꢀtoꢀ85°C.ꢀSpecificationsꢀoverꢀtheꢀ–40°Cꢀtoꢀ125°Cꢀoperatingꢀ
junctionꢀtemperatureꢀrangeꢀareꢀassuredꢀbyꢀdesign,ꢀcharacterizationꢀandꢀ
correlationꢀwithꢀstatisticalꢀprocessꢀcontrols.ꢀTheꢀLTC3857Iꢀisꢀguaranteedꢀ
overꢀtheꢀfullꢀ–40°Cꢀtoꢀ125°Cꢀoperatingꢀjunctionꢀtemperatureꢀrange.
Note 5:ꢀDynamicꢀsupplyꢀcurrentꢀisꢀhigherꢀdueꢀtoꢀtheꢀgateꢀchargeꢀbeingꢀ
deliveredꢀatꢀtheꢀswitchingꢀfrequency.ꢀSeeꢀApplicationsꢀinformation.
Note 6:ꢀRiseꢀandꢀfallꢀtimesꢀareꢀmeasuredꢀusingꢀ10%ꢀandꢀ90%ꢀlevels.ꢀDelayꢀ
timesꢀareꢀmeasuredꢀusingꢀ50%ꢀlevels.
Note 7:ꢀTheꢀminimumꢀon-timeꢀconditionꢀisꢀspecifiedꢀforꢀanꢀinductorꢀ
Note 3:ꢀT ꢀisꢀcalculatedꢀfromꢀtheꢀambientꢀtemperatureꢀT ꢀandꢀpowerꢀ
J
A
peak-to-peakꢀrippleꢀcurrentꢀ≥40%ꢀofꢀI ꢀ(SeeꢀMinimumꢀOn-Timeꢀ
MAX
dissipationꢀP ꢀaccordingꢀtoꢀtheꢀfollowingꢀformula:
D
ConsiderationsꢀinꢀtheꢀApplicationsꢀInformationꢀsection).
ꢀ
T ꢀ=ꢀT ꢀ+ꢀ(P •ꢀ34°C/W)
J A Dꢀ
3857fa
ꢃ
LTC3857
Typical perForMance characTerisTics
Efficiency and Power Loss
Efficiency vs Output Current
Efficiency vs Input Voltage
vs Output Current
100
90
100
90
10000
1000
100
10
98
96
94
92
90
88
86
84
82
80
V
V
= 12V
V
LOAD
= 3.3V
= 5A
IN
OUT
OUT
= 3.3V
I
V
= 5V
IN
FIGURE 13 CIRCUIT
80
80
70
70
V
IN
= 12V
60
50
60
50
40
30
20
10
0
40
30
20
10
0
1
V
= 3.3V
OUT
FIGURE 13 CIRCUIT
0.1
20 25
10 15
INPUT VOLTAGE (V)
0.000010.0001 0.001 0.01
0.1
1
10
1
5
30 35 40
0.000010.0001 0.001 0.01
0.1
1
10
OUTPUT CURRENT (A)
38 57 G01
OUTPUT CURRENT (A)
3857 G02
BURST EFFICIENCY
PULSE-SKIPPING
EFFICIENCY
BURST LOSS
3857 G03
PULSE-SKIPPING
LOSS
CCM EFFICIENCY
CCM LOSS
Load Step
(Forced Continuous Mode)
Load Step
(Pulse-Skipping Mode)
Load Step (Burst Mode Operation)
V
OUT
V
V
OUT
OUT
100mV/DIV
100mV/DIV
100mV/DIV
INDUCTOR
CURRENT
2A/DIV
INDUCTOR
CURRENT
2A/DIV
INDUCTOR
CURRENT
2A/DIV
3857 G06
3857 G04
3857 G05
V
V
= 12V
20µs/DIV
V
V
= 12V
20µs/DIV
V
V
= 12V
20µs/DIV
IN
OUT
IN
OUT
IN
OUT
= 3.3V
= 3.3V
= 3.3V
FIGURE 13 CIRCUIT
FIGURE 13 CIRCUIT
FIGURE 13 CIRCUIT
Inductor Current at Light Load
Soft Start-Up
Tracking Start-Up
FORCED
CONTINUOUS
MODE
V
OUT2
2V/DIV
V
OUT2
2V/DIV
Burst Mode
OPERATION
2A/DIV
V
OUT1
2V/DIV
V
OUT1
2V/DIV
PULSE-
SKIPPING MODE
3857 G07
3857 G09
3857 G08
V
V
LOAD
= 12V
5µs/DIV
20ms/DIV
FIGURE 13 CIRCUIT
20ms/DIV
FIGURE 13 CIRCUIT
IN
= 3.3V
OUT
I
= 200µA
FIGURE 13 CIRCUIT
3857fa
ꢄ
LTC3857
Typical perForMance characTerisTics
Total Input Supply Current
vs Input Voltage
EXTVCC Switchover and INTVCC
Voltages vs Temperature
INTVCC Line Regulation
500
450
400
350
300
250
200
150
100
50
6.0
5.8
5.6
5.4
5.2
5.0
4.8
4.6
4.4
4.2
4.0
5.2
5.1
5.0
4.9
4.8
V
= 3.3V
OUT1
RUN2 = 0V
FIGURE 13 CIRCUIT
INTV
CC
500µA
EXTV RISING
CC
300µA
EXTV FALLING
CC
NO LOAD
0
5
15
20
25
30
35
40
10
55
TEMPERATURE (°C)
130
–45
5
30
80 105
20 25
INPUT VOLTAGE (V)
–20
0
5
10 15
30 35 40
INPUT VOLTAGE (V)
3857 G10
3857 G11
3857 G12
Maximum Current Sense Voltage
vs ITH Voltage
Maximum Current Sense
Threshold vs Duty Cycle
SENSE– Pin Input Bias Current
80
60
40
20
0
80
60
40
20
0
–50
5% DUTY CYCLE
I
= INTV
CC
LIM
–100
–150
–200
–250
–300
–350
–400
–450
–500
–550
–600
PULSE-SKIPPING MODE
I
= FLOAT
LIM
Burst Mode
OPERATION
I
= GND
LIM
I
= GND
LIM
0
–20
–40
I
= FLOAT
LIM
I
= INTV
LIM
CC
FORCED CONTINUOUS MODE
0.8
(V)
1.2
1.4
0
10 20 30 40 50 60 70 80 90 100
0
0.2
0.4 0.6
V
1.0
0
10
15
20
25
5
V
COMMON MODE VOLTAGE (V)
DUTY CYCLE (%)
SENSE
ITH
3857 G13
3857 G14
3857 G15
Foldback Current Limit
Quiescent Current vs Temperature
INTVCC vs Load Current
90
80
70
60
50
40
30
20
10
80
5.20
5.15
5.10
V
= 12V
IN
I
= INTV
LIM
CC
75
70
65
60
55
50
45
I
= FLOAT
= GND
LIM
EXTV = 0V
CC
5.05
5.00
4.95
I
LIM
EXTV = 8.5V
CC
40
0
–20
5
55
80 105 130
20 40 60
120 140
–45
30
0
80 100
160
180
200
0
0.1 0.2 0.3 0.4 0.5
0.9
0.6 0.7 0.8
FEEDBACK VOLTAGE (V)
TEMPERATURE (°C)
LOAD CURRENT (mA)
3857 G17
3857 G18
3857 G16
3857fa
ꢅ
LTC3857
Typical perForMance characTerisTics
Regulated Feedback Voltage
vs Temperature
TRACK/SS Pull-Up Current
vs Temperature
Shutdown (RUN) Threshold
vs Temperature
1.40
1.35
1.30
1.25
800
1.10
1.05
1.00
0.95
806
804
RUN RISING
802
800
798
796
794
RUN FALLING
1.20
1.15
1.10
0.90
792
–45 –20
5
30
55
80 105 130
–45 –20
5
30
55
80
105 130
–45 –20
5
30
55
80 105 130
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
3857 G19
3857 G20
3857 G21
SENSE– Pin Input Current
vs Temperature
Oscillator Frequency
vs Temperature
Shutdown Current
vs Input Voltage
50
0
–50
30
25
20
15
600
550
500
450
V
< INTV – 0.5V
CC
FREQ = INTV
OUT
CC
–100
–150
–200
–250
–300
–350
–400
–450
–500
–550
–600
10
5
400
350
300
FREQ = GND
V
5
> INTV – 0.5V
CC
OUT
0
25
INPUT VOLTAGE (V)
35
40
–45 –20
30
55
80 105 130
5
10
15
20
30
55
TEMPERATURE (°C)
105 130
–45 –20
5
30
80
TEMPERATURE (°C)
3857 G22
3857 G23
3857 G24
Undervoltage Lockout Threshold
vs Temperature
Oscillator Frequency
vs Input Voltage
Shutdown Current 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
20
FREQ = GND
18
16
14
12
10
8
348
346
344
6
4
25
INPUT VOLTAGE (V)
35
40
5
10
15
20
30
–45
5
30
55
80 105 130
–20
5
55
80 105 130
–20
–45
30
TEMPERATURE (°C)
TEMPERATURE (°C)
3857 G26
3857 G25
3857 G27
3857fa
ꢆ
LTC3857
pin FuncTions
–
–
SGND (Pin 6, Exposed Pad Pin 33):ꢀSmall-signalꢀgroundꢀ
commonꢀtoꢀbothꢀcontrollers,ꢀmustꢀbeꢀroutedꢀseparatelyꢀ
fromꢀhighꢀcurrentꢀgroundsꢀtoꢀtheꢀcommonꢀ(–)ꢀterminalsꢀ
SENSE1 , SENSE2 (Pin 1, Pin 9):ꢀTheꢀ(–)ꢀInputꢀtoꢀtheꢀ
Differentialꢀ Currentꢀ Comparators.ꢀ Whenꢀ greaterꢀ thanꢀ
–
INTV ꢀ–ꢀ0.5V,ꢀtheꢀSENSE ꢀpinꢀsuppliesꢀcurrentꢀtoꢀtheꢀ
CC
ofꢀtheꢀC ꢀcapacitors.ꢀTheꢀexposedꢀpadꢀmustꢀbeꢀsolderedꢀ
currentꢀcomparator.
IN
toꢀtheꢀPCBꢀforꢀratedꢀthermalꢀperformance.
FREQ (Pin 2):ꢀTheꢀfrequencyꢀcontrolꢀpinꢀforꢀtheꢀinternalꢀ
RUN1, RUN2 (Pin 7, Pin 8):ꢀDigitalꢀRunꢀControlꢀInputsꢀforꢀ
EachꢀController.ꢀForcingꢀeitherꢀofꢀtheseꢀpinsꢀbelowꢀ1.26Vꢀ
shutsꢀdownꢀthatꢀcontroller.ꢀForcingꢀbothꢀofꢀtheseꢀpinsꢀbelowꢀ
0.7VꢀshutsꢀdownꢀtheꢀentireꢀLTC3857,ꢀreducingꢀquiescentꢀ
currentꢀtoꢀapproximatelyꢀ8µA.ꢀDoꢀnotꢀfloatꢀtheseꢀpins.
VCO.ꢀConnectingꢀtheꢀpinꢀtoꢀGNDꢀforcesꢀtheꢀVCOꢀtoꢀaꢀfixedꢀ
lowꢀfrequencyꢀofꢀ350kHz.ꢀConnectingꢀtheꢀpinꢀtoꢀINTV ꢀ
CC
forcesꢀ theꢀ VCOꢀ toꢀ aꢀ fixedꢀ highꢀ frequencyꢀ ofꢀ 535kHz.ꢀ
Otherꢀ frequenciesꢀ betweenꢀ 50kHzꢀ andꢀ 900kHzꢀ canꢀ beꢀ
programmedꢀusingꢀaꢀresistorꢀbetweenꢀFREQꢀandꢀGND.ꢀ
Anꢀinternalꢀ20µAꢀpull-upꢀcurrentꢀdevelopsꢀtheꢀvoltageꢀtoꢀ
beꢀusedꢀbyꢀtheꢀVCOꢀtoꢀcontrolꢀtheꢀfrequency
INTV (Pin19):ꢀOutputꢀofꢀtheꢀInternalꢀLinearꢀLowꢀDropoutꢀ
CC
Regulator.ꢀTheꢀdriverꢀandꢀcontrolꢀcircuitsꢀareꢀpoweredꢀ
fromꢀthisꢀvoltageꢀsource.ꢀMustꢀbeꢀdecoupledꢀtoꢀpowerꢀ
groundꢀwithꢀaꢀminimumꢀofꢀ4.7µFꢀceramicꢀorꢀotherꢀlowꢀ
PHASMD (Pin 3):ꢀControlꢀInputꢀtoꢀPhaseꢀSelectorꢀwhichꢀ
determinesꢀ theꢀ phaseꢀ relationshipsꢀ betweenꢀ control-
lerꢀ1,ꢀcontrollerꢀ2ꢀandꢀtheꢀCLKOUTꢀsignal.ꢀPullingꢀthisꢀ
pinꢀtoꢀgroundꢀforcesꢀTG2ꢀandꢀCLKOUTꢀtoꢀbeꢀoutꢀofꢀphaseꢀ
180°ꢀandꢀ60°ꢀwithꢀrespectꢀtoꢀTG1.ꢀConnectingꢀthisꢀpinꢀtoꢀ
ESRꢀcapacitor.ꢀDoꢀnotꢀuseꢀtheꢀINTV ꢀpinꢀforꢀanyꢀotherꢀ
CC
purpose.
EXTV (Pin 20):ꢀExternalꢀPowerꢀInputꢀtoꢀanꢀInternalꢀLDOꢀ
CC
INTV ꢀforcesꢀTG2ꢀandꢀCLKOUTꢀtoꢀbeꢀoutꢀofꢀphaseꢀ240°ꢀ
CC
ConnectedꢀtoꢀINTV .ꢀThisꢀLDOꢀsuppliesꢀINTV ꢀpower,ꢀ
CC
CC
andꢀ120°ꢀwithꢀrespectꢀtoꢀTG1.ꢀFloatingꢀthisꢀpinꢀforcesꢀTG2ꢀ
andꢀCLKOUTꢀtoꢀbeꢀoutꢀofꢀphaseꢀ180°ꢀandꢀ90°ꢀwithꢀrespectꢀ
toꢀTG1.ꢀReferꢀtoꢀTableꢀ1.ꢀ
bypassingꢀtheꢀinternalꢀLDOꢀpoweredꢀfromꢀV ꢀwheneverꢀ
IN
EXTV ꢀisꢀhigherꢀthanꢀ4.7V.ꢀSeeꢀEXTV ꢀConnectionꢀinꢀ
CC
CC
theꢀApplicationsꢀInformationꢀsection.ꢀDoꢀnotꢀexceedꢀ14Vꢀ
onꢀthisꢀpin.
CLKOUT (Pin 4):ꢀOutputꢀclockꢀsignalꢀavailableꢀtoꢀdaisy-
chainꢀotherꢀcontrollerꢀICsꢀforꢀadditionalꢀMOSFETꢀdriverꢀ
PGND (Pin 21):ꢀDriverꢀPowerꢀGround.ꢀConnectsꢀtoꢀtheꢀ
stages/phases.ꢀTheꢀoutputꢀlevelsꢀswingꢀfromꢀINTV ꢀtoꢀ
CC
sourcesꢀofꢀbottomꢀ(synchronous)ꢀN-channelꢀMOSFETsꢀ
ground.ꢀ
andꢀtheꢀ(–)ꢀterminal(s)ꢀofꢀC .
IN
PLLIN/MODE (Pin 5):ꢀExternalꢀSynchronizationꢀInputꢀtoꢀ
PhaseꢀDetectorꢀandꢀForcedꢀContinuousꢀModeꢀInput.ꢀWhenꢀ
anꢀexternalꢀclockꢀisꢀappliedꢀtoꢀthisꢀpin,ꢀtheꢀphase-lockedꢀ
loopꢀwillꢀforceꢀtheꢀrisingꢀTG1ꢀsignalꢀtoꢀbeꢀsynchronizedꢀ
withꢀ theꢀ risingꢀ edgeꢀ ofꢀ theꢀ externalꢀ clock.ꢀ Whenꢀ notꢀ
synchronizingꢀtoꢀanꢀexternalꢀclock,ꢀthisꢀinput,ꢀwhichꢀactsꢀ
onꢀbothꢀcontrollers,ꢀdeterminesꢀhowꢀtheꢀLTC3857ꢀoperatesꢀ
atꢀlightꢀloads.ꢀPullingꢀthisꢀpinꢀtoꢀgroundꢀselectsꢀBurstꢀ
Modeꢀoperation.ꢀAnꢀinternalꢀ100kꢀresistorꢀtoꢀgroundꢀalsoꢀ
invokesꢀBurstꢀModeꢀoperationꢀwhenꢀtheꢀpinꢀisꢀfloated.ꢀ
V (Pin 22):ꢀMainꢀSupplyꢀPin.ꢀAꢀbypassꢀcapacitorꢀshouldꢀ
IN
beꢀtiedꢀbetweenꢀthisꢀpinꢀandꢀtheꢀsignalꢀgroundꢀpin.
BG1, BG2 (Pin 23, Pin 18):ꢀHighꢀCurrentꢀGateꢀDrivesꢀ
forꢀBottomꢀ(Synchronous)ꢀN-ChannelꢀMOSFETs.ꢀVoltageꢀ
swingꢀatꢀtheseꢀpinsꢀisꢀfromꢀgroundꢀtoꢀINTV .
CC
BOOST1,BOOST2(Pin24,Pin17):ꢀBootstrappedꢀSuppliesꢀ
toꢀtheꢀTopsideꢀFloatingꢀDrivers.ꢀCapacitorsꢀareꢀconnectedꢀ
betweenꢀtheꢀBOOSTꢀandꢀSWꢀpinsꢀandꢀSchottkyꢀdiodesꢀareꢀ
tiedꢀbetweenꢀtheꢀBOOSTꢀandꢀINTV ꢀpins.ꢀVoltageꢀswingꢀ
CC
TyingꢀthisꢀpinꢀtoꢀINTV ꢀforcesꢀcontinuousꢀinductorꢀcurrentꢀ
CC
atꢀtheꢀBOOSTꢀpinsꢀisꢀfromꢀINTV ꢀtoꢀ(V ꢀ+ꢀINTV ).
CC
IN
CC
operation.ꢀTyingꢀthisꢀpinꢀtoꢀaꢀvoltageꢀgreaterꢀthanꢀ1.2Vꢀandꢀ
SW1, SW2 (Pin 25, Pin 16):ꢀSwitchꢀNodeꢀConnectionsꢀ
toꢀInductors.ꢀ
lessꢀthanꢀINTV ꢀ–ꢀ1.3Vꢀselectsꢀpulse-skippingꢀoperation.ꢀ
CC
Thisꢀcanꢀbeꢀdoneꢀbyꢀaddingꢀaꢀ100kꢀresistorꢀbetweenꢀtheꢀ
PLLIN/MODEꢀpinꢀandꢀINTV .
CC
3857fa
ꢇ
LTC3857
pin FuncTions
I
, I
(Pin 30, Pin 12):ꢀErrorꢀAmplifierꢀOutputsꢀandꢀ
TG1, TG2 (Pin 26, Pin 15):ꢀHighꢀCurrentꢀGateꢀDrivesꢀforꢀ
TH1 TH2
SwitchingꢀRegulatorꢀCompensationꢀPoints.ꢀEachꢀassoci-
atedꢀchannel’sꢀcurrentꢀcomparatorꢀtripꢀpointꢀincreasesꢀ
withꢀthisꢀcontrolꢀvoltage.
TopꢀN-ChannelꢀMOSFETs.ꢀTheseꢀareꢀtheꢀoutputsꢀofꢀfloat-
ingꢀdriversꢀwithꢀaꢀvoltageꢀswingꢀequalꢀtoꢀINTV ꢀ–ꢀ0.5Vꢀ
CC
superimposedꢀonꢀtheꢀswitchꢀnodeꢀvoltageꢀSW.
V
, V (Pin31, Pin11):ꢀReceivesꢀtheꢀremotelyꢀsensedꢀ
PGOOD1, PGOOD2 (Pin 27, Pin 14):ꢀOpen-DrainꢀLogicꢀ
FB1 FB2
feedbackꢀ voltageꢀ forꢀ eachꢀ controllerꢀ fromꢀ anꢀ externalꢀ
Output.ꢀPGOOD1,2ꢀisꢀpulledꢀtoꢀgroundꢀwhenꢀtheꢀvoltageꢀ
resistiveꢀdividerꢀacrossꢀtheꢀoutput.
onꢀtheꢀV ꢀpinꢀisꢀnotꢀwithinꢀ 10%ꢀofꢀitsꢀsetꢀpoint.
FB1,2
+
+
SENSE1 , SENSE2 (Pin 32, Pin 10):ꢀTheꢀ(+)ꢀinputꢀtoꢀtheꢀ
differentialꢀcurrentꢀcomparatorsꢀareꢀnormallyꢀconnectedꢀ
toꢀDCRꢀsensingꢀnetworksꢀorꢀcurrentꢀsensingꢀresistors.ꢀ
I
(Pin 28):ꢀCurrentꢀComparatorꢀSenseꢀVoltageꢀRangeꢀ
LIM
Inputs.ꢀTyingꢀthisꢀpinꢀtoꢀSGND,ꢀFLOATꢀorꢀINTV ꢀsetsꢀtheꢀ
CC
maximumꢀcurrentꢀsenseꢀthresholdꢀtoꢀoneꢀofꢀthreeꢀdifferentꢀ
levelsꢀforꢀbothꢀcomparators.
TheꢀI ꢀpinꢀvoltageꢀandꢀcontrolledꢀoffsetsꢀbetweenꢀtheꢀ
SENSE ꢀandꢀSENSE ꢀpinsꢀinꢀconjunctionꢀwithꢀR
theꢀcurrentꢀtripꢀthreshold.
TH
–
+
ꢀsetꢀ
SENSE
TRACK/SS1, TRACK/SS2 (Pin 29, Pin 13):ꢀ Externalꢀ
TrackingꢀandꢀSoft-StartꢀInput.ꢀTheꢀLTC3857ꢀregulatesꢀtheꢀ
V
ꢀvoltageꢀtoꢀtheꢀsmallerꢀofꢀ0.8Vꢀorꢀtheꢀvoltageꢀonꢀtheꢀ
FB1,2
TRACK/SS1,2ꢀpin.ꢀAnꢀinternalꢀ1µAꢀpull-upꢀcurrentꢀsourceꢀ
isꢀconnectedꢀtoꢀthisꢀpin.ꢀAꢀcapacitorꢀtoꢀgroundꢀatꢀthisꢀ
pinꢀsetsꢀtheꢀrampꢀtimeꢀtoꢀfinalꢀregulatedꢀoutputꢀvoltage.ꢀ
Alternatively,ꢀaꢀresistorꢀdividerꢀonꢀanotherꢀvoltageꢀsupplyꢀ
connectedꢀtoꢀthisꢀpinꢀallowsꢀtheꢀLTC3857ꢀoutputꢀtoꢀtrackꢀ
theꢀotherꢀsupplyꢀduringꢀstart-up.
3857fa
ꢈ
LTC3857
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
30, 12
C
C
0.88V
11V
5.1V
LDO
EN
5.1V
LDO
EN
SHDN
RST
FB
C
C2
R
C
TRACK/SS
29, 13
FOLDBACK
1µA
2(V
)
+
–
0.5µA
C
SHDN
SS
4.7V
RUN
7, 8
33 SGND
19 INTV
CC
3857 FD
3857fa
ꢀ0
LTC3857
operaTion (Refer to the Functional Diagram)
Main Control Loop
toꢀturnꢀonꢀtheꢀtopꢀMOSFETꢀcontinuously.ꢀTheꢀdropoutꢀ
detectorꢀdetectsꢀthisꢀandꢀforcesꢀtheꢀtopꢀMOSFETꢀoffꢀforꢀ
aboutꢀoneꢀtwelfthꢀofꢀtheꢀclockꢀperiodꢀeveryꢀtenthꢀcycleꢀtoꢀ
TheꢀLTC3857ꢀusesꢀaꢀconstantꢀfrequency,ꢀcurrentꢀmodeꢀ
step-downꢀarchitectureꢀwithꢀtheꢀtwoꢀcontrollerꢀchannelsꢀ
operatingꢀ 180ꢀ degreesꢀ outꢀ ofꢀ phase.ꢀ Duringꢀ normalꢀ
operation,ꢀeachꢀexternalꢀtopꢀMOSFETꢀisꢀturnedꢀonꢀwhenꢀ
theꢀclockꢀforꢀthatꢀchannelꢀsetsꢀtheꢀRSꢀlatch,ꢀandꢀisꢀturnedꢀ
offꢀwhenꢀtheꢀmainꢀcurrentꢀcomparator,ꢀICMP,ꢀresetsꢀtheꢀ
RSꢀlatch.ꢀTheꢀpeakꢀinductorꢀcurrentꢀatꢀwhichꢀICMPꢀtripsꢀ
allowꢀC ꢀtoꢀrecharge.
B
Shutdown and Start-Up (RUN1, RUN2 and
TRACK/ SS1, TRACK/SS2 Pins)
TheꢀtwoꢀchannelsꢀofꢀtheꢀLTC3857ꢀcanꢀbeꢀindependentlyꢀ
shutꢀdownꢀusingꢀtheꢀRUN1ꢀandꢀRUN2ꢀpins.ꢀPullingꢀeitherꢀofꢀ
theseꢀpinsꢀbelowꢀ1.26Vꢀshutsꢀdownꢀtheꢀmainꢀcontrolꢀloopꢀ
forꢀthatꢀcontroller.ꢀPullingꢀbothꢀpinsꢀbelowꢀ0.7Vꢀdisablesꢀ
bothꢀcontrollersꢀandꢀmostꢀinternalꢀcircuits,ꢀincludingꢀtheꢀ
andꢀresetsꢀtheꢀlatchꢀisꢀcontrolledꢀbyꢀtheꢀvoltageꢀonꢀtheꢀI ꢀ
TH
pin,ꢀwhichꢀisꢀtheꢀoutputꢀofꢀtheꢀerrorꢀamplifier,ꢀEA.ꢀTheꢀerrorꢀ
amplifierꢀcomparesꢀtheꢀoutputꢀvoltageꢀfeedbackꢀsignalꢀatꢀ
theꢀV ꢀpin,ꢀ(whichꢀisꢀgeneratedꢀwithꢀanꢀexternalꢀresistorꢀ
FB
INTV ꢀLDOs.ꢀInꢀthisꢀstate,ꢀtheꢀLTC3857ꢀdrawsꢀonlyꢀ8µAꢀ
dividerꢀ connectedꢀ acrossꢀ theꢀ outputꢀ voltage,ꢀ V ,ꢀ toꢀ
CC
OUTꢀ
ofꢀquiescentꢀcurrent.
ground)ꢀtoꢀtheꢀinternalꢀ0.800Vꢀreferenceꢀvoltage.ꢀWhenꢀtheꢀ
loadꢀcurrentꢀincreases,ꢀitꢀcausesꢀaꢀslightꢀdecreaseꢀinꢀV ꢀ
FB
TheꢀRUNꢀpinꢀmayꢀbeꢀexternallyꢀpulledꢀupꢀorꢀdrivenꢀdirectlyꢀ
byꢀlogic.ꢀWhenꢀdrivingꢀtheꢀRUNꢀpinꢀwithꢀaꢀlowꢀimpedanceꢀ
source,ꢀdoꢀnotꢀexceedꢀtheꢀabsoluteꢀmaximumꢀratingꢀofꢀ
8V.ꢀTheꢀRUNꢀpinꢀhasꢀanꢀinternalꢀ11Vꢀvoltageꢀclampꢀthatꢀ
allowsꢀtheꢀRUNꢀpinꢀtoꢀbeꢀconnectedꢀthroughꢀaꢀresistorꢀtoꢀaꢀ
relativeꢀtoꢀtheꢀreference,ꢀwhichꢀcausesꢀtheꢀEAꢀtoꢀincreaseꢀ
theꢀI ꢀvoltageꢀuntilꢀtheꢀaverageꢀinductorꢀcurrentꢀmatchesꢀ
TH
theꢀnewꢀloadꢀcurrent.
AfterꢀtheꢀtopꢀMOSFETꢀisꢀturnedꢀoffꢀeachꢀcycle,ꢀtheꢀbottomꢀ
MOSFETꢀisꢀturnedꢀonꢀuntilꢀeitherꢀtheꢀinductorꢀcurrentꢀstartsꢀ
toꢀreverse,ꢀasꢀindicatedꢀbyꢀtheꢀcurrentꢀcomparatorꢀIR,ꢀorꢀ
theꢀbeginningꢀofꢀtheꢀnextꢀclockꢀcycle.
higherꢀvoltageꢀ(forꢀexample,ꢀV ),ꢀsoꢀlongꢀasꢀtheꢀmaximumꢀ
IN
currentꢀintoꢀtheꢀRUNꢀpinꢀdoesꢀnotꢀexceedꢀ100µA.
Theꢀstart-upꢀofꢀeachꢀcontroller’sꢀoutputꢀvoltageꢀV ꢀisꢀ
OUT
controlledꢀbyꢀtheꢀvoltageꢀonꢀtheꢀTRACK/SSꢀpinꢀforꢀthatꢀ
channel.ꢀWhenꢀtheꢀvoltageꢀonꢀtheꢀTRACK/SSꢀpinꢀisꢀlessꢀ
thanꢀtheꢀ0.8Vꢀinternalꢀreference,ꢀtheꢀLTC3857ꢀregulatesꢀ
INTV /EXTV Power
CC
CC
PowerꢀforꢀtheꢀtopꢀandꢀbottomꢀMOSFETꢀdriversꢀandꢀmostꢀ
otherꢀinternalꢀcircuitryꢀisꢀderivedꢀfromꢀtheꢀINTV ꢀpin.ꢀWhenꢀ
theꢀV ꢀvoltageꢀtoꢀtheꢀTRACK/SSꢀpinꢀvoltageꢀinsteadꢀofꢀtheꢀ
CC
FB
theꢀEXTV ꢀpinꢀisꢀleftꢀopenꢀorꢀtiedꢀtoꢀaꢀvoltageꢀlessꢀthanꢀ
0.8Vꢀreference.ꢀThisꢀallowsꢀtheꢀTRACK/SSꢀpinꢀtoꢀbeꢀusedꢀ
toꢀprogramꢀaꢀsoft-startꢀbyꢀconnectingꢀanꢀexternalꢀcapacitorꢀ
fromꢀtheꢀTRACK/SSꢀpinꢀtoꢀSGND.ꢀAnꢀinternalꢀ1µAꢀpull-upꢀ
currentꢀchargesꢀthisꢀcapacitorꢀcreatingꢀaꢀvoltageꢀrampꢀonꢀ
theꢀTRACK/SSꢀpin.ꢀAsꢀtheꢀTRACK/SSꢀvoltageꢀrisesꢀlinearlyꢀ
fromꢀ0Vꢀtoꢀ0.8Vꢀ(andꢀbeyondꢀupꢀtoꢀtheꢀabsoluteꢀmaximumꢀ
CC
4.7V,ꢀtheꢀV ꢀLDOꢀ(lowꢀdropoutꢀlinearꢀregulator)ꢀsuppliesꢀ
IN
5.1VꢀfromꢀV ꢀtoꢀINTV .ꢀIfꢀEXTV ꢀisꢀtakenꢀaboveꢀ4.7V,ꢀ
IN
CC
CC
theꢀV ꢀLDOꢀisꢀturnedꢀoffꢀandꢀanꢀEXTV ꢀLDOꢀisꢀturnedꢀon.ꢀ
IN
CC
Onceꢀenabled,ꢀtheꢀEXTV ꢀLDOꢀsuppliesꢀ5.1VꢀfromꢀEXTV ꢀ
CC
CC
toꢀINTV .ꢀUsingꢀtheꢀEXTV ꢀpinꢀallowsꢀtheꢀINTV ꢀpowerꢀ
CC
CC
CC
toꢀbeꢀderivedꢀfromꢀaꢀhighꢀefficiencyꢀexternalꢀsourceꢀsuchꢀ
ratingꢀofꢀ6V),ꢀtheꢀoutputꢀvoltageꢀV ꢀrisesꢀsmoothlyꢀfromꢀ
OUT
asꢀoneꢀofꢀtheꢀLTC3857ꢀswitchingꢀregulatorꢀoutputs.
zeroꢀtoꢀitsꢀfinalꢀvalue.
Eachꢀ topꢀ MOSFETꢀ driverꢀ isꢀ biasedꢀ fromꢀ theꢀ floatingꢀ
AlternativelyꢀtheꢀTRACK/SSꢀpinꢀcanꢀbeꢀusedꢀtoꢀcauseꢀtheꢀ
bootstrapꢀcapacitor,ꢀC ,ꢀwhichꢀnormallyꢀrechargesꢀduringꢀ
start-upꢀofꢀV ꢀtoꢀtrackꢀthatꢀofꢀanotherꢀsupply.ꢀTypically,ꢀ
B
OUT
eachꢀcycleꢀthroughꢀanꢀexternalꢀdiodeꢀwhenꢀtheꢀtopꢀMOSFETꢀ
thisꢀrequiresꢀconnectingꢀtoꢀtheꢀTRACK/SSꢀpinꢀanꢀexternalꢀ
resistorꢀ dividerꢀ fromꢀ theꢀ otherꢀ supplyꢀ toꢀ groundꢀ (seeꢀ
ApplicationsꢀInformationꢀsection).
turnsꢀoff.ꢀIfꢀtheꢀinputꢀvoltage,ꢀV ,ꢀdecreasesꢀtoꢀaꢀvoltageꢀ
IN
closeꢀtoꢀV ,ꢀtheꢀloopꢀmayꢀenterꢀdropoutꢀandꢀattemptꢀ
OUTꢀ
3857fa
ꢀꢀ
LTC3857
operaTion (Refer to the Functional Diagram)
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping or Forced Continuous Mode)
(PLLIN/MODE Pin)
Inꢀforcedꢀcontinuousꢀoperationꢀorꢀclockedꢀbyꢀanꢀexternalꢀ
clockꢀsourceꢀtoꢀuseꢀtheꢀphase-lockedꢀloopꢀ(seeꢀFrequencyꢀ
SelectionꢀandꢀPhase-LockedꢀLoopꢀsection),ꢀtheꢀinduc-
torꢀcurrentꢀisꢀallowedꢀtoꢀreverseꢀatꢀlightꢀloadsꢀorꢀunderꢀ
largeꢀ transientꢀ conditions.ꢀ Theꢀ peakꢀ inductorꢀ currentꢀ
isꢀdeterminedꢀbyꢀtheꢀvoltageꢀonꢀtheꢀITHꢀpin,ꢀjustꢀasꢀinꢀ
normalꢀoperation.ꢀInꢀthisꢀmode,ꢀtheꢀefficiencyꢀatꢀlightꢀ
loadsꢀisꢀlowerꢀthanꢀinꢀBurstꢀModeꢀoperation.ꢀHowever,ꢀ
continuousꢀoperationꢀhasꢀtheꢀadvantageꢀofꢀlowerꢀoutputꢀ
voltageꢀrippleꢀandꢀlessꢀinterferenceꢀtoꢀaudioꢀcircuitry.ꢀInꢀ
forcedꢀcontinuousꢀmode,ꢀtheꢀoutputꢀrippleꢀisꢀindependentꢀ
ofꢀloadꢀcurrent.
TheꢀLTC3857ꢀcanꢀbeꢀenabledꢀtoꢀenterꢀhighꢀefficiencyꢀBurstꢀ
Modeꢀoperation,ꢀconstantꢀfrequencyꢀpulse-skippingꢀmode,ꢀ
orꢀforcedꢀcontinuousꢀconductionꢀmodeꢀatꢀlowꢀloadꢀcur-
rents.ꢀToꢀselectꢀBurstꢀModeꢀoperation,ꢀtieꢀtheꢀPLLIN/ꢀMODEꢀ
pinꢀtoꢀground.ꢀToꢀselectꢀforcedꢀcontinuousꢀoperation,ꢀtieꢀ
theꢀPLLIN/MODEꢀpinꢀtoꢀINTV .ꢀToꢀselectꢀpulse-skippingꢀ
CC
mode,ꢀtieꢀtheꢀPLLIN/MODEꢀpinꢀtoꢀaꢀDCꢀvoltageꢀgreaterꢀ
thanꢀ1.2VꢀandꢀlessꢀthanꢀINTV ꢀ–ꢀ1.3V.
CC
WhenꢀaꢀcontrollerꢀisꢀenabledꢀforꢀBurstꢀModeꢀoperation,ꢀ
theꢀminimumꢀpeakꢀcurrentꢀinꢀtheꢀinductorꢀisꢀsetꢀtoꢀap-
proximatelyꢀ15%ꢀofꢀtheꢀmaximumꢀsenseꢀvoltageꢀevenꢀ
WhenꢀtheꢀPLLIN/MODEꢀpinꢀisꢀconnectedꢀforꢀpulse-skip-
pingꢀmode,ꢀtheꢀLTC3857ꢀoperatesꢀinꢀPWMꢀpulse-skippingꢀ
modeꢀatꢀlightꢀloads.ꢀInꢀthisꢀmode,ꢀconstantꢀfrequencyꢀ
operationꢀisꢀmaintainedꢀdownꢀtoꢀapproximatelyꢀ1%ꢀofꢀ
designedꢀmaximumꢀoutputꢀcurrent.ꢀAtꢀveryꢀlightꢀloads,ꢀtheꢀ
currentꢀcomparator,ꢀICMP,ꢀmayꢀremainꢀtrippedꢀforꢀseveralꢀ
cyclesꢀandꢀforceꢀtheꢀexternalꢀtopꢀMOSFETꢀtoꢀstayꢀoffꢀforꢀ
theꢀsameꢀnumberꢀofꢀcyclesꢀ(i.e.,ꢀskippingꢀpulses).ꢀTheꢀ
inductorꢀcurrentꢀisꢀnotꢀallowedꢀtoꢀreverseꢀ(discontinuousꢀ
operation).ꢀThisꢀmode,ꢀlikeꢀforcedꢀcontinuousꢀoperation,ꢀ
exhibitsꢀlowꢀoutputꢀrippleꢀasꢀwellꢀasꢀlowꢀaudioꢀnoiseꢀandꢀ
reducedꢀ RFꢀ interferenceꢀ asꢀ comparedꢀ toꢀ Burstꢀ Modeꢀ
operation.ꢀItꢀprovidesꢀhigherꢀlowꢀcurrentꢀefficiencyꢀthanꢀ
forcedꢀcontinuousꢀmode,ꢀbutꢀnotꢀnearlyꢀasꢀhighꢀasꢀBurstꢀ
Modeꢀoperation.
thoughꢀtheꢀvoltageꢀonꢀtheꢀI ꢀpinꢀindicatesꢀaꢀlowerꢀvalue.ꢀ
TH
Ifꢀtheꢀaverageꢀinductorꢀcurrentꢀisꢀhigherꢀthanꢀtheꢀloadꢀ
current,ꢀtheꢀerrorꢀamplifier,ꢀEA,ꢀwillꢀdecreaseꢀtheꢀvoltageꢀ
onꢀtheꢀI ꢀpin.ꢀWhenꢀtheꢀI ꢀvoltageꢀdropsꢀbelowꢀ0.425V,ꢀ
TH
TH
theꢀinternalꢀsleepꢀsignalꢀgoesꢀhighꢀ(enablingꢀsleepꢀmode)ꢀ
andꢀbothꢀexternalꢀMOSFETsꢀareꢀturnedꢀoff.ꢀTheꢀI ꢀpinꢀisꢀ
TH
thenꢀdisconnectedꢀfromꢀtheꢀoutputꢀofꢀtheꢀEAꢀandꢀparkedꢀ
atꢀ0.450V.
Inꢀsleepꢀmode,ꢀmuchꢀofꢀtheꢀinternalꢀcircuitryꢀisꢀturnedꢀoff,ꢀ
reducingꢀtheꢀquiescentꢀcurrentꢀthatꢀtheꢀLTC3857ꢀdraws.ꢀ
Ifꢀoneꢀchannelꢀisꢀshutꢀdownꢀandꢀtheꢀotherꢀchannelꢀisꢀinꢀ
sleepꢀmode,ꢀtheꢀLTC3857ꢀdrawsꢀonlyꢀ50µAꢀofꢀquiescentꢀ
current.ꢀIfꢀbothꢀchannelsꢀareꢀinꢀsleepꢀmode,ꢀtheꢀLTC3857ꢀ
drawsꢀonlyꢀ80µAꢀofꢀquiescentꢀcurrent.ꢀInꢀsleepꢀmode,ꢀ Frequency Selection and Phase-Locked Loop
theꢀloadꢀcurrentꢀisꢀsuppliedꢀbyꢀtheꢀoutputꢀcapacitor.ꢀAsꢀ (FREQ and PLLIN/MODE Pins)
theꢀoutputꢀvoltageꢀdecreases,ꢀtheꢀEA’sꢀoutputꢀbeginsꢀtoꢀ
Theꢀselectionꢀofꢀswitchingꢀfrequencyꢀisꢀaꢀtradeoffꢀbetweenꢀ
rise.ꢀWhenꢀtheꢀoutputꢀvoltageꢀdropsꢀenough,ꢀtheꢀI ꢀpinꢀ
TH
efficiencyꢀ andꢀ componentꢀ size.ꢀ Lowꢀ frequencyꢀ opera-
tionꢀincreasesꢀefficiencyꢀbyꢀreducingꢀMOSFETꢀswitchingꢀ
losses,ꢀbutꢀrequiresꢀlargerꢀinductanceꢀand/orꢀcapacitanceꢀ
toꢀmaintainꢀlowꢀoutputꢀrippleꢀvoltage.
isꢀreconnectedꢀtoꢀtheꢀoutputꢀofꢀtheꢀEA,ꢀtheꢀsleepꢀsignalꢀ
goesꢀlow,ꢀandꢀtheꢀcontrollerꢀresumesꢀnormalꢀoperationꢀ
byꢀturningꢀonꢀtheꢀtopꢀexternalꢀMOSFETꢀonꢀtheꢀnextꢀcycleꢀ
ofꢀtheꢀinternalꢀoscillator.
TheꢀswitchingꢀfrequencyꢀofꢀtheꢀLTC3857’sꢀcontrollersꢀcanꢀ
beꢀselectedꢀusingꢀtheꢀFREQꢀpin.
WhenꢀaꢀcontrollerꢀisꢀenabledꢀforꢀBurstꢀModeꢀoperation,ꢀtheꢀ
inductorꢀcurrentꢀisꢀnotꢀallowedꢀtoꢀreverse.ꢀTheꢀreverseꢀcur-
rentꢀcomparator,ꢀIR,ꢀturnsꢀoffꢀtheꢀbottomꢀexternalꢀMOSFETꢀ
justꢀbeforeꢀtheꢀinductorꢀcurrentꢀreachesꢀzero,ꢀpreventingꢀ
itꢀfromꢀreversingꢀandꢀgoingꢀnegative.ꢀThus,ꢀtheꢀcontrollerꢀ
operatesꢀinꢀdiscontinuousꢀoperation.
IfꢀtheꢀPLLIN/MODEꢀpinꢀisꢀnotꢀbeingꢀdrivenꢀbyꢀanꢀexternalꢀ
clockꢀsource,ꢀtheꢀFREQꢀpinꢀcanꢀbeꢀtiedꢀtoꢀSGND,ꢀtiedꢀtoꢀ
INTV ꢀorꢀprogrammedꢀthroughꢀanꢀexternalꢀresistor.ꢀTyingꢀ
CC
FREQꢀtoꢀSGNDꢀselectsꢀ350kHzꢀwhileꢀtyingꢀFREQꢀtoꢀINTV ꢀ
CC
3857fa
ꢀꢁ
LTC3857
operaTion (Refer to the Functional Diagram)
selectsꢀ535kHz.ꢀPlacingꢀaꢀresistorꢀbetweenꢀFREQꢀandꢀSGNDꢀ
allowsꢀtheꢀfrequencyꢀtoꢀbeꢀprogrammedꢀbetweenꢀ50kHzꢀ
andꢀ900kHz,ꢀasꢀshownꢀinꢀFigureꢀ10.
Table 1
V
CONTROLLER 2 PHASE
CLKOUT PHASE
PHASMD
GND
180°
180°
240°
60°
90°
Floating
Aꢀphase-lockedꢀloopꢀ(PLL)ꢀisꢀavailableꢀonꢀtheꢀLTC3857ꢀ
toꢀsynchronizeꢀtheꢀinternalꢀoscillatorꢀtoꢀanꢀexternalꢀclockꢀ
sourceꢀthatꢀisꢀconnectedꢀtoꢀtheꢀPLLIN/MODEꢀpin.ꢀTheꢀ
phaseꢀdetectorꢀadjustsꢀtheꢀvoltageꢀ(throughꢀanꢀinternalꢀ
lowpassꢀfilter)ꢀofꢀtheꢀVCOꢀinputꢀtoꢀalignꢀtheꢀturn-onꢀofꢀ
controllerꢀ1’sꢀexternalꢀtopꢀMOSFETꢀtoꢀtheꢀrisingꢀedgeꢀofꢀ
theꢀsynchronizingꢀsignal.ꢀThus,ꢀtheꢀturn-onꢀofꢀcontrollerꢀ2’sꢀ
externalꢀtopꢀMOSFETꢀisꢀ180ꢀdegreesꢀoutꢀofꢀphaseꢀtoꢀtheꢀ
risingꢀedgeꢀofꢀtheꢀexternalꢀclockꢀsource.
INTV
120°
CC
Output Overvoltage Protection
Anꢀovervoltageꢀcomparatorꢀguardsꢀagainstꢀtransientꢀover-
shootsꢀasꢀwellꢀasꢀotherꢀmoreꢀseriousꢀconditionsꢀthatꢀmayꢀ
overvoltageꢀtheꢀoutput.ꢀWhenꢀtheꢀV ꢀpinꢀrisesꢀbyꢀmoreꢀ
thanꢀ10%ꢀaboveꢀitsꢀregulationꢀpointꢀofꢀ0.800V,ꢀtheꢀtopꢀ
MOSFETꢀisꢀturnedꢀoffꢀandꢀtheꢀbottomꢀMOSFETꢀisꢀturnedꢀ
onꢀuntilꢀtheꢀovervoltageꢀconditionꢀisꢀcleared.
FB
TheꢀVCOꢀinputꢀvoltageꢀisꢀprebiasedꢀtoꢀtheꢀoperatingꢀfre-
quencyꢀsetꢀbyꢀtheꢀFREQꢀpinꢀbeforeꢀtheꢀexternalꢀclockꢀisꢀ
applied.ꢀIfꢀprebiasedꢀnearꢀtheꢀexternalꢀclockꢀfrequency,ꢀ
theꢀPLLꢀloopꢀonlyꢀneedsꢀtoꢀmakeꢀslightꢀchangesꢀtoꢀtheꢀ
VCOꢀinputꢀinꢀorderꢀtoꢀsynchronizeꢀtheꢀrisingꢀedgeꢀofꢀtheꢀ
externalꢀclock’sꢀtoꢀtheꢀrisingꢀedgeꢀofꢀTG1.ꢀTheꢀabilityꢀtoꢀ
prebiasꢀtheꢀloopꢀfilterꢀallowsꢀtheꢀPLLꢀtoꢀlock-inꢀrapidlyꢀ
withoutꢀdeviatingꢀfarꢀfromꢀtheꢀdesiredꢀfrequency.
Power Good (PGOOD1 and PGOOD2) Pins
EachꢀPGOODꢀpinꢀisꢀconnectedꢀtoꢀanꢀopenꢀdrainꢀofꢀanꢀ
internalꢀN-channelꢀMOSFET.ꢀTheꢀMOSFETꢀturnsꢀonꢀandꢀ
pullsꢀtheꢀPGOODꢀpinꢀlowꢀwhenꢀtheꢀcorrespondingꢀV ꢀpinꢀ
FB
voltageꢀisꢀnotꢀwithinꢀ 10%ꢀofꢀtheꢀ0.8Vꢀreferenceꢀvoltage.ꢀ
TheꢀPGOODꢀpinꢀisꢀalsoꢀpulledꢀlowꢀwhenꢀtheꢀcorrespondingꢀ
RUNꢀpinꢀisꢀlowꢀ(shutꢀdown).ꢀWhenꢀtheꢀV ꢀpinꢀvoltageꢀ
FB
Theꢀtypicalꢀcaptureꢀrangeꢀofꢀtheꢀphase-lockedꢀloopꢀisꢀfromꢀ
approximatelyꢀ55kHzꢀtoꢀ1MHz,ꢀwithꢀaꢀguaranteeꢀoverꢀallꢀ
manufacturingꢀvariationsꢀtoꢀbeꢀbetweenꢀ75kHzꢀandꢀ850kHz.ꢀ
Inꢀotherꢀwords,ꢀtheꢀLTC3857’sꢀPLLꢀisꢀguaranteedꢀtoꢀlockꢀ
toꢀanꢀexternalꢀclockꢀsourceꢀwhoseꢀfrequencyꢀisꢀbetweenꢀ
75kHzꢀandꢀ850kHz.
isꢀwithinꢀtheꢀ 10%ꢀrequirement,ꢀtheꢀMOSFETꢀisꢀturnedꢀ
offꢀandꢀtheꢀpinꢀisꢀallowedꢀtoꢀbeꢀpulledꢀupꢀbyꢀanꢀexternalꢀ
resistorꢀtoꢀaꢀsourceꢀnoꢀgreaterꢀthanꢀ6V.
Foldback Current
Whenꢀtheꢀoutputꢀvoltageꢀfallsꢀtoꢀlessꢀthanꢀ70%ꢀofꢀitsꢀ
nominalꢀlevel,ꢀfoldbackꢀcurrentꢀlimitingꢀisꢀactivated,ꢀpro-
gressivelyꢀloweringꢀtheꢀpeakꢀcurrentꢀlimitꢀinꢀproportionꢀtoꢀ
theꢀseverityꢀofꢀtheꢀovercurrentꢀorꢀshort-circuitꢀcondition.ꢀ
Foldbackꢀcurrentꢀlimitingꢀisꢀdisabledꢀduringꢀtheꢀsoft-startꢀ
TheꢀtypicalꢀinputꢀclockꢀthresholdsꢀonꢀtheꢀPLLIN/MODEꢀ
pinꢀareꢀ1.6Vꢀ(rising)ꢀandꢀ1.1Vꢀ(falling).
PolyPhase® Applications (CLKOUT and PHASMD Pins)
intervalꢀ(asꢀlongꢀasꢀtheꢀV ꢀvoltageꢀisꢀkeepingꢀupꢀwithꢀtheꢀ
TheꢀLTC3857ꢀfeaturesꢀtwoꢀpinsꢀ(CLKOUTꢀandꢀPHASMD)ꢀ
thatꢀallowꢀotherꢀcontrollerꢀICsꢀtoꢀbeꢀdaisy-chainedꢀwithꢀ
theꢀLTC3857ꢀinꢀPolyPhaseꢀapplications.ꢀTheꢀclockꢀoutputꢀ
signalꢀonꢀtheꢀCLKOUTꢀpinꢀcanꢀbeꢀusedꢀtoꢀsynchronizeꢀ
additionalꢀpowerꢀstagesꢀinꢀaꢀmultiphaseꢀpowerꢀsupplyꢀ
solutionꢀfeedingꢀaꢀsingle,ꢀhighꢀcurrentꢀoutputꢀorꢀmultipleꢀ
separateꢀoutputs.ꢀTheꢀPHASMDꢀpinꢀisꢀusedꢀtoꢀadjustꢀtheꢀ
phaseꢀofꢀtheꢀCLKOUTꢀsignalꢀasꢀwellꢀasꢀtheꢀrelativeꢀphasesꢀ
betweenꢀtheꢀtwoꢀinternalꢀcontrollers,ꢀasꢀsummarizedꢀinꢀ
Tableꢀ1.ꢀTheꢀphasesꢀareꢀcalculatedꢀrelativeꢀtoꢀtheꢀzeroꢀ
degreesꢀphaseꢀbeingꢀdefinedꢀasꢀtheꢀrisingꢀedgeꢀofꢀtheꢀtopꢀ
gateꢀdriverꢀoutputꢀofꢀcontrollerꢀ1ꢀ(TG1).
FB
TRACK/SSꢀvoltage).
Theory and Benefits of 2-Phase Operation
Whyꢀtheꢀneedꢀforꢀ2-phaseꢀoperation?ꢀUpꢀuntilꢀtheꢀ2-phaseꢀ
family,ꢀ constant-frequencyꢀ dualꢀ switchingꢀ regulatorsꢀ
operatedꢀ bothꢀ channelsꢀ inꢀ phaseꢀ (i.e.,ꢀ singleꢀ phaseꢀ
operation).ꢀThisꢀmeansꢀthatꢀbothꢀswitchesꢀturnedꢀonꢀatꢀ
theꢀsameꢀtime,ꢀcausingꢀcurrentꢀpulsesꢀofꢀupꢀtoꢀtwiceꢀtheꢀ
amplitudeꢀofꢀthoseꢀforꢀoneꢀregulatorꢀtoꢀbeꢀdrawnꢀfromꢀtheꢀ
inputꢀcapacitorꢀandꢀbattery.ꢀTheseꢀlargeꢀamplitudeꢀcurrentꢀ
3857fa
ꢀꢂ
LTC3857
operaTion (Refer to the Functional Diagram)
pulsesꢀincreasedꢀtheꢀtotalꢀRMSꢀcurrentꢀflowingꢀfromꢀtheꢀ
inputꢀcapacitor,ꢀrequiringꢀtheꢀuseꢀofꢀmoreꢀexpensiveꢀinputꢀ
capacitorsꢀandꢀincreasingꢀbothꢀEMIꢀandꢀlossesꢀinꢀtheꢀinputꢀ
capacitorꢀandꢀbattery.
Ofꢀcourse,ꢀtheꢀimprovementꢀaffordedꢀbyꢀ2-phaseꢀoperationꢀ
isꢀaꢀfunctionꢀofꢀtheꢀdualꢀswitchingꢀregulator’sꢀrelativeꢀdutyꢀ
cyclesꢀwhich,ꢀinꢀturn,ꢀareꢀdependentꢀuponꢀtheꢀinputꢀvoltageꢀ
V ꢀ(DutyꢀCycleꢀ=ꢀV /V ).ꢀFigureꢀ2ꢀshowsꢀhowꢀtheꢀRMSꢀ
IN
OUT IN
inputꢀcurrentꢀvariesꢀforꢀsingle-phaseꢀandꢀ2-phaseꢀoperationꢀ
forꢀ3.3Vꢀandꢀ5Vꢀregulatorsꢀoverꢀaꢀwideꢀinputꢀvoltageꢀrange.
Withꢀ 2-phaseꢀ operation,ꢀ theꢀ twoꢀ channelsꢀ ofꢀ theꢀ dualꢀ
switchingꢀregulatorꢀareꢀoperatedꢀ180ꢀdegreesꢀoutꢀofꢀphase.ꢀ
Thisꢀeffectivelyꢀinterleavesꢀtheꢀcurrentꢀpulsesꢀdrawnꢀbyꢀtheꢀ
switches,ꢀgreatlyꢀreducingꢀtheꢀoverlapꢀtimeꢀwhereꢀtheyꢀaddꢀ
together.ꢀTheꢀresultꢀisꢀaꢀsignificantꢀreductionꢀinꢀtotalꢀRMSꢀ
inputꢀcurrent,ꢀwhichꢀinꢀturnꢀallowsꢀlessꢀexpensiveꢀinputꢀ
capacitorsꢀtoꢀbeꢀused,ꢀreducesꢀshieldingꢀrequirementsꢀforꢀ
EMIꢀandꢀimprovesꢀrealꢀworldꢀoperatingꢀefficiency.
Itꢀcanꢀreadilyꢀbeꢀseenꢀthatꢀtheꢀadvantagesꢀofꢀ2-phaseꢀop-
erationꢀareꢀnotꢀjustꢀlimitedꢀtoꢀaꢀnarrowꢀoperatingꢀrange,ꢀ
forꢀmostꢀapplicationsꢀisꢀthatꢀ2-phaseꢀoperationꢀwillꢀreduceꢀ
theꢀinputꢀcapacitorꢀrequirementꢀtoꢀthatꢀforꢀjustꢀoneꢀchannelꢀ
operatingꢀatꢀmaximumꢀcurrentꢀandꢀ50%ꢀdutyꢀcycle.
Figureꢀ1ꢀcomparesꢀtheꢀinputꢀwaveformsꢀforꢀaꢀsingle-phaseꢀ
dualꢀ switchingꢀ regulatorꢀ toꢀ aꢀ 2-phaseꢀ dualꢀ switchingꢀ
regulator.ꢀAnꢀactualꢀmeasurementꢀofꢀtheꢀRMSꢀinputꢀcur-
rentꢀunderꢀtheseꢀconditionsꢀshowsꢀthatꢀ2-phaseꢀoperationꢀ
3.0
SINGLE PHASE
DUAL CONTROLLER
2.5
2.0
1.5
1.0
0.5
0
droppedꢀtheꢀinputꢀcurrentꢀfromꢀ2.53A
ꢀtoꢀ1.55A
.ꢀ
RMS
RMS
Whileꢀthisꢀisꢀanꢀimpressiveꢀreductionꢀinꢀitself,ꢀrememberꢀ
2
thatꢀtheꢀpowerꢀlossesꢀareꢀproportionalꢀtoꢀI
,ꢀmeaningꢀ
2-PHASE
DUAL CONTROLLER
RMS
thatꢀtheꢀactualꢀpowerꢀwastedꢀisꢀreducedꢀbyꢀaꢀfactorꢀofꢀ2.66.ꢀ
Theꢀreducedꢀinputꢀrippleꢀvoltageꢀalsoꢀmeansꢀlessꢀpowerꢀisꢀ
lostꢀinꢀtheꢀinputꢀpowerꢀpath,ꢀwhichꢀcouldꢀincludeꢀbatter-
ies,ꢀswitches,ꢀtrace/connectorꢀresistancesꢀandꢀprotectionꢀ
circuitry.ꢀImprovementsꢀinꢀbothꢀconductedꢀandꢀradiatedꢀ
EMIꢀalsoꢀdirectlyꢀaccrueꢀasꢀaꢀresultꢀofꢀtheꢀreducedꢀRMSꢀ
inputꢀcurrentꢀandꢀvoltage.
V
O1
V
O2
= 5V/3A
= 3.3V/3A
0
10
20
30
40
INPUT VOLTAGE (V)
3857 F02
Figure 2. RMS Input Current Comparison
5V SWITCH
20V/DIV
3.3V SWITCH
20V/DIV
INPUT CURRENT
5A/DIV
INPUT VOLTAGE
500mV/DIV
3857 F01
I
= 2.53A
I
= 1.55A
IN(MEAS) RMS
IN(MEAS)
RMS
Figure 1. 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
3857fa
ꢀꢃ
LTC3857
applicaTions inForMaTion
TheꢀTypicalꢀApplicationꢀonꢀtheꢀfirstꢀpageꢀisꢀaꢀbasicꢀLTC3857ꢀ
applicationꢀ circuit.ꢀ LTC3857ꢀ canꢀ beꢀ configuredꢀ toꢀ useꢀ
eitherꢀ DCRꢀ (inductorꢀ resistance)ꢀ sensingꢀ orꢀ lowꢀ valueꢀ
resistorꢀ sensing.ꢀ Theꢀ choiceꢀ betweenꢀ theꢀ twoꢀ currentꢀ
sensingꢀschemesꢀisꢀlargelyꢀaꢀdesignꢀtrade-offꢀbetweenꢀ
cost,ꢀ powerꢀ consumptionꢀ andꢀ accuracy.ꢀ DCRꢀ sensingꢀ
isꢀbecomingꢀpopularꢀbecauseꢀitꢀsavesꢀexpensiveꢀcurrentꢀ
sensingꢀresistorsꢀandꢀisꢀmoreꢀpowerꢀefficient,ꢀespeciallyꢀ
inꢀ highꢀ currentꢀ applications.ꢀ However,ꢀ currentꢀ sensingꢀ
resistorsꢀprovideꢀtheꢀmostꢀaccurateꢀcurrentꢀlimitsꢀforꢀtheꢀ
controller.ꢀOtherꢀexternalꢀcomponentꢀselectionꢀisꢀdrivenꢀ
byꢀtheꢀloadꢀrequirement,ꢀandꢀbeginsꢀwithꢀtheꢀselectionꢀofꢀ
Filterꢀcomponentsꢀmutualꢀtoꢀtheꢀsenseꢀlinesꢀshouldꢀbeꢀ
placedꢀcloseꢀtoꢀtheꢀLTC3857,ꢀandꢀtheꢀsenseꢀlinesꢀshouldꢀ
runꢀcloseꢀtogetherꢀtoꢀaꢀKelvinꢀconnectionꢀunderneathꢀtheꢀ
currentꢀsenseꢀelementꢀ(shownꢀinꢀFigureꢀ3).ꢀSensingꢀcur-
rentꢀelsewhereꢀcanꢀeffectivelyꢀaddꢀparasiticꢀinductanceꢀ
andꢀcapacitanceꢀtoꢀtheꢀcurrentꢀsenseꢀelement,ꢀdegradingꢀ
theꢀinformationꢀatꢀtheꢀsenseꢀterminalsꢀandꢀmakingꢀtheꢀ
programmedꢀcurrentꢀlimitꢀunpredictable.ꢀIfꢀinductorꢀDCRꢀ
sensingꢀisꢀusedꢀ(Figureꢀ4b),ꢀsenseꢀresistorꢀR1ꢀshouldꢀbeꢀ
TO SENSE FILTER,
NEXT TO THE CONTROLLER
C
OUT
R
ꢀ(ifꢀR
ꢀisꢀused)ꢀandꢀinductorꢀvalue.ꢀNext,ꢀtheꢀ
SENSE
SENSE
3857 F03
powerꢀMOSFETsꢀandꢀSchottkyꢀdiodesꢀareꢀselected.ꢀFinally,ꢀ
inputꢀandꢀoutputꢀcapacitorsꢀareꢀselected.
INDUCTOR OR R
SENSE
Figure 3. Sense Lines Placement with Inductor or Sense Resistor
Current Limit Programming
V
V
IN
IN
INTV
CC
TheꢀILIMꢀpinꢀisꢀaꢀtri-levelꢀlogicꢀinputꢀwhichꢀsetsꢀtheꢀmaxi-
mumꢀcurrentꢀlimitꢀofꢀtheꢀcontroller.ꢀWhenꢀILIMꢀisꢀgrounded,ꢀ
theꢀmaximumꢀcurrentꢀlimitꢀthresholdꢀvoltageꢀofꢀtheꢀcur-
BOOST
TG
R
SENSE
SW
V
OUT
rentꢀcomparatorꢀisꢀprogrammedꢀtoꢀbeꢀ30mV.ꢀWhenꢀILIM
ꢀ
LTC3857
isꢀfloated,ꢀtheꢀmaximumꢀcurrentꢀlimitꢀthresholdꢀisꢀ50mV.ꢀ
WhenꢀILIMꢀisꢀtiedꢀtoꢀINTVCC,ꢀtheꢀmaximumꢀcurrentꢀlimitꢀ
thresholdꢀisꢀsetꢀtoꢀ75mV.
BG
+
SENSE
PLACE CAPACITOR NEAR
SENSE PINS
–
+
–
SENSE
SGND
SENSE and SENSE Pins
+
–
3857 F04a
TheꢀSENSE ꢀandꢀSENSE ꢀpinsꢀareꢀtheꢀinputsꢀtoꢀtheꢀcur-
rentꢀcomparators.ꢀTheꢀcommonꢀmodeꢀvoltageꢀrangeꢀonꢀ
theseꢀpinsꢀisꢀ0Vꢀtoꢀ24Vꢀ(absꢀmax),ꢀenablingꢀtheꢀLTC3857ꢀ
toꢀregulateꢀoutputꢀvoltagesꢀupꢀtoꢀaꢀnominalꢀ24Vꢀ(allowingꢀ
marginꢀforꢀtolerancesꢀandꢀtransients).ꢀ
(4a) Using a Resistor to Sense Current
V
V
IN
IN
INTV
CC
+
INDUCTOR
DCR
BOOST
TG
TheꢀSENSE ꢀpinꢀisꢀhighꢀimpedanceꢀoverꢀtheꢀfullꢀcommonꢀ
modeꢀrange,ꢀdrawingꢀatꢀmostꢀ 1µA.ꢀThisꢀhighꢀimpedanceꢀ
allowsꢀtheꢀcurrentꢀcomparatorsꢀtoꢀbeꢀusedꢀinꢀinductorꢀ
DCRꢀsensing.
L
SW
V
OUT
LTC3857
BG
–
R1
C1* R2
TheꢀimpedanceꢀofꢀtheꢀSENSE ꢀpinꢀchangesꢀdependingꢀonꢀ
+
SENSE
–
theꢀcommonꢀmodeꢀvoltage.ꢀWhenꢀSENSE ꢀisꢀlessꢀthanꢀ
–
SENSE
INTV ꢀ–ꢀ0.5V,ꢀaꢀsmallꢀcurrentꢀofꢀlessꢀthanꢀ1µAꢀflowsꢀoutꢀ
CC
–
SGND
ofꢀtheꢀpin.ꢀWhenꢀSENSE ꢀisꢀaboveꢀINTV ꢀ+ꢀ0.5V,ꢀaꢀhigherꢀ
CC
3857 F04b
R2
R1 + R2
L
||
(R1 R2) • C1 =
*PLACE C1 NEAR
SENSE PINS
R
= DCR
SENSE(EQ)
currentꢀ(~550µA)ꢀflowsꢀintoꢀtheꢀpin.ꢀBetweenꢀINTV ꢀ–ꢀ0.5Vꢀ
CC
DCR
andꢀINTV ꢀ+ꢀ0.5V,ꢀtheꢀcurrentꢀtransitionsꢀfromꢀtheꢀsmallerꢀ
CC
(4b) Using the Inductor DCR to Sense Current
Figure 4. Current Sensing Methods
currentꢀtoꢀtheꢀhigherꢀcurrent.
3857fa
ꢀꢄ
LTC3857
applicaTions inForMaTion
placedꢀcloseꢀtoꢀtheꢀswitchingꢀnode,ꢀtoꢀpreventꢀnoiseꢀfromꢀ
IfꢀtheꢀexternalꢀR1||R2ꢀ•ꢀC1ꢀtimeꢀconstantꢀisꢀchosenꢀtoꢀbeꢀ
exactlyꢀequalꢀtoꢀtheꢀL/DCRꢀtimeꢀconstant,ꢀtheꢀvoltageꢀdropꢀ
acrossꢀtheꢀexternalꢀcapacitorꢀisꢀequalꢀtoꢀtheꢀdropꢀacrossꢀ
theꢀinductorꢀDCRꢀmultipliedꢀbyꢀR2/(R1ꢀ+ꢀR2).ꢀR2ꢀscalesꢀtheꢀ
voltageꢀacrossꢀtheꢀsenseꢀterminalsꢀforꢀapplicationsꢀwhereꢀ
theꢀDCRꢀisꢀgreaterꢀthanꢀtheꢀtargetꢀsenseꢀresistorꢀvalue.ꢀ
Toꢀproperlyꢀdimensionꢀtheꢀexternalꢀfilterꢀcomponents,ꢀtheꢀ
DCRꢀofꢀtheꢀinductorꢀmustꢀbeꢀknown.ꢀItꢀcanꢀbeꢀmeasuredꢀ
usingꢀaꢀgoodꢀRLCꢀmeter,ꢀbutꢀtheꢀDCRꢀtoleranceꢀisꢀnotꢀ
alwaysꢀtheꢀsameꢀandꢀvariesꢀwithꢀtemperature;ꢀconsultꢀtheꢀ
manufacturers’ꢀdataꢀsheetsꢀforꢀdetailedꢀinformation.
couplingꢀintoꢀsensitiveꢀsmall-signalꢀnodes.
Low Value Resistor Current Sensing
Aꢀtypicalꢀsensingꢀcircuitꢀusingꢀaꢀdiscreteꢀresistorꢀisꢀshownꢀ
inꢀ Figureꢀ 4a.ꢀ R
outputꢀcurrent.
ꢀ isꢀ chosenꢀ basedꢀ onꢀ theꢀ requiredꢀ
SENSE
Theꢀ currentꢀ comparatorꢀ hasꢀ aꢀ maximumꢀ thresholdꢀ
ꢀdeterminedꢀbyꢀtheꢀI ꢀsetting.ꢀTheꢀcurrentꢀ
V
SENSE(MAX)
LIM
comparatorꢀthresholdꢀvoltageꢀsetsꢀtheꢀpeakꢀofꢀtheꢀinduc-
torꢀcurrent,ꢀyieldingꢀaꢀmaximumꢀaverageꢀoutputꢀcurrent,ꢀ
UsingꢀtheꢀinductorꢀrippleꢀcurrentꢀvalueꢀfromꢀtheꢀInductorꢀ
ValueꢀCalculationꢀsection,ꢀtheꢀtargetꢀsenseꢀresistorꢀvalueꢀ
is:
I
,ꢀequalꢀtoꢀtheꢀpeakꢀvalueꢀlessꢀhalfꢀtheꢀpeak-to-peakꢀ
MAX
rippleꢀcurrent,ꢀ∆I .ꢀToꢀcalculateꢀtheꢀsenseꢀresistorꢀvalue,ꢀ
L
useꢀtheꢀequation:
VSENSE(MAX)
VSENSE(MAX)
RSENSE(EQUIV)
=
RSENSE
=
∆IL
∆IL
IMAX
+
IMAX
+
ꢀ
2
ꢀ
2
Toꢀensureꢀthatꢀtheꢀapplicationꢀwillꢀdeliverꢀfullꢀloadꢀcurrentꢀ
overꢀ theꢀ fullꢀ operatingꢀ temperatureꢀ range,ꢀ chooseꢀ theꢀ
minimumꢀvalueꢀforꢀtheꢀmaximumꢀcurrentꢀsenseꢀthresholdꢀ
Whenꢀusingꢀtheꢀcontrollerꢀinꢀveryꢀlowꢀdropoutꢀconditions,ꢀ
theꢀmaximumꢀoutputꢀcurrentꢀlevelꢀwillꢀbeꢀreducedꢀdueꢀtoꢀtheꢀ
internalꢀcompensationꢀrequiredꢀtoꢀmeetꢀstabilityꢀcriterionꢀ
forꢀbuckꢀregulatorsꢀoperatingꢀatꢀgreaterꢀthanꢀ50%ꢀdutyꢀ
factor.ꢀAꢀcurveꢀisꢀprovidedꢀinꢀtheꢀTypicalꢀPerformanceꢀChar-
acteristicsꢀsectionꢀtoꢀestimateꢀthisꢀreductionꢀinꢀpeakꢀoutputꢀ
currentꢀdependingꢀuponꢀtheꢀoperatingꢀdutyꢀfactor.
voltageꢀ (V
)ꢀ inꢀ theꢀ Electricalꢀ Characteristicsꢀ
SENSE(MAX)
tableꢀ(30mV,ꢀ50mVꢀorꢀ75mV,ꢀdependingꢀonꢀtheꢀstateꢀofꢀ
theꢀI ꢀpin).
LIM
Next,ꢀdetermineꢀtheꢀDCRꢀofꢀtheꢀinductor.ꢀWhenꢀprovided,ꢀ
useꢀtheꢀmanufacturer’sꢀmaximumꢀvalue,ꢀusuallyꢀgivenꢀatꢀ
20°C.ꢀIncreaseꢀthisꢀvalueꢀtoꢀaccountꢀforꢀtheꢀtemperatureꢀ
coefficientꢀofꢀcopperꢀresistance,ꢀwhichꢀisꢀapproximatelyꢀ
Inductor DCR Sensing
Forꢀapplicationsꢀrequiringꢀtheꢀhighestꢀpossibleꢀefficiencyꢀ
atꢀhighꢀloadꢀcurrents,ꢀtheꢀLTC3857ꢀisꢀcapableꢀofꢀsensingꢀ
theꢀvoltageꢀdropꢀacrossꢀtheꢀinductorꢀDCR,ꢀasꢀshownꢀinꢀ
Figureꢀ4b.ꢀTheꢀDCRꢀofꢀtheꢀinductorꢀrepresentsꢀtheꢀsmallꢀ
amountꢀofꢀDCꢀresistanceꢀofꢀtheꢀcopperꢀwire,ꢀwhichꢀcanꢀ
beꢀlessꢀthanꢀ1mΩꢀforꢀtoday’sꢀlowꢀvalue,ꢀhighꢀcurrentꢀ
inductors.ꢀInꢀaꢀhighꢀcurrentꢀapplicationꢀrequiringꢀsuchꢀ
anꢀinductor,ꢀpowerꢀlossꢀthroughꢀaꢀsenseꢀresistorꢀwouldꢀ
costꢀseveralꢀpointsꢀofꢀefficiencyꢀcomparedꢀtoꢀinductorꢀ
DCRꢀsensing.
0.4%/°C.ꢀAꢀconservativeꢀvalueꢀforꢀT
ꢀisꢀ100°C.
L(MAX)
ToꢀscaleꢀtheꢀmaximumꢀinductorꢀDCRꢀtoꢀtheꢀdesiredꢀsenseꢀ
resistorꢀ(R )ꢀvalue,ꢀuseꢀtheꢀdividerꢀratio:
D
RSENSE(EQUIV)
RD =
DCRMAX atT
ꢀ
L(MAX)
C1ꢀisꢀusuallyꢀselectedꢀtoꢀbeꢀinꢀtheꢀrangeꢀofꢀ0.1µFꢀtoꢀ0.47µF.ꢀ
ThisꢀforcesꢀR1||ꢀR2ꢀtoꢀaroundꢀ2k,ꢀreducingꢀerrorꢀthatꢀmightꢀ
+
haveꢀbeenꢀcausedꢀbyꢀtheꢀSENSE ꢀpin’sꢀ 1µAꢀcurrent.
3857fa
ꢀꢅ
LTC3857
applicaTions inForMaTion
TheꢀequivalentꢀresistanceꢀR1||ꢀR2ꢀisꢀscaledꢀtoꢀtheꢀroomꢀ
Acceptingꢀ largerꢀ valuesꢀ ofꢀ ∆I ꢀ allowsꢀ theꢀ useꢀ ofꢀ lowꢀ
L
temperatureꢀinductanceꢀandꢀmaximumꢀDCR:
inductances,ꢀbutꢀresultsꢀinꢀhigherꢀoutputꢀvoltageꢀrippleꢀ
andꢀgreaterꢀcoreꢀlosses.ꢀAꢀreasonableꢀstartingꢀpointꢀforꢀ
L
R1||R2 =
settingꢀrippleꢀcurrentꢀisꢀ∆I ꢀ=ꢀ0.3(I
).ꢀTheꢀmaximumꢀ
MAX
L
DCR at 20°C •C1
∆I ꢀoccursꢀatꢀtheꢀmaximumꢀinputꢀvoltage.
L
ꢀ
Theꢀinductorꢀvalueꢀalsoꢀhasꢀsecondaryꢀeffects.ꢀTheꢀtran-
sitionꢀtoꢀBurstꢀModeꢀoperationꢀbeginsꢀwhenꢀtheꢀaverageꢀ
inductorꢀcurrentꢀrequiredꢀresultsꢀinꢀaꢀpeakꢀcurrentꢀbelowꢀ
Theꢀsenseꢀresistorꢀvaluesꢀare:
R1•RD
1–RD
R1||R2
RD
R1=
; R2 =
15%ꢀofꢀtheꢀcurrentꢀlimitꢀdeterminedꢀbyꢀR
.ꢀLowerꢀ
SENSE
ꢀ
inductorꢀvaluesꢀ(higherꢀ∆I )ꢀwillꢀcauseꢀthisꢀtoꢀoccurꢀatꢀ
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
affordꢀtheꢀcoreꢀlossꢀfoundꢀinꢀlowꢀcostꢀpowderedꢀironꢀcores,ꢀ
forcingꢀtheꢀuseꢀofꢀmoreꢀexpensiveꢀferriteꢀorꢀmolypermalloyꢀ
cores.ꢀActualꢀcoreꢀlossꢀisꢀindependentꢀofꢀcoreꢀsizeꢀforꢀaꢀ
fixedꢀinductorꢀvalue,ꢀbutꢀitꢀisꢀveryꢀdependentꢀonꢀinductanceꢀ
valueꢀselected.ꢀAsꢀinductanceꢀincreases,ꢀcoreꢀlossesꢀgoꢀ
down.ꢀUnfortunately,ꢀincreasedꢀinductanceꢀrequiresꢀmoreꢀ
turnsꢀofꢀwireꢀandꢀthereforeꢀcopperꢀlossesꢀwillꢀincrease.
EnsureꢀthatꢀR1ꢀhasꢀaꢀpowerꢀratingꢀhigherꢀthanꢀthisꢀvalue.ꢀ
Ifꢀhighꢀefficiencyꢀisꢀnecessaryꢀatꢀlightꢀloads,ꢀconsiderꢀthisꢀ
powerꢀlossꢀwhenꢀdecidingꢀwhetherꢀtoꢀuseꢀDCRꢀsensingꢀorꢀ
senseꢀresistors.ꢀLightꢀloadꢀpowerꢀlossꢀcanꢀbeꢀmodestlyꢀ
higherꢀwithꢀaꢀDCRꢀnetworkꢀthanꢀwithꢀaꢀsenseꢀresistor,ꢀdueꢀ
toꢀtheꢀextraꢀswitchingꢀlossesꢀincurredꢀthroughꢀR1.ꢀHowever,ꢀ
DCRꢀsensingꢀeliminatesꢀaꢀsenseꢀresistor,ꢀreducesꢀconduc-
tionꢀlossesꢀandꢀprovidesꢀhigherꢀefficiencyꢀatꢀheavyꢀloads.ꢀ
Peakꢀefficiencyꢀisꢀaboutꢀtheꢀsameꢀwithꢀeitherꢀmethod.
Ferriteꢀdesignsꢀhaveꢀveryꢀlowꢀcoreꢀlossꢀandꢀareꢀpreferredꢀ
forꢀ highꢀ switchingꢀ frequencies,ꢀ soꢀ designꢀ goalsꢀ canꢀ
concentrateꢀonꢀcopperꢀlossꢀandꢀpreventingꢀsaturation.ꢀ
Ferriteꢀcoreꢀmaterialꢀsaturatesꢀhard,ꢀwhichꢀmeansꢀthatꢀ
inductanceꢀcollapsesꢀabruptlyꢀwhenꢀtheꢀpeakꢀdesignꢀcurrentꢀ
isꢀexceeded.ꢀThisꢀresultsꢀinꢀanꢀabruptꢀincreaseꢀinꢀinductorꢀ
rippleꢀcurrentꢀandꢀconsequentꢀoutputꢀvoltageꢀripple.ꢀDoꢀ
notꢀallowꢀtheꢀcoreꢀtoꢀsaturate!
Inductor Value Calculation
Theꢀoperatingꢀfrequencyꢀandꢀinductorꢀselectionꢀareꢀinter-
relatedꢀinꢀthatꢀhigherꢀoperatingꢀfrequenciesꢀallowꢀtheꢀuseꢀ
ofꢀsmallerꢀinductorꢀandꢀcapacitorꢀvalues.ꢀSoꢀwhyꢀwouldꢀ
anyoneꢀeverꢀchooseꢀtoꢀoperateꢀatꢀlowerꢀfrequenciesꢀwithꢀ
largerꢀcomponents?ꢀTheꢀanswerꢀisꢀefficiency.ꢀAꢀhigherꢀ
frequencyꢀgenerallyꢀresultsꢀinꢀlowerꢀefficiencyꢀbecauseꢀ
ofꢀMOSFETꢀgateꢀchargeꢀlosses.ꢀInꢀadditionꢀtoꢀthisꢀbasicꢀ
trade-off,ꢀtheꢀeffectꢀofꢀinductorꢀvalueꢀonꢀrippleꢀcurrentꢀandꢀ
lowꢀcurrentꢀoperationꢀmustꢀalsoꢀbeꢀconsidered.
Power MOSFET and Schottky Diode (Optional)
Selection
TwoꢀexternalꢀpowerꢀMOSFETsꢀmustꢀbeꢀselectedꢀforꢀeachꢀ
controllerꢀinꢀtheꢀLTC3857:ꢀoneꢀN-channelꢀMOSFETꢀforꢀtheꢀ
topꢀ(main)ꢀswitch,ꢀandꢀoneꢀN-channelꢀMOSFETꢀforꢀtheꢀ
bottomꢀ(synchronous)ꢀswitch.
Theꢀinductorꢀvalueꢀhasꢀaꢀdirectꢀeffectꢀonꢀrippleꢀcurrent.ꢀ
Theꢀinductorꢀrippleꢀcurrent,ꢀ∆I ,ꢀdecreasesꢀwithꢀhigherꢀ
L
inductanceꢀorꢀhigherꢀfrequencyꢀandꢀincreasesꢀwithꢀhigherꢀ
V :
IN
VOUT
1
∆IL =
VOUT 1–
V
f L
IN
ꢀ
3857fa
ꢀꢆ
LTC3857
applicaTions inForMaTion
Theꢀ peak-to-peakꢀ driveꢀ levelsꢀ areꢀ setꢀ byꢀ theꢀ INTV ꢀ
whereꢀδꢀisꢀtheꢀtemperatureꢀdependencyꢀofꢀR
DR
ꢀandꢀ
CC
DS(ON)
voltage.ꢀ Thisꢀ voltageꢀ isꢀ typicallyꢀ 5.1Vꢀ duringꢀ start-upꢀ
R ꢀ(approximatelyꢀ2Ω)ꢀisꢀtheꢀeffectiveꢀdriverꢀresistanceꢀ
(seeꢀEXTV ꢀPinꢀConnection).ꢀConsequently,ꢀlogic-levelꢀ
atꢀtheꢀMOSFET’sꢀMillerꢀthresholdꢀvoltage.ꢀV
ꢀisꢀtheꢀ
CC
THMIN
thresholdꢀMOSFETsꢀmustꢀbeꢀusedꢀinꢀmostꢀapplications.ꢀ
Theꢀ onlyꢀ exceptionꢀ isꢀ ifꢀ lowꢀ inputꢀ voltageꢀ isꢀ expectedꢀ
typicalꢀMOSFETꢀminimumꢀthresholdꢀvoltage.
2
BothꢀMOSFETsꢀhaveꢀI RꢀlossesꢀwhileꢀtheꢀtopsideꢀN-channelꢀ
equationꢀincludesꢀanꢀadditionalꢀtermꢀforꢀtransitionꢀlosses,ꢀ
(V ꢀ <ꢀ 4V);ꢀ then,ꢀ sub-logicꢀ levelꢀ thresholdꢀ MOSFETsꢀ
IN
(V
ꢀ<ꢀ3V)ꢀshouldꢀbeꢀused.ꢀPayꢀcloseꢀattentionꢀtoꢀtheꢀ
GS(TH)
whichꢀareꢀhighestꢀatꢀhighꢀinputꢀvoltages.ꢀForꢀV ꢀ<ꢀ20Vꢀ
IN
BV ꢀspecificationꢀforꢀtheꢀMOSFETsꢀasꢀwell;ꢀmanyꢀofꢀtheꢀ
DSS
theꢀhighꢀcurrentꢀefficiencyꢀgenerallyꢀimprovesꢀwithꢀlargerꢀ
logicꢀlevelꢀMOSFETsꢀareꢀlimitedꢀtoꢀ30Vꢀorꢀless.
MOSFETs,ꢀwhileꢀforꢀV ꢀ>ꢀ20Vꢀtheꢀtransitionꢀlossesꢀrapidlyꢀ
IN
SelectionꢀcriteriaꢀforꢀtheꢀpowerꢀMOSFETsꢀincludeꢀtheꢀon-
increaseꢀtoꢀtheꢀpointꢀthatꢀtheꢀuseꢀofꢀaꢀhigherꢀR
ꢀdeviceꢀ
DS(ON)
resistance,ꢀ R ,ꢀ Millerꢀ capacitance,ꢀ C ,ꢀ inputꢀ
DS(ON) MILLER
withꢀlowerꢀC
ꢀactuallyꢀprovidesꢀhigherꢀefficiency.ꢀTheꢀ
MILLER
voltageꢀandꢀmaximumꢀoutputꢀcurrent.ꢀMillerꢀcapacitance,ꢀ
,ꢀcanꢀbeꢀapproximatedꢀfromꢀtheꢀgateꢀchargeꢀcurveꢀ
synchronousꢀMOSFETꢀlossesꢀareꢀgreatestꢀatꢀhighꢀinputꢀ
voltageꢀwhenꢀtheꢀtopꢀswitchꢀdutyꢀfactorꢀisꢀlowꢀorꢀduringꢀ
aꢀshort-circuitꢀwhenꢀtheꢀsynchronousꢀswitchꢀisꢀonꢀcloseꢀ
toꢀ100%ꢀofꢀtheꢀperiod.
C
MILLER
usuallyꢀ providedꢀ onꢀ theꢀ MOSFETꢀ manufacturers’ꢀ dataꢀ
sheet.ꢀC ꢀisꢀequalꢀtoꢀtheꢀincreaseꢀinꢀgateꢀchargeꢀ
MILLER
alongꢀtheꢀhorizontalꢀaxisꢀwhileꢀtheꢀcurveꢀisꢀapproximatelyꢀ
Theꢀtermꢀ(1+ꢀδ)ꢀisꢀgenerallyꢀgivenꢀforꢀaꢀMOSFETꢀinꢀtheꢀ
flatꢀdividedꢀbyꢀtheꢀspecifiedꢀchangeꢀinꢀV .ꢀThisꢀresultꢀisꢀ
DS
formꢀofꢀaꢀnormalizedꢀR
ꢀvsꢀTemperatureꢀcurve,ꢀbutꢀ
DS(ON)
thenꢀmultipliedꢀbyꢀtheꢀratioꢀofꢀtheꢀapplicationꢀappliedꢀV ꢀ
DS
δꢀ=ꢀ0.005/°Cꢀcanꢀbeꢀusedꢀasꢀanꢀapproximationꢀforꢀlowꢀ
toꢀtheꢀgateꢀchargeꢀcurveꢀspecifiedꢀV .ꢀWhenꢀtheꢀICꢀisꢀ
DS
voltageꢀMOSFETs.
operatingꢀinꢀcontinuousꢀmodeꢀtheꢀdutyꢀcyclesꢀforꢀtheꢀtopꢀ
Theꢀ optionalꢀ Schottkyꢀ diodesꢀ D1ꢀ andꢀ D2ꢀ shownꢀ inꢀ
Figureꢀ11ꢀ conductꢀ duringꢀ theꢀ dead-timeꢀ betweenꢀ theꢀ
conductionꢀofꢀtheꢀtwoꢀpowerꢀMOSFETs.ꢀThisꢀpreventsꢀ
theꢀbodyꢀdiodeꢀofꢀtheꢀbottomꢀMOSFETꢀfromꢀturningꢀon,ꢀ
storingꢀ chargeꢀ duringꢀ theꢀ dead-timeꢀ andꢀ requiringꢀ aꢀ
reverseꢀrecoveryꢀperiodꢀthatꢀcouldꢀcostꢀasꢀmuchꢀasꢀ3%ꢀ
andꢀbottomꢀMOSFETsꢀareꢀgivenꢀby:
VOUT
Main Switch Duty Cycle =
V
IN
V − VOUT
IN
Synchronous Switch Duty Cycle =
V
IN
ꢀ
inꢀefficiencyꢀatꢀhighꢀV .ꢀAꢀ1Aꢀtoꢀ3AꢀSchottkyꢀisꢀgenerallyꢀ
IN
aꢀgoodꢀcompromiseꢀforꢀbothꢀregionsꢀofꢀoperationꢀdueꢀ
toꢀtheꢀrelativelyꢀsmallꢀaverageꢀcurrent.ꢀLargerꢀdiodesꢀ
resultꢀinꢀadditionalꢀtransitionꢀlossesꢀdueꢀtoꢀtheirꢀlargerꢀ
junctionꢀcapacitance.
Theꢀ MOSFETꢀ powerꢀ dissipationsꢀ atꢀ maximumꢀ outputꢀ
currentꢀareꢀgivenꢀby:
VOUT
2
PMAIN
=
=
I
1+ δ R
+
(
MAX) (
)
DS(ON)
V
IN
C and C
Selection
IN
OUT
IMAX
2
2
V
R
C
•
(
)
(
DR )(
)
IN
MILLER
TheꢀselectionꢀofꢀC ꢀisꢀsimplifiedꢀbyꢀtheꢀ2-phaseꢀarchitec-
IN
tureꢀandꢀitsꢀimpactꢀonꢀtheꢀworst-caseꢀRMSꢀcurrentꢀdrawnꢀ
throughꢀtheꢀinputꢀnetworkꢀ(battery/fuse/capacitor).ꢀItꢀcanꢀ
beꢀshownꢀthatꢀtheꢀworst-caseꢀcapacitorꢀRMSꢀcurrentꢀoc-
cursꢀwhenꢀonlyꢀoneꢀcontrollerꢀisꢀoperating.ꢀTheꢀcontrollerꢀ
1
1
+
f
( )
VINTVCC – VTHMIN VTHMIN
V – VOUT
2
withꢀtheꢀhighestꢀ(V )(I )ꢀproductꢀneedsꢀtoꢀbeꢀusedꢀ
IN
OUT OUT
PSYNC
I
1+ δ R
(
MAX) (
)
DS(ON)
inꢀtheꢀformulaꢀshownꢀinꢀEquationꢀ(1)ꢀtoꢀdetermineꢀtheꢀ
V
IN
ꢀ
3857fa
ꢀꢇ
LTC3857
applicaTions inForMaTion
maximumꢀRMSꢀcapacitorꢀcurrentꢀrequirement.ꢀIncreas-
ingꢀtheꢀoutputꢀcurrentꢀdrawnꢀfromꢀtheꢀotherꢀcontrollerꢀ
willꢀactuallyꢀdecreaseꢀtheꢀinputꢀRMSꢀrippleꢀcurrentꢀfromꢀ
itsꢀmaximumꢀvalue.ꢀTheꢀout-of-phaseꢀtechniqueꢀtypicallyꢀ
reducesꢀ theꢀ inputꢀ capacitor’sꢀ RMSꢀ rippleꢀ currentꢀ byꢀ aꢀ
factorꢀofꢀ30%ꢀtoꢀ70%ꢀwhenꢀcomparedꢀtoꢀaꢀsingleꢀphaseꢀ
powerꢀsupplyꢀsolution.
TheꢀdrainsꢀofꢀtheꢀtopꢀMOSFETsꢀshouldꢀbeꢀplacedꢀwithinꢀ
1cmꢀofꢀeachꢀotherꢀandꢀshareꢀaꢀcommonꢀC (s).ꢀSeparatingꢀ
IN
theꢀdrainsꢀandꢀC ꢀmayꢀproduceꢀundesirableꢀvoltageꢀandꢀ
IN
currentꢀresonancesꢀatꢀV .
IN
Aꢀsmallꢀ(0.1µFꢀtoꢀ1µF)ꢀbypassꢀcapacitorꢀbetweenꢀtheꢀchipꢀ
V ꢀpinꢀandꢀground,ꢀplacedꢀcloseꢀtoꢀtheꢀLTC3857,ꢀisꢀalsoꢀ
IN
suggested.ꢀAꢀ10ΩꢀresistorꢀplacedꢀbetweenꢀC ꢀ(C1)ꢀandꢀ
IN
Inꢀcontinuousꢀmode,ꢀtheꢀsourceꢀcurrentꢀofꢀtheꢀtopꢀMOSFETꢀ
isꢀaꢀsquareꢀwaveꢀofꢀdutyꢀcycleꢀ(V )/(V ).ꢀToꢀpreventꢀ
theꢀV ꢀpinꢀprovidesꢀfurtherꢀisolationꢀbetweenꢀtheꢀtwoꢀ
IN
channels.
OUT
IN
largeꢀvoltageꢀtransients,ꢀaꢀlowꢀESRꢀcapacitorꢀsizedꢀforꢀtheꢀ
maximumꢀRMSꢀcurrentꢀofꢀoneꢀchannelꢀmustꢀbeꢀused.ꢀTheꢀ
maximumꢀRMSꢀcapacitorꢀcurrentꢀisꢀgivenꢀby:
TheꢀselectionꢀofꢀC ꢀisꢀdrivenꢀbyꢀtheꢀeffectiveꢀseriesꢀ
OUT
resistanceꢀ(ESR).ꢀTypically,ꢀonceꢀtheꢀESRꢀrequirementꢀ
isꢀsatisfied,ꢀtheꢀcapacitanceꢀisꢀadequateꢀforꢀfiltering.ꢀTheꢀ
1/2
outputꢀrippleꢀ(∆V )ꢀisꢀapproximatedꢀby:
IMAX
OUT
CIN Required IRMS
≈
V
OUT )(
V – V
IN OUT
(1)
ꢀ
(
)
V
IN
1
∆VOUT ≈ ∆IL ESR +
ThisꢀformulaꢀhasꢀaꢀmaximumꢀatꢀV ꢀ=ꢀ2V ,ꢀwhereꢀI
ꢀ
8 • f •C
OUT
IN
OUTꢀ
RMS
ꢀ
=ꢀI /2.ꢀThisꢀsimpleꢀworst-caseꢀconditionꢀisꢀcommonlyꢀ
OUT
whereꢀfꢀisꢀtheꢀoperatingꢀfrequency,ꢀC ꢀisꢀtheꢀoutputꢀ
OUT
usedꢀforꢀdesignꢀbecauseꢀevenꢀsignificantꢀdeviationsꢀdoꢀnotꢀ
offerꢀmuchꢀrelief.ꢀNoteꢀthatꢀcapacitorꢀmanufacturers’ꢀrippleꢀ
currentꢀratingsꢀareꢀoftenꢀbasedꢀonꢀonlyꢀ2000ꢀhoursꢀofꢀlife.ꢀ
Thisꢀmakesꢀitꢀadvisableꢀtoꢀfurtherꢀderateꢀtheꢀcapacitor,ꢀorꢀ
toꢀchooseꢀaꢀcapacitorꢀratedꢀatꢀaꢀhigherꢀtemperatureꢀthanꢀ
required.ꢀSeveralꢀcapacitorsꢀmayꢀbeꢀparalleledꢀtoꢀmeetꢀ
sizeꢀorꢀheightꢀrequirementsꢀinꢀtheꢀdesign.ꢀDueꢀtoꢀtheꢀhighꢀ
operatingꢀfrequencyꢀofꢀtheꢀLTC3857,ꢀceramicꢀcapacitorsꢀ
capacitanceꢀandꢀ∆I ꢀisꢀtheꢀrippleꢀcurrentꢀinꢀtheꢀinductor.ꢀ
L
Theꢀoutputꢀrippleꢀisꢀhighestꢀatꢀmaximumꢀinputꢀvoltageꢀ
sinceꢀ∆I ꢀincreasesꢀwithꢀinputꢀvoltage.
L
Setting Output Voltage
TheꢀLTC3857ꢀoutputꢀvoltagesꢀareꢀeachꢀsetꢀbyꢀanꢀexternalꢀ
feedbackꢀresistorꢀdividerꢀcarefullyꢀplacedꢀacrossꢀtheꢀout-
put,ꢀasꢀshownꢀinꢀFigureꢀ5.ꢀTheꢀregulatedꢀoutputꢀvoltageꢀ
isꢀdeterminedꢀby:
canꢀalsoꢀbeꢀusedꢀforꢀC .ꢀAlwaysꢀconsultꢀtheꢀmanufacturerꢀ
IN
ifꢀthereꢀisꢀanyꢀquestion.
TheꢀbenefitꢀofꢀtheꢀLTC3857ꢀ2-phaseꢀoperationꢀcanꢀbeꢀcal-
culatedꢀbyꢀusingꢀEquationꢀ1ꢀforꢀtheꢀhigherꢀpowerꢀcontrollerꢀ
andꢀthenꢀcalculatingꢀtheꢀlossꢀthatꢀwouldꢀhaveꢀresultedꢀifꢀ
bothꢀcontrollerꢀchannelsꢀswitchedꢀonꢀatꢀtheꢀsameꢀtime.ꢀ
TheꢀtotalꢀRMSꢀpowerꢀlostꢀisꢀlowerꢀwhenꢀbothꢀcontrollersꢀ
areꢀoperatingꢀdueꢀtoꢀtheꢀreducedꢀoverlapꢀofꢀcurrentꢀpulsesꢀ
requiredꢀthroughꢀtheꢀinputꢀcapacitor’sꢀESR.ꢀThisꢀisꢀwhyꢀ
theꢀinputꢀcapacitor’sꢀrequirementꢀcalculatedꢀaboveꢀforꢀtheꢀ
worst-caseꢀcontrollerꢀisꢀadequateꢀforꢀtheꢀdualꢀcontrollerꢀ
design.ꢀAlso,ꢀtheꢀinputꢀprotectionꢀfuseꢀresistance,ꢀbatteryꢀ
resistance,ꢀandꢀPCꢀboardꢀtraceꢀresistanceꢀlossesꢀareꢀalsoꢀ
reducedꢀdueꢀtoꢀtheꢀreducedꢀpeakꢀcurrentsꢀinꢀaꢀ2-phaseꢀ
system.ꢀTheꢀoverallꢀbenefitꢀofꢀaꢀmultiphaseꢀdesignꢀwillꢀ
onlyꢀbeꢀfullyꢀrealizedꢀwhenꢀtheꢀsourceꢀimpedanceꢀofꢀtheꢀ
powerꢀsupply/batteryꢀisꢀincludedꢀinꢀtheꢀefficiencyꢀtesting.ꢀ
RB
R
VOUT = 0.8V 1+
A
ꢀ
Toꢀimproveꢀtheꢀfrequencyꢀresponse,ꢀaꢀfeedforwardꢀca-
pacitor,ꢀC ,ꢀmayꢀbeꢀused.ꢀGreatꢀcareꢀshouldꢀbeꢀtakenꢀtoꢀ
FFꢀ
FB
routeꢀtheꢀV ꢀlineꢀawayꢀfromꢀnoiseꢀsources,ꢀsuchꢀasꢀtheꢀ
inductorꢀorꢀtheꢀSWꢀline.
V
OUT
R
C
FF
1/2 LTC3857
B
V
FB
R
A
3857 F05
Figure 5. Setting Output Voltage
3857fa
ꢀꢈ
LTC3857
applicaTions inForMaTion
Tracking and Soft-Start (TRACK/SS Pins)
V
V
X(MASTER)
Theꢀstart-upꢀofꢀeachꢀV ꢀisꢀcontrolledꢀbyꢀtheꢀvoltageꢀonꢀ
OUT
theꢀrespectiveꢀTRACK/SSꢀpin.ꢀWhenꢀtheꢀvoltageꢀonꢀtheꢀ
TRACK/SSꢀpinꢀisꢀlessꢀthanꢀtheꢀinternalꢀ0.8Vꢀreference,ꢀ
OUT(SLAVE)
theꢀLTC3857ꢀregulatesꢀtheꢀV ꢀpinꢀvoltageꢀtoꢀtheꢀvoltageꢀ
FB
onꢀtheꢀTRACK/SSꢀpinꢀinsteadꢀofꢀ0.8V.ꢀTheꢀTRACK/SSꢀpinꢀ
canꢀbeꢀusedꢀtoꢀprogramꢀanꢀexternalꢀsoft-startꢀfunctionꢀorꢀ
toꢀallowꢀV ꢀtoꢀtrackꢀanotherꢀsupplyꢀduringꢀstart-up.
OUT
3857 F07a
Soft-startꢀisꢀenabledꢀbyꢀsimplyꢀconnectingꢀaꢀcapacitorꢀ
fromꢀtheꢀTRACK/SSꢀpinꢀtoꢀground,ꢀasꢀshownꢀinꢀFigureꢀ6.ꢀ
Anꢀ internalꢀ 1µAꢀ currentꢀ sourceꢀ chargesꢀ theꢀ capacitor,ꢀ
providingꢀaꢀlinearꢀrampingꢀvoltageꢀatꢀtheꢀTRACK/SSꢀpin.ꢀ
TIME
(7a) Coincident Tracking
V
V
X(MASTER)
OUT(SLAVE)
TheꢀLTC3857ꢀwillꢀregulateꢀtheꢀV ꢀpinꢀ(andꢀhenceꢀV )ꢀ
FB
OUT
accordingꢀtoꢀtheꢀvoltageꢀonꢀtheꢀTRACK/SSꢀpin,ꢀallowingꢀ
V
ꢀtoꢀriseꢀsmoothlyꢀfromꢀ0Vꢀtoꢀitsꢀfinalꢀregulatedꢀvalue.ꢀ
OUT
Theꢀtotalꢀsoft-startꢀtimeꢀwillꢀbeꢀapproximately:
0.8V
1µA
tSS = CSS
•
ꢀ
3857 F07b
TIME
1/2 LTC3857
TRACK/SS
(7b) Ratiometric Tracking
C
SS
Figure 7. Two Different Modes of Output Voltage Tracking
SGND
3857 F06
V
V
OUT
x
Figure 6. Using the TRACK/SS Pin to Program Soft-Start
1/2 LTC3857
R
B
V
FB
Alternatively,ꢀtheꢀTRACK/SSꢀpinꢀcanꢀbeꢀusedꢀtoꢀtrackꢀtwoꢀ
(orꢀmore)ꢀsuppliesꢀduringꢀstart-up,ꢀasꢀshownꢀqualita-
tivelyꢀinꢀFiguresꢀ7aꢀandꢀ7b.ꢀToꢀdoꢀthis,ꢀaꢀresistorꢀdividerꢀ
R
A
R
R
TRACKB
TRACK/SS
3857 F08
shouldꢀbeꢀconnectedꢀfromꢀtheꢀmasterꢀsupplyꢀ(V )ꢀtoꢀtheꢀ
TRACKA
X
TRACK/SSꢀpinꢀofꢀtheꢀslaveꢀsupplyꢀ(V ),ꢀasꢀshownꢀinꢀ
OUT
Figureꢀ8.ꢀDuringꢀstart-upꢀV ꢀwillꢀtrackꢀV ꢀaccordingꢀ
OUT
X
Figure 8. Using the TRACK/SS Pin for Tracking
toꢀtheꢀratioꢀsetꢀbyꢀtheꢀresistorꢀdivider:
RTRACKA +RTRACKB
RA +RB
VX
RA
=
•
VOUT RTRACKA
ꢀ
Forꢀcoincidentꢀtrackingꢀ(V ꢀ=ꢀV ꢀduringꢀstart-up):
OUT
X
ꢀ R ꢀ=ꢀR
A
TRACKA
TRACKB
ꢀ R ꢀ=ꢀR
B
3857fa
ꢁ0
EXTV ꢀConnectedꢀtoꢀanꢀExternalꢀSupply.ꢀIfꢀanꢀexternalꢀ
2.ꢀ
3.ꢀ
EXTV ꢀConnectedꢀDirectlyꢀtoꢀV .ꢀThisꢀisꢀtheꢀnormalꢀ
CC OUTꢀ
connectionꢀforꢀaꢀ5Vꢀtoꢀ14Vꢀregulatorꢀandꢀprovidesꢀtheꢀ
highestꢀefficiency.
LTC3857
applicaTions inForMaTion
INTV Regulators
isꢀlessꢀthanꢀ5.1V,ꢀtheꢀLDOꢀisꢀinꢀdropoutꢀandꢀtheꢀINTV ꢀ
CC
CC
voltageꢀisꢀapproximatelyꢀequalꢀtoꢀEXTV .ꢀWhenꢀEXTV ꢀ
CC
CC
TheꢀLTC3857ꢀfeaturesꢀtwoꢀseparateꢀinternalꢀP-channelꢀlowꢀ
dropoutꢀlinearꢀregulatorsꢀ(LDO)ꢀthatꢀsupplyꢀpowerꢀatꢀtheꢀ
INTVCCꢀpinꢀfromꢀeitherꢀtheꢀVINꢀsupplyꢀpinꢀorꢀtheꢀEXTVCCꢀ
isꢀgreaterꢀthanꢀ5.1V,ꢀupꢀtoꢀanꢀabsoluteꢀmaximumꢀofꢀ14V,ꢀ
INTV ꢀisꢀregulatedꢀtoꢀ5.1V.
CC
pinꢀ dependingꢀ onꢀ theꢀ connectionꢀ ofꢀ theꢀ EXTVCCꢀ pin.ꢀ UsingꢀtheꢀEXTV ꢀLDOꢀallowsꢀtheꢀMOSFETꢀdriverꢀandꢀcon-
CC
INTVCCꢀpowersꢀtheꢀgateꢀdriversꢀandꢀmuchꢀofꢀtheꢀLTC3857’sꢀ trolꢀpowerꢀtoꢀbeꢀderivedꢀfromꢀoneꢀofꢀtheꢀLTC3857’sꢀswitch-
internalꢀcircuitry.ꢀTheꢀVINꢀLDOꢀandꢀtheꢀEXTVCCꢀLDOꢀregulateꢀ ingꢀregulatorꢀoutputsꢀ(4.7Vꢀ≤ꢀV ꢀ≤ꢀ14V)ꢀduringꢀnormalꢀ
OUT
INTV ꢀtoꢀ5.1V.ꢀEachꢀofꢀtheseꢀcanꢀsupplyꢀaꢀpeakꢀcurrentꢀofꢀ operationꢀandꢀfromꢀtheꢀV ꢀLDOꢀwhenꢀtheꢀoutputꢀisꢀoutꢀofꢀ
CC
IN
50mAꢀandꢀmustꢀbeꢀbypassedꢀtoꢀgroundꢀwithꢀaꢀminimumꢀ regulationꢀ(e.g.,ꢀstart-up,ꢀshort-circuit).ꢀIfꢀmoreꢀcurrentꢀ
ofꢀ4.7µFꢀceramicꢀcapacitor.ꢀNoꢀmatterꢀwhatꢀtypeꢀofꢀbulkꢀ isꢀrequiredꢀthroughꢀtheꢀEXTV ꢀLDOꢀthanꢀisꢀspecified,ꢀanꢀ
CC
capacitorꢀisꢀused,ꢀanꢀadditionalꢀ1µFꢀceramicꢀcapacitorꢀ externalꢀSchottkyꢀdiodeꢀcanꢀbeꢀaddedꢀbetweenꢀtheꢀEXTV ꢀ
CC
placedꢀdirectlyꢀadjacentꢀtoꢀtheꢀINTV ꢀandꢀPGNDꢀpinsꢀisꢀ andꢀINTV ꢀpins.ꢀInꢀthisꢀcase,ꢀdoꢀnotꢀapplyꢀmoreꢀthanꢀ6Vꢀ
CC
CC
highlyꢀrecommended.ꢀGoodꢀbypassingꢀisꢀneededꢀtoꢀsupplyꢀ toꢀtheꢀEXTV ꢀpinꢀandꢀmakeꢀsureꢀthatꢀEXTV ꢀ≤ꢀV .
CC
CC
IN
theꢀhighꢀtransientꢀcurrentsꢀrequiredꢀbyꢀtheꢀMOSFETꢀgateꢀ
driversꢀandꢀtoꢀpreventꢀinteractionꢀbetweenꢀtheꢀchannels.
Significantꢀefficiencyꢀandꢀthermalꢀgainsꢀcanꢀbeꢀrealizedꢀ
byꢀpoweringꢀINTV ꢀfromꢀtheꢀoutput,ꢀsinceꢀtheꢀV ꢀcur-
CC
IN
HighꢀinputꢀvoltageꢀapplicationsꢀinꢀwhichꢀlargeꢀMOSFETsꢀ rentꢀresultingꢀfromꢀtheꢀdriverꢀandꢀcontrolꢀcurrentsꢀwillꢀbeꢀ
areꢀ beingꢀ drivenꢀ atꢀ highꢀ frequenciesꢀ mayꢀ causeꢀ theꢀ scaledꢀbyꢀaꢀfactorꢀofꢀ(DutyꢀCycle)/(SwitcherꢀEfficiency).ꢀ
maximumꢀjunctionꢀtemperatureꢀratingꢀforꢀtheꢀLTC3857ꢀ Forꢀ5Vꢀtoꢀ14Vꢀregulatorꢀoutputs,ꢀthisꢀmeansꢀconnectingꢀ
toꢀbeꢀexceeded.ꢀTheꢀINTV ꢀcurrent,ꢀwhichꢀisꢀdominatedꢀ theꢀEXTV ꢀpinꢀdirectlyꢀtoꢀV .ꢀTyingꢀtheꢀEXTV ꢀpinꢀtoꢀ
CC
CC
OUTꢀ
CC
byꢀtheꢀgateꢀchargeꢀcurrent,ꢀmayꢀbeꢀsuppliedꢀbyꢀeitherꢀtheꢀ anꢀ8.5Vꢀsupplyꢀreducesꢀtheꢀjunctionꢀtemperatureꢀinꢀtheꢀ
V ꢀLDOꢀorꢀtheꢀEXTV ꢀLDO.ꢀWhenꢀtheꢀvoltageꢀonꢀtheꢀ previousꢀexampleꢀfromꢀ125°Cꢀto:
IN
CC
EXTV ꢀpinꢀisꢀlessꢀthanꢀ4.7V,ꢀtheꢀV ꢀLDOꢀisꢀenabled.ꢀPowerꢀ
CC
IN
ꢀ T ꢀ=ꢀ70°Cꢀ+ꢀ(32mA)(8.5V)(43°C/W)ꢀ=ꢀ82°C
J
dissipationꢀforꢀtheꢀICꢀinꢀthisꢀcaseꢀisꢀhighestꢀandꢀisꢀequalꢀ
toꢀV ꢀ•ꢀI .ꢀTheꢀgateꢀchargeꢀcurrentꢀisꢀdependentꢀ
However,ꢀforꢀ3.3Vꢀandꢀotherꢀlowꢀvoltageꢀoutputs,ꢀaddi-
IN
INTVCC
tionalꢀcircuitryꢀisꢀrequiredꢀtoꢀderiveꢀINTV ꢀpowerꢀfromꢀ
onꢀ operatingꢀ frequencyꢀ asꢀ discussedꢀ inꢀ theꢀ Efficiencyꢀ
Considerationsꢀ section.ꢀ Theꢀ junctionꢀ temperatureꢀ canꢀ
beꢀestimatedꢀbyꢀusingꢀtheꢀequationsꢀgivenꢀinꢀNoteꢀ3ꢀofꢀ
theꢀElectricalꢀCharacteristics.ꢀForꢀexample,ꢀtheꢀLTC3857ꢀ
CC
theꢀoutput.
Theꢀfollowingꢀlistꢀsummarizesꢀtheꢀfourꢀpossibleꢀconnec-
tionsꢀforꢀEXTV :
CC
INTV ꢀcurrentꢀisꢀlimitedꢀtoꢀlessꢀthanꢀ32mAꢀfromꢀaꢀ40Vꢀ
CC
1.ꢀEXTV ꢀLeftꢀOpenꢀ(orꢀGrounded).ꢀThisꢀwillꢀcauseꢀINTV ꢀ
CC
CC
supplyꢀwhenꢀnotꢀusingꢀtheꢀEXTV ꢀsupplyꢀatꢀ70°Cꢀambi-
CC
toꢀbeꢀpoweredꢀfromꢀtheꢀinternalꢀ5.1Vꢀregulatorꢀresult-
ingꢀinꢀanꢀefficiencyꢀpenaltyꢀofꢀupꢀtoꢀ10%ꢀatꢀhighꢀinputꢀ
voltages.
entꢀtemperature:
ꢀ T ꢀ=ꢀ70°Cꢀ+ꢀ(32mA)(40V)(43°C/W)ꢀ=ꢀ125°C
J
Toꢀpreventꢀtheꢀmaximumꢀjunctionꢀtemperatureꢀfromꢀbe-
ingꢀexceeded,ꢀtheꢀinputꢀsupplyꢀcurrentꢀmustꢀbeꢀcheckedꢀ
whileꢀoperatingꢀinꢀforcedꢀcontinuousꢀmodeꢀ(PLLIN/MODEꢀ
=ꢀINTV )ꢀatꢀmaximumꢀV .
CC
IN
CC
WhenꢀtheꢀvoltageꢀappliedꢀtoꢀEXTV ꢀrisesꢀaboveꢀ4.7V,ꢀtheꢀ
CC
supplyꢀisꢀavailableꢀinꢀtheꢀ5Vꢀtoꢀ14Vꢀrange,ꢀitꢀmayꢀbeꢀ
V ꢀLDOꢀisꢀturnedꢀoffꢀandꢀtheꢀEXTV ꢀLDOꢀisꢀenabled.ꢀTheꢀ
IN
CC
usedꢀtoꢀpowerꢀEXTV ꢀprovidingꢀitꢀisꢀcompatibleꢀwithꢀtheꢀ
CC
EXTV ꢀLDOꢀremainsꢀonꢀasꢀlongꢀasꢀtheꢀvoltageꢀappliedꢀtoꢀ
CC
MOSFETꢀgateꢀdriveꢀrequirements.ꢀEnsureꢀthatꢀEXTV ꢀ
CC
EXTV ꢀremainsꢀaboveꢀ4.5V.ꢀTheꢀEXTV ꢀLDOꢀattemptsꢀ
CC
CC
<ꢀV .
IN
toꢀregulateꢀtheꢀINTV ꢀvoltageꢀtoꢀ5.1V,ꢀsoꢀwhileꢀEXTV ꢀ
CC
CC
3857fa
ꢁꢀ
4.ꢀ
EXTV ꢀConnectedꢀtoꢀanꢀOutput-DerivedꢀBoostꢀNetwork.ꢀ
LTC3857
applicaTions inForMaTion
Fault Conditions: Current Limit and Current Foldback
C
IN
TheꢀLTC3857ꢀincludesꢀcurrentꢀfoldbackꢀtoꢀhelpꢀlimitꢀloadꢀ
currentꢀwhenꢀtheꢀoutputꢀisꢀshortedꢀtoꢀground.ꢀIfꢀtheꢀoutputꢀ
voltageꢀfallsꢀbelowꢀ70%ꢀofꢀitsꢀnominalꢀoutputꢀlevel,ꢀthenꢀ
theꢀmaximumꢀsenseꢀvoltageꢀisꢀprogressivelyꢀloweredꢀtoꢀ
aboutꢀhalfꢀofꢀitsꢀmaximumꢀselectedꢀvalue.ꢀUnderꢀshort-
circuitꢀconditionsꢀwithꢀveryꢀlowꢀdutyꢀcycles,ꢀtheꢀLTC3857ꢀ
willꢀbeginꢀcycleꢀskippingꢀinꢀorderꢀtoꢀlimitꢀtheꢀshort-circuitꢀ
current.ꢀ Inꢀ thisꢀ situationꢀ theꢀ bottomꢀ MOSFETꢀ willꢀ beꢀ
dissipatingꢀmostꢀofꢀtheꢀpowerꢀbutꢀlessꢀthanꢀinꢀnormalꢀ
operation.ꢀTheꢀshort-circuitꢀrippleꢀcurrentꢀisꢀdeterminedꢀbyꢀ
BAT85
BAT85
BAT85
V
IN
MTOP
MBOT
VN2222LL
TG1
1/2 LTC3857
L
R
SENSE
V
EXTV
SW
OUT
CC
C
BG1
OUT
3857 F09
PGND
theꢀminimumꢀon-time,ꢀt
,ꢀofꢀtheꢀLTC3857ꢀ(≈90ns),ꢀ
ON(MIN)
Figure 9. Capacitive Charge Pump for EXTVCC
theꢀinputꢀvoltageꢀandꢀinductorꢀvalue:
IN
V
CC
∆IL(SC) = tON(MIN)
L
Forꢀ3.3Vꢀandꢀotherꢀlowꢀvoltageꢀregulators,ꢀefficiencyꢀ
ꢀ
gainsꢀcanꢀstillꢀbeꢀrealizedꢀbyꢀconnectingꢀEXTV ꢀtoꢀanꢀ
CC
Theꢀresultingꢀaverageꢀshort-circuitꢀcurrentꢀis:
output-derivedꢀvoltageꢀthatꢀhasꢀbeenꢀboostedꢀtoꢀgreaterꢀ
thanꢀ4.7V.ꢀThisꢀcanꢀbeꢀdoneꢀwithꢀtheꢀcapacitiveꢀchargeꢀ
1
2
ISC ≈ 50% •ILIM(MAX) – ∆IL(SC)
pumpꢀshownꢀinꢀFigureꢀ9.ꢀEnsureꢀthatꢀEXTV ꢀ<ꢀV .
CC
IN
ꢀ
Topside MOSFET Driver Supply (C , D )
B
B
Fault Conditions: Overvoltage Protection (Crowbar)
Externalꢀbootstrapꢀcapacitors,ꢀC ,ꢀconnectedꢀtoꢀtheꢀBOOSTꢀ
B
Theꢀovervoltageꢀcrowbarꢀisꢀdesignedꢀtoꢀblowꢀaꢀsystemꢀ
inputꢀfuseꢀwhenꢀtheꢀoutputꢀvoltageꢀofꢀtheꢀregulatorꢀrisesꢀ
muchꢀhigherꢀthanꢀnominalꢀlevels.ꢀTheꢀcrowbarꢀcausesꢀhugeꢀ
currentsꢀtoꢀflow,ꢀthatꢀblowꢀtheꢀfuseꢀtoꢀprotectꢀagainstꢀaꢀ
shortedꢀtopꢀMOSFETꢀifꢀtheꢀshortꢀoccursꢀwhileꢀtheꢀcontrol-
lerꢀisꢀoperating.
pinsꢀsupplyꢀtheꢀgateꢀdriveꢀvoltagesꢀforꢀtheꢀtopsideꢀMOSFETs.ꢀ
CapacitorꢀC ꢀinꢀtheꢀFunctionalꢀDiagramꢀisꢀchargedꢀthoughꢀ
B
externalꢀdiodeꢀD ꢀfromꢀINTV ꢀwhenꢀtheꢀSWꢀpinꢀisꢀlow.ꢀ
B
CC
WhenꢀoneꢀofꢀtheꢀtopsideꢀMOSFETsꢀisꢀtoꢀbeꢀturnedꢀon,ꢀtheꢀ
driverꢀplacesꢀtheꢀC ꢀvoltageꢀacrossꢀtheꢀgate-sourceꢀofꢀtheꢀ
B
desiredꢀMOSFET.ꢀThisꢀenhancesꢀtheꢀtopꢀMOSFETꢀswitchꢀ
Aꢀcomparatorꢀmonitorsꢀtheꢀoutputꢀforꢀovervoltageꢀcondi-
tions.ꢀTheꢀcomparatorꢀdetectsꢀfaultsꢀgreaterꢀthanꢀ10%ꢀ
aboveꢀtheꢀnominalꢀoutputꢀvoltage.ꢀWhenꢀthisꢀconditionꢀ
isꢀsensed,ꢀtheꢀtopꢀMOSFETꢀisꢀturnedꢀoffꢀandꢀtheꢀbottomꢀ
MOSFETꢀisꢀturnedꢀonꢀuntilꢀtheꢀovervoltageꢀconditionꢀisꢀ
cleared.ꢀTheꢀbottomꢀMOSFETꢀremainsꢀonꢀcontinuouslyꢀ
andꢀturnsꢀitꢀon.ꢀTheꢀswitchꢀnodeꢀvoltage,ꢀSW,ꢀrisesꢀtoꢀV ꢀ
IN
andꢀtheꢀBOOSTꢀpinꢀfollows.ꢀWithꢀtheꢀtopsideꢀMOSFETꢀ
on,ꢀtheꢀboostꢀvoltageꢀisꢀaboveꢀtheꢀinputꢀsupply:ꢀV
ꢀ=ꢀ
BOOST
V ꢀ+ꢀV
.ꢀTheꢀvalueꢀofꢀtheꢀboostꢀcapacitor,ꢀC ,ꢀneedsꢀ
IN
INTVCC
B
toꢀbeꢀ100ꢀtimesꢀthatꢀofꢀtheꢀtotalꢀinputꢀcapacitanceꢀofꢀtheꢀ
topsideꢀMOSFET(s).ꢀTheꢀreverseꢀbreakdownꢀofꢀtheꢀexternalꢀ
forꢀasꢀlongꢀasꢀtheꢀovervoltageꢀconditionꢀpersists;ꢀifꢀV
ꢀ
SchottkyꢀdiodeꢀmustꢀbeꢀgreaterꢀthanꢀV
.ꢀ
OUT
IN(MAX)
returnsꢀtoꢀaꢀsafeꢀlevel,ꢀnormalꢀoperationꢀautomaticallyꢀ
resumes.ꢀ
Whenꢀadjustingꢀtheꢀgateꢀdriveꢀlevel,ꢀtheꢀfinalꢀarbiterꢀisꢀtheꢀ
totalꢀinputꢀcurrentꢀforꢀtheꢀregulator.ꢀIfꢀaꢀchangeꢀisꢀmadeꢀ
andꢀtheꢀinputꢀcurrentꢀdecreases,ꢀthenꢀtheꢀefficiencyꢀhasꢀ
improved.ꢀIfꢀthereꢀisꢀnoꢀchangeꢀinꢀinputꢀcurrent,ꢀthenꢀthereꢀ
isꢀnoꢀchangeꢀinꢀefficiency.
AꢀshortedꢀtopꢀMOSFETꢀwillꢀresultꢀinꢀaꢀhighꢀcurrentꢀconditionꢀ
whichꢀwillꢀopenꢀtheꢀsystemꢀfuse.ꢀTheꢀswitchingꢀregulatorꢀ
willꢀregulateꢀproperlyꢀwithꢀaꢀleakyꢀtopꢀMOSFETꢀbyꢀalteringꢀ
theꢀdutyꢀcycleꢀtoꢀaccommodateꢀtheꢀleakage.
3857fa
ꢁꢁ
LTC3857
applicaTions inForMaTion
Phase-Locked Loop and Frequency Synchronization
1000
900
800
700
600
500
400
300
200
100
0
TheꢀLTC3857ꢀhasꢀanꢀinternalꢀphase-lockedꢀloopꢀ(PLL)ꢀ
comprisedꢀofꢀaꢀphaseꢀfrequencyꢀdetector,ꢀaꢀlowpassꢀfilter,ꢀ
andꢀaꢀvoltage-controlledꢀoscillatorꢀ(VCO).ꢀThisꢀallowsꢀtheꢀ
turn-onꢀofꢀtheꢀtopꢀMOSFETꢀofꢀcontrollerꢀ1ꢀtoꢀbeꢀlockedꢀtoꢀ
theꢀrisingꢀedgeꢀofꢀanꢀexternalꢀclockꢀsignalꢀappliedꢀtoꢀtheꢀ
PLLIN/MODEꢀpin.ꢀTheꢀturn-onꢀofꢀcontrollerꢀ2’sꢀtopꢀMOSFETꢀ
isꢀthusꢀ180ꢀdegreesꢀoutꢀofꢀphaseꢀwithꢀtheꢀexternalꢀclock.ꢀ
Theꢀphaseꢀdetectorꢀisꢀanꢀedgeꢀsensitiveꢀdigitalꢀtypeꢀthatꢀ
providesꢀzeroꢀdegreesꢀphaseꢀshiftꢀbetweenꢀtheꢀexternalꢀ
andꢀinternalꢀoscillators.ꢀThisꢀtypeꢀofꢀphaseꢀdetectorꢀdoesꢀ
notꢀexhibitꢀfalseꢀlockꢀtoꢀharmonicsꢀofꢀtheꢀexternalꢀclock.
15 25 35 45 55 65 75 85 95 105 115 125
FREQ PIN RESISTOR (kΩ)
3857 F10
Figure 10. Relationship Between Oscillator Frequency
and Resistor Value at the FREQ Pin
Ifꢀtheꢀexternalꢀclockꢀfrequencyꢀisꢀgreaterꢀthanꢀtheꢀinter-
nalꢀoscillator’sꢀfrequency,ꢀf ,ꢀthenꢀcurrentꢀisꢀsourcedꢀ
OSC
continuouslyꢀ fromꢀ theꢀ phaseꢀ detectorꢀ output,ꢀ pullingꢀ
andꢀsynchronization.ꢀAlthoughꢀitꢀisꢀnotꢀrequiredꢀthatꢀtheꢀ
free-runningꢀfrequencyꢀbeꢀnearꢀexternalꢀclockꢀfrequency,ꢀ
doingꢀsoꢀwillꢀpreventꢀtheꢀoperatingꢀfrequencyꢀfromꢀpassingꢀ
throughꢀaꢀlargeꢀrangeꢀofꢀfrequenciesꢀasꢀtheꢀPLLꢀlocks.ꢀ
upꢀ theꢀ VCOꢀ input.ꢀ Whenꢀ theꢀ externalꢀ clockꢀ frequencyꢀ
isꢀlessꢀthanꢀf ,ꢀcurrentꢀisꢀsunkꢀcontinuously,ꢀpullingꢀ
OSC
downꢀ theꢀ VCOꢀ input.ꢀ Ifꢀ theꢀ externalꢀ andꢀ internalꢀ fre-
quenciesꢀ areꢀ theꢀ sameꢀ butꢀ exhibitꢀ aꢀ phaseꢀ difference,ꢀ
theꢀ currentꢀ sourcesꢀ turnꢀ onꢀ forꢀ anꢀ amountꢀ ofꢀ timeꢀ
correspondingꢀtoꢀtheꢀphaseꢀdifference.ꢀTheꢀvoltageꢀatꢀtheꢀ
VCOꢀinputꢀisꢀadjustedꢀuntilꢀtheꢀphaseꢀandꢀfrequencyꢀofꢀ
theꢀinternalꢀandꢀexternalꢀoscillatorsꢀareꢀidentical.ꢀAtꢀtheꢀ
stableꢀoperatingꢀpoint,ꢀtheꢀphaseꢀdetectorꢀoutputꢀisꢀhighꢀ
Tableꢀ2ꢀsummarizesꢀtheꢀdifferentꢀstatesꢀinꢀwhichꢀtheꢀFREQꢀ
pinꢀcanꢀbeꢀused.
Table 2
FREQ PIN
PLLIN/MODE PIN
DCꢀVoltage
FREQUENCY
350kHz
0V
impedanceꢀandꢀtheꢀinternalꢀfilterꢀcapacitor,ꢀC ,ꢀholdsꢀtheꢀ
LPꢀ
INTV
DCꢀVoltage
535kHz
CC
voltageꢀatꢀtheꢀVCOꢀinput.
Resistor
DCꢀVoltage
50kHz-900kHz
NoteꢀthatꢀtheꢀLTC3857ꢀcanꢀonlyꢀbeꢀsynchronizedꢀtoꢀanꢀ
externalꢀ clockꢀ whoseꢀ frequencyꢀ isꢀ withinꢀ rangeꢀ ofꢀ theꢀ
LTC3857’sꢀinternalꢀVCO,ꢀwhichꢀisꢀnominallyꢀ55kHzꢀtoꢀ1MHz.ꢀ
Thisꢀisꢀguaranteedꢀtoꢀbeꢀbetweenꢀ75kHzꢀandꢀ850kHz.
AnyꢀofꢀtheꢀAbove
ExternalꢀClock
Phase–Lockedꢀtoꢀ
ExternalꢀClock
Minimum On-Time Considerations
Minimumꢀon-time,ꢀt ,ꢀisꢀtheꢀsmallestꢀtimeꢀdurationꢀ
thatꢀtheꢀLTC3857ꢀisꢀcapableꢀofꢀturningꢀonꢀtheꢀtopꢀMOSFET.ꢀ
Itꢀisꢀdeterminedꢀbyꢀinternalꢀtimingꢀdelaysꢀandꢀtheꢀgateꢀ
chargeꢀrequiredꢀtoꢀturnꢀonꢀtheꢀtopꢀMOSFET.ꢀLowꢀdutyꢀ
cycleꢀapplicationsꢀmayꢀapproachꢀthisꢀminimumꢀon-timeꢀ
limitꢀandꢀcareꢀshouldꢀbeꢀtakenꢀtoꢀensureꢀthat:
ON(MIN)
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
synchronizationꢀfrequency.ꢀTheꢀVCO’sꢀinputꢀvoltageꢀisꢀ
prebiasedꢀatꢀaꢀfrequencyꢀcorrespondingꢀtoꢀtheꢀfrequencyꢀ
setꢀbyꢀtheꢀFREQꢀpin.ꢀOnceꢀprebiased,ꢀtheꢀPLLꢀonlyꢀneedsꢀ
toꢀadjustꢀtheꢀfrequencyꢀslightlyꢀtoꢀachieveꢀphaseꢀlockꢀ
VOUT
tON(MIN)
<
V
IN
f
ꢀ
3857fa
ꢁꢂ
LTC3857
applicaTions inForMaTion
Ifꢀtheꢀdutyꢀcycleꢀfallsꢀbelowꢀwhatꢀcanꢀbeꢀaccommodatedꢀ
byꢀtheꢀminimumꢀon-time,ꢀtheꢀcontrollerꢀwillꢀbeginꢀtoꢀskipꢀ
cycles.ꢀTheꢀoutputꢀvoltageꢀwillꢀcontinueꢀtoꢀbeꢀregulated,ꢀ
butꢀtheꢀrippleꢀvoltageꢀandꢀcurrentꢀwillꢀincrease.
outꢀofꢀINTV ꢀthatꢀisꢀtypicallyꢀmuchꢀlargerꢀthanꢀtheꢀ
CC
controlꢀcircuitꢀcurrent.ꢀInꢀcontinuousꢀmode,ꢀI
ꢀ
GATECHG
=ꢀf(Q ꢀ+ꢀQ ),ꢀwhereꢀQ ꢀandꢀQ ꢀareꢀtheꢀgateꢀchargesꢀofꢀ
T
B
T
B
theꢀtopsideꢀandꢀbottomꢀsideꢀMOSFETs.
Theꢀminimumꢀon-timeꢀforꢀtheꢀLTC3857ꢀisꢀapproximatelyꢀ
95ns.ꢀHowever,ꢀasꢀtheꢀpeakꢀsenseꢀvoltageꢀdecreasesꢀtheꢀ
minimumꢀon-timeꢀgraduallyꢀincreasesꢀupꢀtoꢀaboutꢀ130ns.ꢀ
Thisꢀisꢀofꢀparticularꢀconcernꢀinꢀforcedꢀcontinuousꢀapplica-
tionsꢀwithꢀlowꢀrippleꢀcurrentꢀatꢀlightꢀloads.ꢀIfꢀtheꢀdutyꢀcycleꢀ
dropsꢀbelowꢀtheꢀminimumꢀon-timeꢀlimitꢀinꢀthisꢀsituation,ꢀ
aꢀsignificantꢀamountꢀofꢀcycleꢀskippingꢀcanꢀoccurꢀwithꢀcor-
respondinglyꢀlargerꢀcurrentꢀandꢀvoltageꢀripple.
ꢀ SupplyingꢀINTV ꢀfromꢀanꢀoutput-derivedꢀpowerꢀsourceꢀ
CC
throughꢀ EXTV ꢀ willꢀ scaleꢀ theꢀ V ꢀ currentꢀ requiredꢀ
CC
IN
forꢀtheꢀdriverꢀandꢀcontrolꢀcircuitsꢀbyꢀaꢀfactorꢀofꢀ(Dutyꢀ
Cycle)/(Efficiency).ꢀForꢀexample,ꢀinꢀaꢀ20Vꢀtoꢀ5Vꢀapplica-
tion,ꢀ10mAꢀofꢀINTV ꢀcurrentꢀresultsꢀinꢀapproximatelyꢀ
CC
2.5mAꢀofꢀV ꢀcurrent.ꢀThisꢀreducesꢀtheꢀmidcurrentꢀlossꢀ
IN
fromꢀ10%ꢀorꢀmoreꢀ(ifꢀtheꢀdriverꢀwasꢀpoweredꢀdirectlyꢀ
fromꢀV )ꢀtoꢀonlyꢀaꢀfewꢀpercent.
IN
2
3.ꢀI RꢀlossesꢀareꢀpredictedꢀfromꢀtheꢀDCꢀresistancesꢀofꢀtheꢀ
Efficiency Considerations
fuseꢀ(ifꢀused),ꢀMOSFET,ꢀinductor,ꢀcurrentꢀsenseꢀresis-
torꢀandꢀinputꢀandꢀoutputꢀcapacitorꢀESR.ꢀInꢀcontinuousꢀ
modeꢀtheꢀaverageꢀoutputꢀcurrentꢀflowsꢀthroughꢀLꢀandꢀ
Theꢀpercentꢀefficiencyꢀofꢀaꢀswitchingꢀregulatorꢀisꢀequalꢀtoꢀ
theꢀoutputꢀpowerꢀdividedꢀbyꢀtheꢀinputꢀpowerꢀtimesꢀ100%.ꢀ
Itꢀisꢀoftenꢀusefulꢀtoꢀanalyzeꢀindividualꢀlossesꢀtoꢀdetermineꢀ
whatꢀisꢀlimitingꢀtheꢀefficiencyꢀandꢀwhichꢀchangeꢀwouldꢀ
produceꢀtheꢀmostꢀimprovement.ꢀPercentꢀefficiencyꢀcanꢀ
beꢀexpressedꢀas:
R
,ꢀbutꢀisꢀchoppedꢀbetweenꢀtheꢀtopsideꢀMOSFETꢀ
SENSE
andꢀtheꢀsynchronousꢀMOSFET.ꢀIfꢀtheꢀtwoꢀMOSFETsꢀhaveꢀ
approximatelyꢀtheꢀsameꢀR
,ꢀthenꢀtheꢀresistanceꢀ
DS(ON)
ofꢀoneꢀMOSFETꢀcanꢀsimplyꢀbeꢀsummedꢀwithꢀtheꢀresis-
2
tancesꢀofꢀL,ꢀR
example,ꢀifꢀeachꢀR
ꢀandꢀESRꢀtoꢀobtainꢀI Rꢀlosses.ꢀForꢀ
SENSE
ꢀ %Efficiencyꢀ=ꢀ100%ꢀ–ꢀ(L1ꢀ+ꢀL2ꢀ+ꢀL3ꢀ+ꢀ...)
ꢀ=ꢀ30mΩ,ꢀR ꢀ=ꢀ50mΩ,ꢀR
ꢀ
DS(ON)
L
SENSE
whereꢀL1,ꢀL2,ꢀetc.ꢀareꢀtheꢀindividualꢀlossesꢀasꢀaꢀpercent-
ageꢀofꢀinputꢀpower.
=ꢀ10mΩꢀandꢀR ꢀ=ꢀ40mΩꢀ(sumꢀofꢀbothꢀinputꢀandꢀ
ESR
outputꢀcapacitanceꢀlosses),ꢀthenꢀtheꢀtotalꢀresistanceꢀ
isꢀ130mΩ.ꢀThisꢀresultsꢀinꢀlossesꢀrangingꢀfromꢀ3%ꢀtoꢀ
13%ꢀasꢀtheꢀoutputꢀcurrentꢀincreasesꢀfromꢀ1Aꢀtoꢀ5Aꢀforꢀ
aꢀ5Vꢀoutput,ꢀorꢀaꢀ4%ꢀtoꢀ20%ꢀlossꢀforꢀaꢀ3.3Vꢀoutput.ꢀ
Althoughꢀallꢀdissipativeꢀelementsꢀinꢀtheꢀcircuitꢀproduceꢀ
losses,ꢀfourꢀmainꢀsourcesꢀusuallyꢀaccountꢀforꢀmostꢀofꢀtheꢀ
lossesꢀinꢀLTC3857ꢀcircuits:ꢀ1)ꢀICꢀV ꢀcurrent,ꢀ2)ꢀINTV
IN
CCꢀ
2
EfficiencyꢀvariesꢀasꢀtheꢀinverseꢀsquareꢀofꢀV ꢀforꢀtheꢀ
regulatorꢀ current,ꢀ 3)ꢀ I Rꢀ losses,ꢀ 4)ꢀ topsideꢀ MOSFETꢀ
OUT
sameꢀexternalꢀcomponentsꢀandꢀoutputꢀpowerꢀlevel.ꢀTheꢀ
combinedꢀeffectsꢀofꢀincreasinglyꢀlowerꢀoutputꢀvoltagesꢀ
andꢀhigherꢀcurrentsꢀrequiredꢀbyꢀhighꢀperformanceꢀdigitalꢀ
systemsꢀisꢀnotꢀdoublingꢀbutꢀquadruplingꢀtheꢀimportanceꢀ
ofꢀlossꢀtermsꢀinꢀtheꢀswitchingꢀregulatorꢀsystem!
transitionꢀlosses.
1.ꢀTheꢀV ꢀcurrentꢀisꢀtheꢀDCꢀinputꢀsupplyꢀcurrentꢀgivenꢀ
IN
inꢀtheꢀElectricalꢀCharacteristicsꢀtable,ꢀwhichꢀexcludesꢀ
MOSFETꢀdriverꢀandꢀcontrolꢀcurrents.ꢀV ꢀcurrentꢀtypi-
IN
callyꢀresultsꢀinꢀaꢀsmallꢀ(<0.1%)ꢀloss.
4.ꢀTransitionꢀlossesꢀapplyꢀonlyꢀtoꢀtheꢀtopsideꢀMOSFET(s),ꢀ
andꢀbecomeꢀsignificantꢀonlyꢀwhenꢀoperatingꢀatꢀhighꢀ
2.ꢀINTV ꢀcurrentꢀisꢀtheꢀsumꢀofꢀtheꢀMOSFETꢀdriverꢀandꢀ
CC
controlꢀcurrents.ꢀTheꢀMOSFETꢀdriverꢀcurrentꢀresultsꢀ
fromꢀ switchingꢀ theꢀ gateꢀ capacitanceꢀ ofꢀ theꢀ powerꢀ
MOSFETs.ꢀEachꢀtimeꢀaꢀMOSFETꢀgateꢀisꢀswitchedꢀfromꢀ
lowꢀtoꢀhighꢀtoꢀlowꢀagain,ꢀaꢀpacketꢀofꢀcharge,ꢀdQ,ꢀmovesꢀ
inputꢀ voltagesꢀ (t
ypicallyꢀ 15Vꢀ orꢀ greater).ꢀ Transitionꢀ
lossesꢀcanꢀbeꢀestimatedꢀfrom:
ꢀ ꢀ TransitionꢀLossꢀ=ꢀ(1.7)ꢀ•ꢀV ꢀ•ꢀ2ꢀ•ꢀI
ꢀ•ꢀC ꢀ•ꢀf
IN
O(MAX)
RSS
fromꢀINTV ꢀtoꢀground.ꢀTheꢀresultingꢀdQ/dtꢀisꢀaꢀcurrentꢀ
CC
3857fa
ꢁꢃ
LTC3857
applicaTions inForMaTion
ꢀ Otherꢀhiddenꢀlossesꢀsuchꢀasꢀcopperꢀtraceꢀandꢀinternalꢀ
batteryꢀresistancesꢀcanꢀaccountꢀforꢀanꢀadditionalꢀ5%ꢀ
toꢀ10%ꢀefficiencyꢀdegradationꢀinꢀportableꢀsystems.ꢀItꢀ
isꢀveryꢀimportantꢀtoꢀincludeꢀtheseꢀsystemꢀlevelꢀlossesꢀ
duringꢀtheꢀdesignꢀphase.ꢀTheꢀinternalꢀbatteryꢀandꢀfuseꢀ
resistanceꢀlossesꢀcanꢀbeꢀminimizedꢀbyꢀmakingꢀsureꢀthatꢀ
TheꢀI ꢀseriesꢀR -C ꢀfilterꢀsetsꢀtheꢀdominantꢀpole-zeroꢀ
TH
C
C
loopꢀcompensation.ꢀTheꢀvaluesꢀcanꢀbeꢀmodifiedꢀslightlyꢀ
(fromꢀ0.5ꢀtoꢀ2ꢀtimesꢀtheirꢀsuggestedꢀvalues)ꢀtoꢀoptimizeꢀ
transientꢀresponseꢀonceꢀtheꢀfinalꢀPCꢀlayoutꢀisꢀdoneꢀandꢀ
theꢀparticularꢀoutputꢀcapacitorꢀtypeꢀandꢀvalueꢀhaveꢀbeenꢀ
determined.ꢀTheꢀoutputꢀcapacitorsꢀneedꢀtoꢀbeꢀselectedꢀ
becauseꢀtheꢀvariousꢀtypesꢀandꢀvaluesꢀdetermineꢀtheꢀloopꢀ
gainꢀandꢀphase.ꢀAnꢀoutputꢀcurrentꢀpulseꢀofꢀ20%ꢀtoꢀ80%ꢀ
ofꢀfull-loadꢀcurrentꢀhavingꢀaꢀriseꢀtimeꢀofꢀ1µsꢀtoꢀ10µsꢀwillꢀ
C ꢀhasꢀadequateꢀchargeꢀstorageꢀandꢀveryꢀlowꢀESRꢀatꢀ
IN
theꢀswitchingꢀfrequency.ꢀAꢀ25Wꢀsupplyꢀwillꢀtypicallyꢀ
requireꢀ aꢀ minimumꢀ ofꢀ 20µFꢀ toꢀ 40µFꢀ ofꢀ capacitanceꢀ
havingꢀaꢀmaximumꢀofꢀ20mΩꢀtoꢀ50mΩꢀofꢀESR.ꢀTheꢀ
LTC3857ꢀ2-phaseꢀarchitectureꢀtypicallyꢀhalvesꢀthisꢀinputꢀ
capacitanceꢀ requirementꢀ overꢀ competingꢀ solutions.ꢀ
Otherꢀ lossesꢀ includingꢀ Schottkyꢀ conductionꢀ lossesꢀ
duringꢀdead-timeꢀandꢀinductorꢀcoreꢀlossesꢀgenerallyꢀ
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)ꢀ
isꢀwhyꢀitꢀisꢀbetterꢀtoꢀlookꢀatꢀtheꢀI ꢀpinꢀsignalꢀwhichꢀisꢀinꢀ
TH
loadꢀcurrent.ꢀWhenꢀaꢀloadꢀstepꢀoccurs,ꢀV ꢀshiftsꢀbyꢀ
OUT
theꢀfeedbackꢀloopꢀandꢀisꢀtheꢀfilteredꢀandꢀcompensatedꢀ
anꢀamountꢀequalꢀtoꢀ∆I
ꢀ(ESR),ꢀwhereꢀESRꢀisꢀtheꢀef-
LOAD
controlꢀloopꢀresponse.ꢀ
fectiveꢀseriesꢀresistanceꢀofꢀC .ꢀ∆I
ꢀalsoꢀbeginsꢀtoꢀ
OUTꢀ
LOAD
TheꢀgainꢀofꢀtheꢀloopꢀwillꢀbeꢀincreasedꢀbyꢀincreasingꢀR ꢀ
C
chargeꢀorꢀdischargeꢀC ꢀgeneratingꢀtheꢀfeedbackꢀerrorꢀ
OUT
andꢀtheꢀbandwidthꢀofꢀtheꢀloopꢀwillꢀbeꢀincreasedꢀbyꢀde-
signalꢀthatꢀforcesꢀtheꢀregulatorꢀtoꢀadaptꢀtoꢀtheꢀcurrentꢀ
creasingꢀC .ꢀIfꢀR ꢀisꢀincreasedꢀbyꢀtheꢀsameꢀfactorꢀthatꢀC ꢀ
C
C
C
changeꢀandꢀreturnꢀV ꢀtoꢀitsꢀsteady-stateꢀvalue.ꢀDuringꢀ
OUT
OUT
isꢀdecreased,ꢀtheꢀzeroꢀfrequencyꢀwillꢀbeꢀkeptꢀtheꢀsame,ꢀ
therebyꢀkeepingꢀtheꢀphaseꢀshiftꢀtheꢀsameꢀinꢀtheꢀmostꢀ
criticalꢀfrequencyꢀrangeꢀofꢀtheꢀfeedbackꢀloop.ꢀTheꢀoutputꢀ
voltageꢀsettlingꢀbehaviorꢀisꢀrelatedꢀtoꢀtheꢀstabilityꢀofꢀtheꢀ
closed-loopꢀsystemꢀandꢀwillꢀdemonstrateꢀtheꢀactualꢀoverallꢀ
supplyꢀperformance.
thisꢀrecoveryꢀtimeꢀV ꢀcanꢀbeꢀmonitoredꢀforꢀexcessiveꢀ
overshootꢀ orꢀ ringing,ꢀ whichꢀ wouldꢀ indicateꢀ aꢀ stabilityꢀ
problem.ꢀOPTI-LOOPꢀcompensationꢀallowsꢀtheꢀtransientꢀ
responseꢀtoꢀbeꢀoptimizedꢀoverꢀaꢀwideꢀrangeꢀofꢀoutputꢀ
capacitanceꢀandꢀESRꢀvalues.ꢀThe availability of the I pin
TH
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
marginꢀand/orꢀdampingꢀfactorꢀcanꢀbeꢀestimatedꢀusingꢀtheꢀ
percentageꢀofꢀovershootꢀseenꢀatꢀthisꢀpin.ꢀTheꢀbandwidthꢀ
canꢀalsoꢀbeꢀestimatedꢀbyꢀexaminingꢀtheꢀriseꢀtimeꢀatꢀtheꢀ
Aꢀsecond,ꢀmoreꢀsevereꢀtransientꢀisꢀcausedꢀbyꢀswitchingꢀ
inꢀloadsꢀwithꢀlargeꢀ(>1µF)ꢀsupplyꢀbypassꢀcapacitors.ꢀTheꢀ
dischargedꢀbypassꢀcapacitorsꢀareꢀeffectivelyꢀputꢀinꢀparallelꢀ
withꢀC ,ꢀcausingꢀaꢀrapidꢀdropꢀinꢀV .ꢀNoꢀregulatorꢀcanꢀ
OUTꢀ
OUTꢀ
alterꢀitsꢀdeliveryꢀofꢀcurrentꢀquicklyꢀenoughꢀtoꢀpreventꢀthisꢀ
suddenꢀstepꢀchangeꢀinꢀoutputꢀvoltageꢀifꢀtheꢀloadꢀswitchꢀ
resistanceꢀisꢀlowꢀandꢀitꢀisꢀdrivenꢀquickly.ꢀIfꢀtheꢀratioꢀofꢀ
pin.ꢀ Theꢀ I ꢀ externalꢀ componentsꢀ shownꢀ inꢀ Figureꢀ 13ꢀ
C
ꢀtoꢀC ꢀisꢀgreaterꢀthanꢀ1:50,ꢀtheꢀswitchꢀriseꢀtimeꢀ
TH
LOAD
OUT
circuitꢀwillꢀprovideꢀanꢀadequateꢀstartingꢀpointꢀforꢀmostꢀ
shouldꢀbeꢀcontrolledꢀsoꢀthatꢀtheꢀloadꢀriseꢀtimeꢀisꢀlimitedꢀ
applications.
3857fa
ꢁꢄ
LTC3857
applicaTions inForMaTion
toꢀapproximatelyꢀ25ꢀ•ꢀC
.ꢀThusꢀaꢀ10µFꢀcapacitorꢀwouldꢀ TheꢀpowerꢀdissipationꢀonꢀtheꢀtopsideꢀMOSFETꢀcanꢀbeꢀeasilyꢀ
LOAD
requireꢀaꢀ250µsꢀriseꢀtime,ꢀlimitingꢀtheꢀchargingꢀcurrentꢀ estimated.ꢀChoosingꢀaꢀFairchildꢀFDS6982SꢀdualꢀMOSFETꢀ
toꢀaboutꢀ200mA.
resultsꢀin:ꢀR
ꢀ=ꢀ0.035Ω/0.022Ω,ꢀC
ꢀ=ꢀ215pF.ꢀAtꢀ
DS(ON)
MILLER
maximumꢀinputꢀvoltageꢀwithꢀT(estimated)ꢀ=ꢀ50°C:
Design Example
2
3.3V
22V
PMAIN
=
5A 1+ 0.005 50°C – 25°C
(
)
(
)(
)
Asꢀaꢀdesignꢀexampleꢀforꢀoneꢀchannel,ꢀassumeꢀV ꢀ=ꢀ12Vꢀ
IN
MAX
(nominal),ꢀ V ꢀ =ꢀ 22Vꢀ (max),ꢀ V ꢀ =ꢀ 3.3V,ꢀ I ꢀ =ꢀ 5A,ꢀ
IN
OUT
2 5A
V
ꢀ=ꢀ75mVꢀandꢀfꢀ=ꢀ350kHz.
0.035Ω + 22V
2.5Ω 215pF •
(
) (
)
1
(
)(
)
SENSE(MAX)
2
Theꢀinductanceꢀvalueꢀisꢀchosenꢀfirstꢀbasedꢀonꢀaꢀ30%ꢀrippleꢀ
currentꢀassumption.ꢀTheꢀhighestꢀvalueꢀofꢀrippleꢀcurrentꢀ
occursꢀatꢀtheꢀmaximumꢀinputꢀvoltage.ꢀTieꢀtheꢀFREQꢀpinꢀ
toꢀ GND,ꢀ generatingꢀ 350kHzꢀ operation.ꢀ Theꢀ minimumꢀ
inductanceꢀforꢀ30%ꢀrippleꢀcurrentꢀis:
1
+
350kHz = 331mW
(
)
5V – 2.3V 2.3V
ꢀ
Aꢀshort-circuitꢀtoꢀgroundꢀwillꢀresultꢀinꢀaꢀfoldedꢀbackꢀcur-
rentꢀof:
VOUT
ƒ •L
VOUT
95ns 22V
(
)
32mV
0.01Ω 2
1
∆IL(NOM)
=
1–
ISC =
–
= 2.98A
V
IN(NOM)
4.7µH
ꢀ
ꢀ
Aꢀ4.7µHꢀinductorꢀwillꢀproduceꢀ29%ꢀrippleꢀcurrent.ꢀTheꢀ
peakꢀinductorꢀcurrentꢀwillꢀbeꢀtheꢀmaximumꢀDCꢀvalueꢀplusꢀ
oneꢀhalfꢀtheꢀrippleꢀcurrent,ꢀorꢀ5.73A.ꢀIncreasingꢀtheꢀrippleꢀ
currentꢀwillꢀalsoꢀhelpꢀensureꢀthatꢀtheꢀminimumꢀon-timeꢀ
ofꢀ95nsꢀisꢀnotꢀviolated.ꢀTheꢀminimumꢀon-timeꢀoccursꢀatꢀ
withꢀaꢀtypicalꢀvalueꢀofꢀR
ꢀandꢀδꢀ=ꢀ(0.005/°C)(25°C)ꢀ
DS(ON)
=ꢀ0.125.ꢀTheꢀresultingꢀpowerꢀdissipatedꢀinꢀtheꢀbottomꢀ
MOSFETꢀis:
2
PSYNC = 2.98 1.125 0.022Ω = 220mW
maximumꢀV :
ꢀ
IN
whichꢀisꢀlessꢀthanꢀunderꢀfull-loadꢀconditions.
VOUT
IN(MAX)ƒ
3.3V
tON(MIN)
=
=
= 429ns
V
22V 350kHz
C ꢀisꢀchosenꢀforꢀanꢀRMSꢀcurrentꢀratingꢀofꢀatꢀleastꢀ3Aꢀatꢀ
IN
ꢀ
temperatureꢀassumingꢀonlyꢀthisꢀchannelꢀisꢀon.ꢀC ꢀisꢀ
OUT
TheꢀequivalentꢀR
ꢀresistorꢀvalueꢀcanꢀbeꢀcalculatedꢀbyꢀ
chosenꢀwithꢀanꢀESRꢀofꢀ0.02Ωꢀforꢀlowꢀoutputꢀripple.ꢀTheꢀ
outputꢀrippleꢀinꢀcontinuousꢀmodeꢀwillꢀbeꢀhighestꢀatꢀtheꢀ
maximumꢀinputꢀvoltage.ꢀTheꢀoutputꢀvoltageꢀrippleꢀdueꢀtoꢀ
ESRꢀisꢀapproximately:
SENSE
usingꢀtheꢀminimumꢀvalueꢀforꢀtheꢀmaximumꢀcurrentꢀsenseꢀ
thresholdꢀ(64mV):
64mV
5.73A
RSENSE
≤
≈ 0.01Ω
ꢀ V ꢀ=ꢀR ꢀ(∆I )ꢀ=ꢀ0.02Ω(1.45A)ꢀ=ꢀ29mV
ORIPPLE ESR L P-P
ꢀ
Choosingꢀ1%ꢀresistors:ꢀRAꢀ=ꢀ25kꢀandꢀRBꢀ=ꢀ80.6kꢀyieldsꢀ
anꢀoutputꢀvoltageꢀofꢀ3.33V.
3857fa
ꢁꢅ
1.ꢀ
AreꢀtheꢀtopꢀN-channelꢀMOSFETsꢀMTOP1ꢀandꢀMTOP2ꢀ
locatedꢀwithinꢀ1cmꢀofꢀeachꢀotherꢀwithꢀaꢀcommonꢀdrainꢀ
LTC3857
applicaTions inForMaTion
PC Board Layout Checklist
2.ꢀAreꢀtheꢀsignalꢀandꢀpowerꢀgroundsꢀkeptꢀseparate?ꢀTheꢀ
combinedꢀICꢀsignalꢀgroundꢀpinꢀandꢀtheꢀgroundꢀreturnꢀ
Whenꢀlayingꢀoutꢀtheꢀprintedꢀcircuitꢀboard,ꢀtheꢀfollowingꢀ
checklistꢀshouldꢀbeꢀusedꢀtoꢀensureꢀproperꢀoperationꢀofꢀ
theꢀIC.ꢀTheseꢀitemsꢀareꢀalsoꢀillustratedꢀgraphicallyꢀinꢀtheꢀ
layoutꢀdiagramꢀofꢀFigureꢀ11.ꢀFigureꢀ12ꢀillustratesꢀtheꢀcurrentꢀ
waveformsꢀpresentꢀinꢀtheꢀvariousꢀbranchesꢀofꢀtheꢀ2-phaseꢀ
synchronousꢀregulatorsꢀoperatingꢀinꢀtheꢀcontinuousꢀmode.ꢀ
Checkꢀtheꢀfollowingꢀinꢀyourꢀlayout:
ofꢀC
ꢀmustꢀreturnꢀtoꢀtheꢀcombinedꢀC ꢀ(–)ꢀter-
INTVCC
OUT
minals.ꢀTheꢀpathꢀformedꢀbyꢀtheꢀtopꢀN-channelꢀMOSFET,ꢀ
SchottkyꢀdiodeꢀandꢀtheꢀC ꢀcapacitorꢀshouldꢀhaveꢀshortꢀ
IN
leadsꢀandꢀPCꢀtraceꢀlengths.ꢀTheꢀoutputꢀcapacitorꢀ(–)ꢀ
terminalsꢀshouldꢀbeꢀconnectedꢀasꢀcloseꢀasꢀpossibleꢀ
toꢀtheꢀ(–)ꢀterminalsꢀofꢀtheꢀinputꢀcapacitorꢀbyꢀplacingꢀ
theꢀcapacitorsꢀnextꢀtoꢀeachꢀotherꢀandꢀawayꢀfromꢀtheꢀ
Schottkyꢀloopꢀdescribedꢀabove.
3.ꢀDoꢀtheꢀLTC3857ꢀV ꢀpins’ꢀresistiveꢀdividersꢀconnectꢀtoꢀ
FB
connectionꢀatꢀC ?ꢀDoꢀnotꢀattemptꢀtoꢀsplitꢀtheꢀinputꢀ
IN
theꢀ(+)ꢀterminalsꢀofꢀC ?ꢀTheꢀresistiveꢀdividerꢀmustꢀbeꢀ
OUT
decouplingꢀforꢀtheꢀtwoꢀchannelsꢀasꢀitꢀcanꢀcauseꢀaꢀlargeꢀ
resonantꢀloop.
connectedꢀbetweenꢀtheꢀ(+)ꢀterminalꢀofꢀC ꢀandꢀsignalꢀ
OUT
ground.ꢀTheꢀfeedbackꢀresistorꢀconnectionsꢀshouldꢀnotꢀ
beꢀalongꢀtheꢀhighꢀcurrentꢀinputꢀfeedsꢀfromꢀtheꢀinputꢀ
capacitor(s).
R
PU2
TRACK/SS1
V
PULL-UP
PGOOD2
I
PGOOD2
PGOOD1
TG1
R
TH1
PU1
V
PULL-UP
V
PGOOD1
FB1
L1
R
SENSE
+
–
V
SENSE1
SENSE1
FREQ
OUT1
SW1
LTC3857
C
B1
M1
M2
D1
BOOST1
BG1
PHASMD
CLKOUT
PLLIN/MODE
RUN1
R
IN
C
C
OUT1
V
IN
f
1µF
IN
+
C
CERAMIC
VIN
PGND
GND
RUN2
+
EXTV
INTV
CC
CC
C
+
IN
V
C
SGND
IN
INTVCC
–
SENSE2
OUT2
1µF
CERAMIC
+
BG2
SENSE2
M4
L2
M3
D2
BOOST2
V
FB2
TH2
C
B2
SW2
TG2
I
R
SENSE
V
OUT2
TRACK/SS2
ILIM
3857 F11
Figure 11. Recommended Printed Circuit Layout Diagram
3857fa
ꢁꢆ
4.ꢀAreꢀtheꢀSENSE ꢀandꢀSENSE ꢀleadsꢀroutedꢀtogetherꢀwithꢀ 6.ꢀ
minimumꢀPCꢀtraceꢀspacing?ꢀTheꢀfilterꢀcapacitorꢀbetweenꢀ
Keepꢀtheꢀswitchingꢀnodesꢀ(SW1,ꢀSW2),ꢀtopꢀgateꢀnodesꢀ
(TG1,ꢀTG2),ꢀandꢀboostꢀnodesꢀ(BOOST1,ꢀBOOST2)ꢀawayꢀ
fromꢀ sensitiveꢀ small-signalꢀ nodes,ꢀ especiallyꢀ fromꢀ
theꢀoppositesꢀchannel’sꢀvoltageꢀandꢀcurrentꢀsensingꢀ
feedbackꢀpins.ꢀAllꢀofꢀtheseꢀnodesꢀhaveꢀveryꢀlargeꢀandꢀ
fastꢀmovingꢀsignalsꢀandꢀthereforeꢀshouldꢀbeꢀkeptꢀonꢀ
theꢀoutput sideꢀofꢀtheꢀLTC3857ꢀandꢀoccupyꢀminimumꢀ
PCꢀtraceꢀarea.
LTC3857
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.
3857 F12
Figure 12. Branch Current Waveforms
–
+
+
–
SENSE ꢀandꢀSENSE ꢀshouldꢀbeꢀasꢀcloseꢀasꢀpossibleꢀ
toꢀtheꢀIC.ꢀEnsureꢀaccurateꢀcurrentꢀsensingꢀwithꢀKelvinꢀ
connectionsꢀatꢀtheꢀSENSEꢀresistor.
5.ꢀIsꢀtheꢀINTV ꢀdecouplingꢀcapacitorꢀconnectedꢀcloseꢀ
CC
toꢀtheꢀIC,ꢀbetweenꢀtheꢀINTV ꢀandꢀtheꢀpowerꢀgroundꢀ
CC
pins?ꢀThisꢀcapacitorꢀcarriesꢀtheꢀMOSFETꢀdrivers’ꢀcur-
rentꢀpeaks.ꢀAnꢀadditionalꢀ1µFꢀceramicꢀcapacitorꢀplacedꢀ 7.ꢀUseꢀaꢀmodifiedꢀstargroundꢀꢀtechnique:ꢀaꢀlowꢀimpedance,ꢀ
immediatelyꢀnextꢀtoꢀtheꢀINTV ꢀandꢀPGNDꢀpinsꢀcanꢀhelpꢀ
improveꢀnoiseꢀperformanceꢀsubstantially.
largeꢀcopperꢀareaꢀcentralꢀgroundingꢀpointꢀonꢀtheꢀsameꢀ
sideꢀofꢀtheꢀPCꢀboardꢀasꢀtheꢀinputꢀandꢀoutputꢀcapacitorsꢀ
CC
withꢀtie-insꢀforꢀtheꢀbottomꢀofꢀtheꢀINTV ꢀdecouplingꢀ
CC
capacitor,ꢀtheꢀbottomꢀofꢀtheꢀvoltageꢀfeedbackꢀresistiveꢀ
dividerꢀandꢀtheꢀSGNDꢀpinꢀofꢀtheꢀIC.
3857fa
ꢁꢇ
LTC3857
applicaTions inForMaTion
PC Board Layout Debugging
Investigateꢀwhetherꢀanyꢀproblemsꢀexistꢀonlyꢀatꢀhigherꢀout-
putꢀcurrentsꢀorꢀonlyꢀatꢀhigherꢀinputꢀvoltages.ꢀIfꢀproblemsꢀ
coincideꢀwithꢀhighꢀinputꢀvoltagesꢀandꢀlowꢀoutputꢀcurrents,ꢀ
lookꢀforꢀcapacitiveꢀcouplingꢀbetweenꢀtheꢀBOOST,ꢀSW,ꢀTG,ꢀ
andꢀpossiblyꢀBGꢀconnectionsꢀandꢀtheꢀsensitiveꢀvoltageꢀ
andꢀcurrentꢀpins.ꢀTheꢀcapacitorꢀplacedꢀacrossꢀtheꢀcurrentꢀ
sensingꢀpinsꢀneedsꢀtoꢀbeꢀplacedꢀimmediatelyꢀadjacentꢀtoꢀ
theꢀpinsꢀofꢀtheꢀIC.ꢀThisꢀcapacitorꢀhelpsꢀtoꢀminimizeꢀtheꢀ
effectsꢀofꢀdifferentialꢀnoiseꢀinjectionꢀdueꢀtoꢀhighꢀfrequencyꢀ
capacitiveꢀ coupling.ꢀ Ifꢀ problemsꢀ areꢀ encounteredꢀ withꢀ
highꢀcurrentꢀoutputꢀloadingꢀatꢀlowerꢀinputꢀvoltages,ꢀlookꢀ
Startꢀwithꢀoneꢀcontrollerꢀonꢀatꢀaꢀtime.ꢀItꢀisꢀhelpfulꢀtoꢀuseꢀ
aꢀDC-50MHzꢀcurrentꢀprobeꢀtoꢀmonitorꢀtheꢀcurrentꢀinꢀtheꢀ
inductorꢀ whileꢀ testingꢀ theꢀ circuit.ꢀ Monitorꢀ theꢀ outputꢀ
switchingꢀnodeꢀ(SWꢀpin)ꢀtoꢀsynchronizeꢀtheꢀoscilloscopeꢀ
toꢀtheꢀinternalꢀoscillatorꢀandꢀprobeꢀtheꢀactualꢀoutputꢀvoltageꢀ
asꢀwell.ꢀCheckꢀforꢀproperꢀperformanceꢀoverꢀtheꢀoperatingꢀ
voltageꢀandꢀcurrentꢀrangeꢀexpectedꢀinꢀtheꢀapplication.ꢀTheꢀ
frequencyꢀofꢀoperationꢀshouldꢀbeꢀmaintainedꢀoverꢀtheꢀinputꢀ
voltageꢀrangeꢀdownꢀtoꢀdropoutꢀandꢀuntilꢀtheꢀoutputꢀloadꢀ
dropsꢀbelowꢀtheꢀlowꢀcurrentꢀoperationꢀthreshold—typi-
callyꢀ15%ꢀofꢀtheꢀmaximumꢀdesignedꢀcurrentꢀlevelꢀinꢀBurstꢀ
Modeꢀoperation.
forꢀinductiveꢀcouplingꢀbetweenꢀC ,ꢀSchottkyꢀandꢀtheꢀtopꢀ
IN
MOSFETꢀcomponentsꢀtoꢀtheꢀsensitiveꢀcurrentꢀandꢀvoltageꢀ
sensingꢀtraces.ꢀInꢀaddition,ꢀinvestigateꢀcommonꢀgroundꢀ
pathꢀvoltageꢀpickupꢀbetweenꢀtheseꢀcomponentsꢀandꢀtheꢀ
SGNDꢀpinꢀofꢀtheꢀIC.
Theꢀdutyꢀcycleꢀpercentageꢀshouldꢀbeꢀmaintainedꢀfromꢀcycleꢀ
toꢀcycleꢀinꢀaꢀwell-designed,ꢀlowꢀnoiseꢀPCBꢀimplementa-
tion.ꢀVariationꢀinꢀtheꢀdutyꢀcycleꢀatꢀaꢀsubharmonicꢀrateꢀcanꢀ
suggestꢀnoiseꢀpickupꢀatꢀtheꢀcurrentꢀorꢀvoltageꢀsensingꢀ
inputsꢀorꢀinadequateꢀloopꢀcompensation.ꢀOvercompen-
sationꢀofꢀtheꢀloopꢀcanꢀbeꢀusedꢀtoꢀtameꢀaꢀpoorꢀPCꢀlayoutꢀ
ifꢀregulatorꢀbandwidthꢀoptimizationꢀisꢀnotꢀrequired.ꢀOnlyꢀ
afterꢀeachꢀcontrollerꢀisꢀcheckedꢀforꢀitsꢀindividualꢀperfor-
manceꢀshouldꢀbothꢀcontrollersꢀbeꢀturnedꢀonꢀatꢀtheꢀsameꢀ
time.ꢀAꢀparticularlyꢀdifficultꢀregionꢀofꢀoperationꢀisꢀwhenꢀ
oneꢀcontrollerꢀchannelꢀisꢀnearingꢀitsꢀcurrentꢀcomparatorꢀ
tripꢀpointꢀwhenꢀtheꢀotherꢀchannelꢀisꢀturningꢀonꢀitsꢀtopꢀ
MOSFET.ꢀThisꢀoccursꢀaroundꢀ50%ꢀdutyꢀcycleꢀonꢀeitherꢀ
channelꢀdueꢀtoꢀtheꢀphasingꢀofꢀtheꢀinternalꢀclocksꢀandꢀmayꢀ
causeꢀminorꢀdutyꢀcycleꢀjitter.
Anꢀembarrassingꢀproblem,ꢀwhichꢀcanꢀbeꢀmissedꢀinꢀanꢀ
otherwiseꢀproperlyꢀworkingꢀswitchingꢀregulator,ꢀresultsꢀ
whenꢀtheꢀcurrentꢀsensingꢀleadsꢀareꢀhookedꢀupꢀbackwards.ꢀ
Theꢀoutputꢀvoltageꢀunderꢀthisꢀimproperꢀhookupꢀwillꢀstillꢀ
beꢀmaintainedꢀbutꢀtheꢀadvantagesꢀofꢀcurrentꢀmodeꢀcontrolꢀ
willꢀnotꢀbeꢀrealized.ꢀCompensationꢀofꢀtheꢀvoltageꢀloopꢀwillꢀ
beꢀ muchꢀ moreꢀ sensitiveꢀ toꢀ componentꢀ selection.ꢀ Thisꢀ
behaviorꢀcanꢀbeꢀinvestigatedꢀbyꢀtemporarilyꢀshortingꢀoutꢀ
theꢀcurrentꢀsensingꢀresistor—don’tꢀworry,ꢀtheꢀregulatorꢀ
willꢀstillꢀmaintainꢀcontrolꢀofꢀtheꢀoutputꢀvoltage.
Reduceꢀ V ꢀ fromꢀ itsꢀ nominalꢀ levelꢀ toꢀ verifyꢀ operationꢀ
IN
ofꢀtheꢀregulatorꢀinꢀdropout.ꢀCheckꢀtheꢀoperationꢀofꢀtheꢀ
undervoltageꢀlockoutꢀcircuitꢀbyꢀfurtherꢀloweringꢀV ꢀwhileꢀ
IN
monitoringꢀtheꢀoutputsꢀtoꢀverifyꢀoperation.
3857fa
ꢁꢈ
LTC3857
applicaTions inForMaTion
R
B1
INTV
215k
CC
LTC3857
+
100k
C
15pF
SENSE1
F1
PGOOD2
C1
1nF
100k
–
R
A1
SENSE1
PGOOD1
BG1
68.1k
L1
3.3µH
MBOT1
MTOP1
V
FB1
V
3.3V
5A
OUT1
C
150pF
ITH1A
SW1
R
C
C
SENSE1
6mΩ
OUT1
B1
BOOST1
TG1
150µF
0.47µF
R
ITH1
15k
I
TH1
D1
D2
C
ITH1
820pF
C
SS1
0.1µF
V
IN
V
IN
9V TO 38V
C
IN
TRACK/SS1
22µF
I
INTV
CC
LIM
C
INT
PHSMD
4.7µF
CLKOUT
PLLIN/MODE
SGND
PGND
MTOP2
MBOT2
EXTV
TG2
CC
RUN1
RUN2
FREQ
C
B2
BOOST2
0.47µF
L2
7.2µH
R
SENSE2
8mΩ
C
0.1µF
SS2
V
8.5V
3A
OUT2
SW2
BG2
TRACK/SS2
C
C
680pF
OUT2
ITH2
R
27k
150µF
ITH2
I
TH2
C
100pF
C2
ITH2A
V
FB2
R
A2
–
+
SENSE2
44.2k
C
1nF
F2
39pF
SENSE2
R
B2
422k
3857 F13
C
, C
: SANYO 10TPD150M
OUT1 OUT2
D1, D2: CENTRAL SEMI CMDSH-4E
L1: SUMIDA CDEP105-3R2M
L2: SUMIDA CDEP105-7R2M
MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP
Figure 13. High Efficiency Dual 3.3V/8.5V Step-Down Converter
3857fa
ꢂ0
LTC3857
Typical applicaTions
High Efficiency Dual 2.5V/3.3V Step-Down Converter
R
B1
INTV
143k
CC
LTC3857
+
100k
C
22pF
SENSE1
F1
PGOOD2
C1
1nF
100k
–
R
A1
SENSE1
PGOOD1
BG1
68.1k
L1
2.4µH
MBOT1
MTOP1
V
FB1
V
2.5V
5A
OUT1
C
100pF
ITH1A
SW1
R
C
C
SENSE1
6mΩ
OUT1
B1
BOOST1
TG1
150µF
0.47µF
R
ITH1
22k
I
TH1
D1
D2
C
ITH1
820pF
C
SS1
0.01µF
V
IN
V
IN
4.5V TO 38V
C
IN
TRACK/SS1
22µF
I
INTV
CC
LIM
C
INT
PHSMD
4.7µF
CLKOUT
PLLIN/MODE
SGND
PGND
MTOP2
MBOT2
EXTV
TG2
CC
RUN1
RUN2
FREQ
C
B2
BOOST2
0.47µF
L2
3.2µH
R
SENSE2
6mΩ
C
0.01µF
SS2
V
3.3V
5A
OUT2
SW2
BG2
TRACK/SS2
C
C
820pF
OUT2
ITH2
R
15k
150µF
ITH2
I
TH2
C
150pF
C2
ITH2A
V
FB2
R
A2
–
+
SENSE2
68.1k
C
1nF
F2
15pF
SENSE2
R
B2
215k
3857 TA02
C
, C
: SANYO 4TPE150M
OUT1 OUT2
D1, D2: CENTRAL SEMI CMDSH-4E
L1: SUMIDA CDEP105-2R5
L2: SUMIDA CDEP105-3R2M
MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP
3857fa
ꢂꢀ
LTC3857
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
100pF
ITH1A
SW1
R
C
C
SENSE1
9mΩ
OUT1
B1
BOOST1
TG1
47µF
0.47µF
R
ITH1
10k
I
TH1
D1
D2
LTC3857
C
SS1
0.01µF
C
ITH1
680pF
V
IN
V
IN
12.5V TO 38V
C
IN
TRACK/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
TRACK/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 CDEP105-8R8M
L2: SUMIDA CDEP105-4R3M
MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP
C
1nF
F2
15pF
SENSE2
R
B2
392k
3857 TA03
3857fa
ꢂꢁ
LTC3857
Typical applicaTions
High Efficiency Dual 24V/5V Step-Down Converter
R
B1
487k
INTV
CC
100k
100k
+
–
C
SENSE1
SENSE1
F1
PGOOD2
C1
1nF
18pF
R
A1
PGOOD1
BG1
16.9k
L1
22µH
MBOT1
MTOP1
V
FB1
V
24V
1A
OUT1
C
100pF
ITH1A
SW1
R
C
C
SENSE1
OUT1
B1
BOOST1
TG1
25mΩ
22µF
0.47µF
R
46k
ITH1
s2
I
TH1
CERAMIC
C
0.01µF
D1
D2
SS1
LTC3857
C
680pF
ITH1
V
IN
V
TRACK/SS1
IN
28V TO 38V
C
I
INTV
CC
IN
LIM
C
INT
22µF
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
TRACK/SS2
C
C
680pF
OUT2
ITH2
R
17k
150µF
ITH2
I
TH2
C
100pF
C2
ITH2A
V
FB2
R
A2
–
+
SENSE2
75k
C
: SANYO 10TPD150M
OUT2
L1: SUMIDA CDR7D43MN
C
1nF
F2
15pF
L2: SUMIDA CDEP105-4R3M
SENSE2
MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP
R
B2
3858 TA04
392k
3857fa
ꢂꢂ
LTC3857
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
200pF
ITH1A
SW1
C
R
OUT1
C
SENSE1
B1
BOOST1
TG1
220µF
3.5mΩ
0.47µF
R
ITH1
3.93k
s2
I
TH1
D1
D2
LTC3857
C
1000pF
ITH1
C
SS1
0.01µF
V
IN
V
IN
12V
C
IN
TRACK/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
3.5mΩ
C
0.01µF
SS2
V
1.2V
8A
OUT2
SW2
BG2
TRACK/SS2
C
OUT2
C
1000pF
ITH2
220µF
R
3.93k
ITH2
s2
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
3858 TA05
57.6k
3857fa
ꢂꢃ
LTC3857
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
200pF
ITH1A
1V
C
OUT1 8A
C
B1
BOOST1
TG1
220µF
0.47µF
R
ITH1
3.93k
s2
I
TH1
D1
D2
LTC3857
C
1000pF
ITH1
C
SS1
0.01µF
V
IN
V
IN
12V
C
IN
TRACK/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
TRACK/SS2
C
OUT2 8A
C
1000pF
ITH2
220µF
R
3.93k
ITH2
s2
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
3857 TA06
57.6k
3857fa
ꢂꢄ
LTC3857
package DescripTion
UH Package
32-Lead Plastic QFN (5mm × 5mm)
(ReferenceꢀLTCꢀDWGꢀ#ꢀ05-08-1693ꢀRevꢀD)
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
3857fa
ꢂꢅ
LTC3857
revision hisTory
REV
DATE
DESCRIPTION
PAGE NUMBER
A
12/09 ChangeꢀtoꢀAbsoluteꢀMaximumꢀRatings
ChangeꢀtoꢀOrderꢀInformation
2
2
ChangeꢀtoꢀElectricalꢀCharacteristics
ChangeꢀtoꢀTypicalꢀPerformanceꢀCharacteristics
ChangeꢀtoꢀPinꢀFunctions
2,ꢀ3,ꢀ4
6
8,ꢀ9
TextꢀChangesꢀtoꢀOperationsꢀSection
TextꢀChangesꢀtoꢀApplicationsꢀInformationꢀSection
ChangeꢀtoꢀTableꢀ2
11,ꢀ12,ꢀ13
21,ꢀ22,ꢀ23,ꢀ24,ꢀ26
23
27
38
ChangeꢀtoꢀFigureꢀ11
ChangesꢀtoꢀRelatedꢀParts
3857fa
InformationꢀfurnishedꢀbyꢀLinearꢀTechnologyꢀCorporationꢀisꢀbelievedꢀtoꢀbeꢀaccurateꢀandꢀreliable.ꢀ
However,ꢀnoꢀresponsibilityꢀisꢀassumedꢀforꢀitsꢀuse.ꢀLinearꢀTechnologyꢀCorporationꢀmakesꢀnoꢀrepresenta-
tionꢀthatꢀtheꢀinterconnectionꢀofꢀitsꢀcircuitsꢀasꢀdescribedꢀhereinꢀwillꢀnotꢀinfringeꢀonꢀexistingꢀpatentꢀrights.
ꢂꢆ
LTC3857
relaTeD parTs
PART NUMBER
DESCRIPTION
COMMENTS
LTC3858/LTC3858-1 LowꢀI ,ꢀDualꢀOutputꢀ2-PhaseꢀSynchronousꢀStep-Downꢀ Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ50kHzꢀtoꢀ900kHz,ꢀꢀ
Q
DC/DCꢀControllerꢀwithꢀ99%ꢀDutyꢀCycle
4Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ24V,ꢀI ꢀ=ꢀ170µA,
IN OUT Q
LTC3868/LTC3868-1 LowꢀI ,ꢀDualꢀOutputꢀ2-PhaseꢀSynchronousꢀStep-Downꢀ Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ50kHzꢀtoꢀ900kHz,ꢀꢀ
Q
DC/DCꢀControllerꢀwithꢀ99%ꢀDutyꢀCycle
4Vꢀ≤ꢀV ꢀ≤ꢀ24V,ꢀꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ14V,ꢀI ꢀ=ꢀ170µA,
IN OUT Q
LTC3834/LTC3834-1 LowꢀI ,ꢀSynchronousꢀStep-DownꢀDC/DCꢀController
Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ140kHzꢀtoꢀ650kHz,ꢀꢀ
4Vꢀ≤ꢀV ꢀ≤ꢀ36V,ꢀꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ10V,ꢀI ꢀ=ꢀ30µA,
Q
IN
OUT
Q
LTC3835/LTC3835-1 LowꢀI ,ꢀSynchronousꢀStep-DownꢀDC/DCꢀController
Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ140kHzꢀtoꢀ650kHz,ꢀ
4Vꢀ≤ꢀV ꢀ≤ꢀ36V,ꢀꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ10V,ꢀI ꢀ=ꢀ80µA,
Q
IN
OUT
Q
LT3845
LT3800
LTC3824
LowꢀI ,ꢀHighꢀVoltageꢀSynchronousꢀStep-Downꢀꢀ
AdjustableꢀFixedꢀOperatingꢀFrequencyꢀ100kHzꢀtoꢀ500kHz,ꢀ
4Vꢀ≤ꢀV ꢀ≤ꢀ60V,ꢀ1.23Vꢀ≤ꢀV ꢀ≤ꢀ36V,ꢀI ꢀ=ꢀ120µA,ꢀTSSOP-16
Q
DC/DCꢀController
IN
OUT
Q
LowꢀI ,ꢀHighꢀVoltageꢀSynchronousꢀStep-Downꢀꢀ
Fixedꢀ200kHzꢀOperatingꢀFrequency,ꢀ4Vꢀ≤ꢀV ꢀ≤ꢀ60V,ꢀ1.23Vꢀ≤ꢀV ꢀ≤ꢀ36V,ꢀ
IN OUT
I ꢀ=ꢀ100µA,ꢀTSSOP-16
Q
Q
DC/DCꢀController
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
ꢀorꢀDCRꢀCurrentꢀSensingꢀandꢀ 4Vꢀ≤ꢀV ꢀ≤ꢀ30V,ꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ5.25V
SENSE IN OUT
Tracking
LTC3855
Dual,ꢀMultiphase,ꢀSynchronousꢀDC/DCꢀStep-Downꢀ
ControllerꢀwithꢀDiffampꢀandꢀDCRꢀTemperatureꢀ
Compensation
Phase-LockableꢀFixedꢀFrequencyꢀ250kHzꢀtoꢀ770kHz,ꢀꢀ
4.5Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ12.5V
IN
OUT
LTC3853
TripleꢀOutput,ꢀMultiphaseꢀSynchronousꢀStep-Downꢀ
Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ250kHzꢀtoꢀ750kHz,ꢀ
DC/DCꢀController,ꢀR
Tracking
ꢀorꢀDCRꢀCurrentꢀSensingꢀandꢀ 4Vꢀ≤ꢀV ꢀ≤ꢀ24V,ꢀV ꢀUpꢀtoꢀ13.5V
IN OUT
SENSE
LTC3854
LTC3775
SmallꢀFootprintꢀWideꢀV ꢀRangeꢀSynchronousꢀꢀ
Fixedꢀ400kHzꢀOperatingꢀFrequencyꢀ4.5Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀꢀ
IN
IN
Step-DownꢀDC/DCꢀController
0.8Vꢀ≤ꢀV ꢀ≤ꢀ5.25V,ꢀ2mmꢀ×ꢀ3mmꢀQFN-12,ꢀMSOP-12
OUT
HighꢀFrequencyꢀSynchronousꢀVoltageꢀModeꢀStep-Downꢀ FastꢀTransientꢀResponse,ꢀt
DC/DCꢀController
ꢀ=ꢀ30ns,ꢀ4Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀ
ON(MIN) IN
0.6Vꢀ≤ꢀV ꢀ≤ꢀ0.8V ,ꢀMSOP-16E,ꢀ3mmꢀ×ꢀ3mmꢀQFN-16
OUT IN
LTC3851A/ꢀ
LTC3851A-1
NoꢀR ™ꢀWideꢀV ꢀRangeꢀSynchronousꢀStep-Downꢀ Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ250kHzꢀtoꢀ750kHz,ꢀꢀ
SENSE
IN
DC/DCꢀController
4Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ5.25V,ꢀMSOP-16E,ꢀ3mmꢀ×ꢀ3mmꢀQFN-16,ꢀ
IN
OUT
SSOP-16
LTC3878/LTC3879
LTM4600HV
NoꢀR
ꢀConstantꢀOn-TimeꢀSynchronousꢀStep-Downꢀ VeryꢀFastꢀTransientꢀResponse,ꢀt
ꢀ=ꢀ43ns,ꢀ4Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀꢀ
SENSE
ON(MIN) IN
DC/DCꢀController
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
3857fa
LT 0110 REV A • 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
相关型号：
LTC3857IGN-1#PBF
LTC3857-1 - Low IQ, Dual, 2-Phase Synchronous Step-Down Controller; Package: SSOP; Pins: 28; Temperature Range: -40°C to 85°C
Linear
LTC3857IGN-1#TRPBF
LTC3857-1 - Low IQ, Dual, 2-Phase Synchronous Step-Down Controller; Package: SSOP; Pins: 28; Temperature Range: -40°C to 85°C
Linear
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