LTC3857-1 [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： | LTC3857-1 |
| 厂家： | 
Linear |
| 描述： | Low IQ, Dual, 2-Phase Synchronous Step-Down Controller |
| 文件： | 总38页 (文件大小：514K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC3857-1
Low I , Dual, 2-Phase
Q
Synchronous Step-Down
Controller
FeaTures
DescripTion
TheꢀLTC®3857-1ꢀisꢀaꢀhighꢀperformanceꢀdualꢀstep-downꢀ
switchingꢀregulatorꢀcontrollerꢀthatꢀdrivesꢀallꢀN-channelꢀ
synchronousꢀpowerꢀMOSFETꢀstages.ꢀAꢀconstantꢀfrequencyꢀ
currentꢀmodeꢀarchitectureꢀallowsꢀaꢀphase-lockableꢀfre-
quencyꢀofꢀupꢀtoꢀ850kHz.ꢀPowerꢀlossꢀandꢀnoiseꢀdueꢀtoꢀtheꢀ
ESRꢀofꢀtheꢀinputꢀcapacitorꢀESRꢀareꢀminimizedꢀbyꢀoperatingꢀ
theꢀtwoꢀcontrollerꢀoutputꢀstagesꢀoutꢀofꢀphase.
n
ꢀ Low Operating I : 50µA (One Channel On)
Q
n
n
n
n
ꢀ Wide Output Voltage Range: 0.8V ≤ V
≤ 24V
OUT
ꢀ Wide V Range: 4V to 38V
IN
ꢀ R
or DCR Current Sensing
SENSE
ꢀ Out-of-PhaseꢀControllersꢀReduceꢀRequiredꢀInputꢀ
CapacitanceꢀandꢀPowerꢀSupplyꢀInducedꢀNoise
®
n
n
n
n
ꢀ OPTI-LOOP ꢀCompensationꢀMinimizesꢀC
OUT
ꢀ Phase-LockableꢀFrequencyꢀ(75kHz-850kHz)
ꢀ ProgrammableꢀFixedꢀFrequencyꢀ(50kHz-900kHz)
ꢀ SelectableꢀContinuous,ꢀPulse-SkippingꢀorꢀLowꢀRippleꢀ
BurstꢀMode®ꢀOperationꢀatꢀLightꢀLoads
Theꢀ50μAꢀno-loadꢀquiescentꢀcurrentꢀextendsꢀoperatingꢀlifeꢀ
inꢀbattery-poweredꢀsystems.ꢀTheꢀLTC3857-1ꢀfeaturesꢀaꢀpre-
cisionꢀ0.8Vꢀreferenceꢀandꢀaꢀpowerꢀgoodꢀoutputꢀindicator.ꢀAꢀ
wideꢀ4Vꢀtoꢀ38Vꢀinputꢀsupplyꢀrangeꢀencompassesꢀaꢀwideꢀrangeꢀ
ofꢀintermediateꢀbusꢀvoltagesꢀandꢀbatteryꢀchemistries.
n
n
n
n
n
n
n
n
ꢀ VeryꢀLowꢀDropoutꢀOperation:ꢀ99%ꢀDutyꢀCycle
ꢀ AdjustableꢀOutputꢀVoltageꢀSoft-StartꢀorꢀTracking
ꢀ PowerꢀGoodꢀOutputꢀVoltageꢀMonitor
IndependentꢀTRACK/SSꢀpinsꢀforꢀeachꢀcontrollerꢀrampꢀtheꢀ
outputꢀvoltagesꢀduringꢀstart-up.ꢀCurrentꢀfoldbackꢀlimitsꢀ
MOSFETꢀheatꢀdissipationꢀduringꢀshort-circuitꢀconditions.ꢀ
TheꢀPLLIN/MODEꢀpinꢀselectsꢀamongꢀBurstꢀModeꢀopera-
tion,ꢀpulse-skippingꢀmode,ꢀorꢀcontinuousꢀinductorꢀcurrentꢀ
modeꢀatꢀlightꢀloads.
ꢀ OutputꢀOvervoltageꢀProtection
ꢀ LowꢀShutdownꢀI :ꢀ<8µA
ꢀ InternalꢀLDOꢀPowersꢀGateꢀDriveꢀfromꢀV ꢀorꢀEXTV
ꢀ NoꢀCurrentꢀFoldbackꢀDuringꢀStart-Up
ꢀ NarrowꢀSSOPꢀPackage
Q
IN
CC
Forꢀaꢀleadlessꢀ32-pinꢀQFNꢀpackageꢀwithꢀtheꢀadditionalꢀfea-
turesꢀofꢀadjustableꢀcurrentꢀlimit,ꢀclockꢀout,ꢀphaseꢀmodula-
tionꢀandꢀtwoꢀPGOODꢀoutputs,ꢀseeꢀtheꢀLTC3857ꢀdataꢀsheet.
L,ꢀLT,ꢀLTC,ꢀLTM,ꢀBurstꢀMode,ꢀOPTI-LOOP,ꢀµModule,ꢀLinearꢀTechnologyꢀandꢀtheꢀLinearꢀlogoꢀ
applicaTions
n
ꢀ AutomotiveꢀAlways-OnꢀSystems
ꢀ BatteryꢀOperatedꢀDigitalꢀDevices
n
areꢀregisteredꢀtrademarksꢀandꢀNoꢀR
ꢀandꢀUltraFastꢀareꢀtrademarksꢀofꢀLinearꢀTechnologyꢀ
SENSE
n
ꢀ DistributedꢀDCꢀPowerꢀSystems
Corporation.ꢀAllꢀotherꢀtrademarksꢀareꢀtheꢀpropertyꢀofꢀtheirꢀrespectiveꢀowners.ꢀProtectedꢀbyꢀ
U.S.ꢀPatents,ꢀincludingꢀ5481178,ꢀ5929620,ꢀ6177787,ꢀ6144194,ꢀ5408150,ꢀ6580258,ꢀ5705919,ꢀ
6100678.
Typical applicaTion
High Efficiency Dual 3.3V/8.5V Step-Down Converter
V
Efficiency and Power Loss
IN
9V TO 38V
22µF
50V
vs Output Current
1µF
100
90
10000
1000
100
10
V
INTV
CC
IN
V
V
= 12V
IN
OUT
TG1
TG2
= 3.3V
0.1µF
0.1µF
FIGURE 13 CIRCUIT
BOOST1
SW1
BOOST2
SW2
80
3.3µH
7.2µH
70
BG1
BG2
60
50
LTC3857-1
PGND
+
+
40
30
20
10
0
SENSE1
SENSE2
0.010Ω
193k
0.007Ω
1
–
–
V
8.5V
3.5A
SENSE2
OUT2
SENSE1
V
OUT1
3.3V
5A
V
FB1
V
FB2
62.5k
I
I
TH2
TH1
TRACK/SS1 SGND TRACK/SS2
0.1µF
0.1
150µF
680pF
15k
680pF
150µF
0.000010.0001 0.001 0.01
0.1
1
10
20k
OUTPUT CURRENT (A)
20k
15k
0.1µF
3857 TA01b
38571 TA01
38571fa
ꢀ
RUN1,ꢀRUN2................................................ –0.3Vꢀtoꢀ8V
SENSE2 ꢀVoltages
...................................... –0.3Vꢀtoꢀ28V
EXTV ꢀ...................................................... –0.3Vꢀtoꢀ14V
PGOOD1ꢀVoltageꢀ
TRACK/SS1,ꢀTRACK/SS2ꢀVoltagesꢀ
OperatingꢀJunctionꢀTemperatureꢀRangeꢀ
(Noteꢀ2).................................................. –40°Cꢀtoꢀ125°C
MaximumꢀJunctionꢀTemperatureꢀ(Noteꢀ3)ꢀ............ 125°C
StorageꢀTemperatureꢀRange................... –65°Cꢀtoꢀ150°C
......................................... –0.3Vꢀtoꢀ6V
.............. –0.3Vꢀtoꢀ6V
ꢀ BOOST1,ꢀBOOST2ꢀ................................. –0.3Vꢀtoꢀ46V
LTC3857-1
absoluTe MaxiMuM raTings
pin conFiguraTion
(Note 1)
TOP VIEW
InputꢀSupplyꢀVoltageꢀ(V )ꢀ......................... –0.3Vꢀtoꢀ40V
IN
TopsideꢀDriverꢀVoltagesꢀ
1
TRACK/SS1
PGOOD1
TG1
28
27
26
25
24
23
22
21
20
19
18
17
16
15
I
TH1
2
3
V
FB1
+
SENSE1
SENSE1
SwitchꢀVoltageꢀ(SW1,ꢀSW2)ꢀꢀ........................ –5Vꢀtoꢀ40V
(BOOST1-SW1),ꢀ(BOOST2-SW2),ꢀINTV ꢀ... –0.3Vꢀtoꢀ6V
–
4
SW1
CC
5
BOOST1
BG1
FREQ
PLLIN/MODE
SGND
6
ꢀ MaximumꢀCurrentꢀSourcedꢀintoꢀPinꢀ
7
V
IN
ꢀ fromꢀSourceꢀ>8V...............................................100µA
8
PGND
RUN1
+
–
+
–
SENSE1 ,ꢀSENSE2 ,ꢀSENSE1
9
EXTV
CC
RUN2
–
10
11
12
13
14
INTV
CC
SENSE2
PLLIN/MODE,ꢀFREQꢀVoltagesꢀꢀ.............. –0.3VꢀtoꢀINTV
+
CC
BG2
SENSE2
CC
BOOST2
SW2
V
FB2
TH2
I
,ꢀI ,V ,ꢀV ꢀVoltagesꢀ...................... –0.3Vꢀtoꢀ6V
TH1 TH2 FB1 FB2
I
TG2
TRACK/SS2
GN PACKAGE
28-LEAD PLASTIC SSOP
ꢀ
T
ꢀ=ꢀ125°C,ꢀθ ꢀ=ꢀ90°C/W
JA
JMAX
orDer inForMaTion
LEAD FREE FINISH
LTC3857EGN-1#PBF
LTC3857IGN-1#PBF
TAPE AND REEL
PART MARKING*
LTC3857GN-1
LTC3857GN-1
PACKAGE DESCRIPTION
28-LeadꢀPlasticꢀSSOP
28-LeadꢀPlasticꢀSSOP
TEMPERATURE RANGE
–40°Cꢀtoꢀ125°C
LTC3857EGN-1#TRPBF
LTC3857IGN-1#TRPBF
–40°Cꢀtoꢀ125°C
ConsultꢀLTCꢀMarketingꢀforꢀpartsꢀspecifiedꢀwithꢀwiderꢀoperatingꢀtemperatureꢀranges.ꢀꢀ*Theꢀtemperatureꢀgradeꢀisꢀidentifiedꢀbyꢀaꢀlabelꢀonꢀtheꢀshippingꢀcontainer.
ConsultꢀLTCꢀMarketingꢀforꢀinformationꢀonꢀnon-standardꢀleadꢀbasedꢀfinishꢀparts.
Forꢀmoreꢀinformationꢀonꢀleadꢀfreeꢀpartꢀmarking,ꢀgoꢀto:ꢀhttp://www.linear.com/leadfree/ꢀꢀ
Forꢀmoreꢀinformationꢀonꢀtapeꢀandꢀreelꢀspecifications,ꢀgoꢀto:ꢀhttp://www.linear.com/tapeandreel/
38571fa
ꢁ
LTC3857-1
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
V
ReferenceꢀVoltageꢀLineꢀRegulation
OutputꢀVoltageꢀLoadꢀRegulation
(Noteꢀ4)ꢀV ꢀ=ꢀ4.5Vꢀtoꢀ38V
0.002
0.02
%/V
REFLNREG
LOADREG
IN
(Note4)ꢀ
ꢀ
ꢀ
ꢀ
%
l
l
MeasuredꢀinꢀServoꢀLoop,ꢀꢀ
0.01
0.1
∆I ꢀVoltageꢀ=ꢀ1.2Vꢀtoꢀ0.7V
TH
(Note4)ꢀ
ꢀ
ꢀ
ꢀ
%
MeasuredꢀinꢀServoꢀLoop,ꢀꢀ
–0.01
–0.1
∆I ꢀVoltageꢀ=ꢀ1.2Vꢀtoꢀ2V
TH
g
ꢀ
TransconductanceꢀAmplifierꢀg
InputꢀDCꢀSupplyꢀCurrent
(Noteꢀ4)ꢀI
ꢀ=ꢀ1.2V,ꢀSink/Sourceꢀ=ꢀ5µA
TH1,2
2
2
mmho
mA
m1,2
m
I
Q
(Noteꢀ5)
Pulse-SkippingꢀorꢀForcedꢀContinuousꢀ
Modeꢀ(OneꢀChannelꢀOn)
RUN1ꢀ=ꢀ5VꢀandꢀRUN2ꢀ=ꢀ0Vꢀorꢀꢀ
RUN1ꢀ=ꢀ0VꢀandꢀRUN2ꢀ=ꢀ5V,ꢀꢀ
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
1.0
1.26
50
%
µA
V
MAX
I
V
1.4
TRACK/SS1,2
TRACK1,2
l
l
V
V
V
ꢀOn
V
,ꢀV ꢀRising
RUN1 RUN2
1.21
1.31
RUN1,2
ꢀHyst RUNꢀPinꢀHysteresis
mV
mV
RUN1,2
MaximumꢀCurrentꢀSenseꢀThreshold
V
ꢀ=ꢀ0.7V,ꢀV –, –ꢀ=ꢀ3.3V
FB1,2 SENSE1 2
43
50
57
SENSE(MAX)
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
38571fa
ꢂ
LTC3857-1
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
TG/BGꢀt
TopꢀGateꢀOffꢀtoꢀBottomꢀGateꢀOnꢀDelayꢀ
SynchronousꢀSwitch-OnꢀDelayꢀTime
C ꢀ=ꢀ3300pFꢀEachꢀDriver
LOAD
30
ns
1D
BG/TGꢀt
BottomꢀGateꢀOffꢀtoꢀTopꢀGateꢀOnꢀDelayꢀ
TopꢀSwitch-OnꢀDelayꢀTime
C
ꢀ=ꢀ3300pFꢀEachꢀDriver
30
95
ns
ns
1D
LOAD
t
MinimumꢀOn-Time
(Noteꢀ7)
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 Output
V
PGOOD1ꢀVoltageꢀLow
PGOOD1ꢀLeakageꢀCurrent
PGOOD1ꢀTripꢀLevel
I
ꢀ=ꢀ2mA
PGOOD
0.2
0.4
1
V
PGL
I
V
V
ꢀ=ꢀ5V
PGOOD
µA
PGOOD
V
ꢀwithꢀRespectꢀtoꢀSetꢀRegulatedꢀVoltageꢀ
FB
FB
ꢀ
ꢀ
ꢀ
ꢀ
PG
ꢀV ꢀRampingꢀNegativeꢀ
ꢀHysteresis
–13
–10ꢀ
2.5
–7
%ꢀ
%
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:ꢀTheꢀLTC3857-1ꢀisꢀtestedꢀinꢀaꢀfeedbackꢀloopꢀthatꢀservosꢀV
ꢀtoꢀ
ITH1,2
aꢀspecifiedꢀvoltageꢀandꢀmeasuresꢀtheꢀresultantꢀV .ꢀTheꢀspecificationꢀatꢀ
FB1,2
85°Cꢀisꢀnotꢀtestedꢀinꢀproduction.ꢀThisꢀspecificationꢀisꢀassuredꢀbyꢀdesign,ꢀ
characterizationꢀandꢀcorrelationꢀtoꢀproductionꢀtestingꢀatꢀ125°C.
Note 2:ꢀTheꢀLTC3857E-1ꢀisꢀguaranteedꢀtoꢀmeetꢀperformanceꢀspecificationsꢀ
fromꢀ0°Cꢀtoꢀ85°C.ꢀSpecificationsꢀoverꢀtheꢀ–40°Cꢀtoꢀ125°Cꢀoperatingꢀ
junctionꢀtemperatureꢀrangeꢀareꢀassuredꢀbyꢀdesign,ꢀcharacterizationꢀandꢀ
correlationꢀwithꢀstatisticalꢀprocessꢀcontrols.ꢀTheꢀLTC3857I-1ꢀisꢀguaranteedꢀ
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 •ꢀ90°C/W)
J A Dꢀ
38571fa
ꢃ
LTC3857-1
Typical perForMance characTerisTics
Efficiency and Power Loss
Efficiency vs Output Current
Efficiency vs Input Voltage
vs Output Current
100
90
10000
1000
100
10
100
90
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)
1
5
30 35 40
0.000010.0001 0.001 0.01
0.1
1
10
0.000010.0001 0.001 0.01
0.1
1
10
38571 G01
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
38571 G02
BURST EFFICIENCY
PULSE-SKIPPING
EFFICIENCY
BURST LOSS
38571 G03
PULSE-SKIPPING
LOSS
CCM EFFICIENCY
CCM LOSS
Load Step
(Forced Continuous Mode)
Load Step
(Pulse-Skipping Mode)
Load Step (Burst Mode Operation)
V
V
V
OUT
OUT
OUT
100mV/DIV
100mV/DIV
100mV/DIV
INDUCTOR
CURRENT
2A/DIV
INDUCTOR
CURRENT
2A/DIV
INDUCTOR
CURRENT
2A/DIV
38571 G04
38571 G05
38571 G06
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
38571 G08
38571 G07
38571 G09
20ms/DIV
FIGURE 13 CIRCUIT
V
V
LOAD
= 12V
5µs/DIV
20ms/DIV
FIGURE 13 CIRCUIT
IN
= 3.3V
OUT
I
= 200µA
FIGURE 13 CIRCUIT
38571fa
ꢄ
LTC3857-1
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
–45
5
30
55
80
130
20 25
INPUT VOLTAGE (V)
10
–20
105
0
5
10 15
30 35 40
INPUT VOLTAGE (V)
TEMPERATURE (°C)
38571 G10
38571 G11
38571 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
–50
80
60
40
20
0
5% DUTY CYCLE
–100
–150
–200
–250
–300
–350
–400
–450
–500
–550
–600
PULSE-SKIPPING MODE
Burst Mode
OPERATION
0
–20
–40
FORCED CONTINUOUS MODE
0.8
(V)
1.2
1.4
0
10
COMMON MODE VOLTAGE (V)
15
20
25
0
0.2
0.4 0.6
V
1.0
5
0
10 20 30 40 50 60 70 80 90 100
V
DUTY CYCLE (%)
SENSE
ITH
38571 G13
38571 G14
38571 G15
INTVCC and EXTVCC
vs Load Current
Foldback Current Limit
Quiescent Current vs Temperature
80
5.20
5.15
5.10
90
80
70
60
50
40
30
20
10
V
IN
= 12V
75
70
65
60
55
50
45
EXTV = 0V
CC
5.05
5.00
4.95
EXTV = 8.5V
CC
40
0
–20
5
55
80 105 130
0
20 40 60
120 140 160 180 200
–45
30
80 100
0
0.1 0.2 0.3 0.4 0.5
0.9
0.6 0.7 0.8
TEMPERATURE (°C)
LOAD CURRENT (mA)
FEEDBACK VOLTAGE (V)
38571 G17
38571 G18
38571 G16
38571fa
ꢅ
LTC3857-1
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
1.10
1.05
1.00
0.95
800
806
804
RUN RISING
802
800
798
796
794
RUN FALLING
1.20
1.15
1.10
0.90
792
55
TEMPERATURE (°C)
105 130
–45 –20
5
30
80
–45 –20
5
30
55
80 105 130
–20
5
55
80 105 130
–45
30
TEMPERATURE (°C)
TEMPERATURE (°C)
38571 G20
38571 G19
3857 G21
SENSE– Pin Input Current
vs Temperature
Shutdown Current
vs Input Voltage
Oscillator Frequency
vs Temperature
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
5
10
15
20
30
55
TEMPERATURE (°C)
105 130
–45 –20
5
30
80
–45 –20
30
55
80 105 130
TEMPERATURE (°C)
38571 G22
38571 G23
38571 G24
Oscillator Frequency
vs Input Voltage
Undervoltage Lockout Threshold
vs Temperature
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
–45
5
30
55
80 105 130
5
10
15
20
30
–20
5
55
TEMPERATURE (°C)
80 105 130
–20
–45
30
TEMPERATURE (°C)
38571 G25
38571 G26
38571 G27
38571fa
ꢆ
LTC3857-1
pin FuncTions
TH1 TH2
SwitchingꢀRegulatorꢀCompensationꢀPoints.ꢀEachꢀassoci-
atedꢀchannel’sꢀcurrentꢀcomparatorꢀtripꢀpointꢀincreasesꢀ
withꢀthisꢀcontrolꢀvoltage.
I
, I
(Pin 1, Pin 13):ꢀErrorꢀAmplifierꢀOutputsꢀandꢀ
operation.ꢀTyingꢀthisꢀpinꢀtoꢀaꢀvoltageꢀgreaterꢀthanꢀ1.2Vꢀandꢀ
lessꢀthanꢀINTV ꢀ–ꢀ1.3Vꢀselectsꢀpulse-skippingꢀoperation.ꢀ
CC
Thisꢀcanꢀbeꢀdoneꢀbyꢀaddingꢀaꢀ100kꢀresistorꢀbetweenꢀtheꢀ
PLLIN/MODEꢀpinꢀandꢀINTV .
CC
V
, V (Pin 2, Pin 12):ꢀReceivesꢀtheꢀremotelyꢀsensedꢀ
SGND (Pin 7):ꢀ Small-signalꢀ groundꢀ commonꢀ toꢀ bothꢀ
controllers,ꢀmustꢀbeꢀroutedꢀseparatelyꢀfromꢀhighꢀcur-
rentꢀgroundsꢀtoꢀtheꢀcommonꢀ(–)ꢀterminalsꢀofꢀtheꢀCINꢀ
capacitors.ꢀ
FB1 FB2
feedbackꢀ voltageꢀ forꢀ eachꢀ controllerꢀ fromꢀ anꢀ externalꢀ
resistiveꢀdividerꢀacrossꢀtheꢀoutput.
+
+
SENSE1 , SENSE2 (Pin 3, Pin 11):ꢀTheꢀ(+)ꢀinputꢀtoꢀtheꢀ
differentialꢀcurrentꢀcomparatorsꢀareꢀnormallyꢀconnectedꢀ
toꢀDCRꢀsensingꢀnetworksꢀorꢀcurrentꢀsensingꢀresistors.ꢀ
TheꢀI ꢀpinꢀvoltageꢀandꢀcontrolledꢀoffsetsꢀbetweenꢀtheꢀ
SENSE ꢀandꢀSENSE ꢀpinsꢀinꢀconjunctionꢀwithꢀR
theꢀcurrentꢀtripꢀthreshold.
RUN1, RUN2 (Pin 8, Pin 9):ꢀDigitalꢀRunꢀControlꢀInputsꢀ
forꢀEachꢀController.ꢀForcingꢀeitherꢀofꢀtheseꢀpinsꢀbelowꢀ
1.26Vꢀshutsꢀdownꢀthatꢀcontroller.ꢀForcingꢀbothꢀofꢀtheseꢀ
pinsꢀbelowꢀ0.7VꢀshutsꢀdownꢀtheꢀentireꢀLTC3857-1,ꢀreduc-
ingꢀquiescentꢀcurrentꢀtoꢀapproximatelyꢀ8µA.ꢀDoꢀnotꢀfloatꢀ
theseꢀpins.
TH
–
+
setꢀ
SENSEꢀ
–
–
SENSE1 , SENSE2 (Pin 4, Pin 10):ꢀ Theꢀ (–)ꢀ Inputꢀ toꢀ
theꢀDifferentialꢀCurrentꢀComparators.ꢀWhenꢀgreaterꢀthanꢀ
INTV (Pin19):ꢀOutputꢀofꢀtheꢀInternalꢀLinearꢀLowꢀDropoutꢀ
CC
–
INTV ꢀ–ꢀ0.5V,ꢀtheꢀSENSE ꢀpinꢀsuppliesꢀcurrentꢀtoꢀtheꢀ
Regulator.ꢀTheꢀdriverꢀandꢀcontrolꢀcircuitsꢀareꢀpoweredꢀ
fromꢀthisꢀvoltageꢀsource.ꢀMustꢀbeꢀdecoupledꢀtoꢀpowerꢀ
groundꢀwithꢀaꢀminimumꢀofꢀ4.7µFꢀceramicꢀorꢀotherꢀlowꢀ
CC
currentꢀcomparator.
FREQ (Pin 5):ꢀTheꢀFrequencyꢀControlꢀPinꢀforꢀtheꢀInternalꢀ
ESRꢀcapacitor.ꢀDoꢀnotꢀuseꢀtheꢀINTV ꢀpinꢀforꢀanyꢀotherꢀ
CC
VCO.ꢀConnectingꢀtheꢀpinꢀtoꢀGNDꢀforcesꢀtheꢀVCOꢀtoꢀaꢀfixedꢀ
purpose.
lowꢀfrequencyꢀofꢀ350kHz.ꢀConnectingꢀtheꢀpinꢀtoꢀINTV ꢀ
CC
forcesꢀ theꢀ VCOꢀ toꢀ aꢀ fixedꢀ highꢀ frequencyꢀ ofꢀ 535kHz.ꢀ
Otherꢀ frequenciesꢀ betweenꢀ 50kHzꢀ andꢀ 900kHzꢀ canꢀ beꢀ
programmedꢀusingꢀaꢀresistorꢀbetweenꢀFREQꢀandꢀGND.ꢀ
Anꢀinternalꢀ20µAꢀpull-upꢀcurrentꢀdevelopsꢀtheꢀvoltageꢀtoꢀ
beꢀusedꢀbyꢀtheꢀVCOꢀtoꢀcontrolꢀtheꢀfrequencyꢀ
EXTV (Pin 20):ꢀExternalꢀPowerꢀInputꢀtoꢀanꢀInternalꢀLDOꢀ
CC
ConnectedꢀtoꢀINTV .ꢀThisꢀLDOꢀsuppliesꢀINTV ꢀpower,ꢀ
CC
CC
bypassingꢀtheꢀinternalꢀLDOꢀpoweredꢀfromꢀV ꢀwheneverꢀ
IN
EXTV ꢀisꢀhigherꢀthanꢀ4.7V.ꢀSeeꢀEXTV ꢀConnectionꢀinꢀ
CC
CC
theꢀApplicationsꢀInformationꢀsection.ꢀDoꢀnotꢀexceedꢀ14Vꢀ
onꢀthisꢀpin.
PLLIN/MODE (Pin 6):ꢀExternalꢀSynchronizationꢀInputꢀtoꢀ
PhaseꢀDetectorꢀandꢀForcedꢀContinuousꢀModeꢀInput.ꢀWhenꢀ
anꢀexternalꢀclockꢀisꢀappliedꢀtoꢀthisꢀpin,ꢀtheꢀphase-lockedꢀ
loopꢀwillꢀforceꢀtheꢀrisingꢀTG1ꢀsignalꢀtoꢀbeꢀsynchronizedꢀ
withꢀtheꢀrisingꢀedgeꢀofꢀtheꢀexternalꢀclock.ꢀWhenꢀnotꢀsyn-
chronizingꢀtoꢀanꢀexternalꢀclock,ꢀthisꢀinput,ꢀwhichꢀactsꢀonꢀ
bothꢀcontrollers,ꢀdeterminesꢀhowꢀtheꢀLTC3857-1ꢀoperatesꢀ
atꢀlightꢀloads.ꢀPullingꢀthisꢀpinꢀtoꢀgroundꢀselectsꢀBurstꢀ
Modeꢀoperation.ꢀAnꢀinternalꢀ100kꢀresistorꢀtoꢀgroundꢀalsoꢀ
invokesꢀBurstꢀModeꢀoperationꢀwhenꢀtheꢀpinꢀisꢀfloated.ꢀ
PGND (Pin 21):ꢀDriverꢀPowerꢀGround.ꢀConnectsꢀtoꢀtheꢀ
sourcesꢀofꢀbottomꢀ(synchronous)ꢀN-channelꢀMOSFETsꢀ
andꢀtheꢀ(–)ꢀterminal(s)ꢀofꢀC .
IN
V (Pin 22):ꢀMainꢀSupplyꢀPin.ꢀAꢀbypassꢀcapacitorꢀshouldꢀ
IN
beꢀtiedꢀbetweenꢀthisꢀpinꢀandꢀtheꢀsignalꢀgroundꢀpin.
BG1, BG2 (Pin 23, Pin 18):ꢀHighꢀCurrentꢀGateꢀDrivesꢀ
forꢀBottomꢀ(Synchronous)ꢀN-ChannelꢀMOSFETs.ꢀVoltageꢀ
swingꢀatꢀtheseꢀpinsꢀisꢀfromꢀgroundꢀtoꢀINTV .
CC
TyingꢀthisꢀpinꢀtoꢀINTV ꢀforcesꢀcontinuousꢀinductorꢀcurrentꢀ
CC
38571fa
ꢇ
LTC3857-1
pin FuncTions
BOOST1,BOOST2(Pin24,Pin17):ꢀBootstrappedꢀSuppliesꢀ PGOOD1 (Pin 27):ꢀOpen-DrainꢀLogicꢀOutput.ꢀPGOOD1ꢀisꢀ
toꢀtheꢀTopsideꢀFloatingꢀDrivers.ꢀCapacitorsꢀareꢀconnectedꢀ pulledꢀtoꢀgroundꢀwhenꢀtheꢀvoltageꢀonꢀtheꢀV ꢀpinꢀisꢀnotꢀ
FB1
betweenꢀtheꢀBOOSTꢀandꢀSWꢀpinsꢀandꢀSchottkyꢀdiodesꢀareꢀ withinꢀ 10%ꢀofꢀitsꢀsetꢀpoint.
tiedꢀbetweenꢀtheꢀBOOSTꢀandꢀINTVCCꢀpins.ꢀVoltageꢀswingꢀ
TRACK/SS1, TRACK/SS2(Pin28, Pin14):ExternalꢀTrack-
atꢀtheꢀBOOSTꢀpinsꢀisꢀfromꢀINTVCCꢀtoꢀ(VINꢀ+ꢀINTVCC).
ingꢀandꢀSoft-StartꢀInput.ꢀTheꢀLTC3857-1ꢀregulatesꢀtheꢀ
SW1, SW2 (Pin 25, Pin 16):ꢀSwitchꢀNodeꢀConnectionsꢀ
toꢀInductors.ꢀ
V
FB1,2
ꢀvoltageꢀtoꢀtheꢀsmallerꢀofꢀ0.8Vꢀorꢀtheꢀvoltageꢀonꢀtheꢀ
TRACK/SS1,2ꢀpin.ꢀAnꢀinternalꢀ1µAꢀpull-upꢀcurrentꢀsourceꢀ
isꢀconnectedꢀtoꢀthisꢀpin.ꢀAꢀcapacitorꢀtoꢀgroundꢀatꢀthisꢀ
pinꢀsetsꢀtheꢀrampꢀtimeꢀtoꢀfinalꢀregulatedꢀoutputꢀvoltage.ꢀ
Alternatively,ꢀaꢀresistorꢀdividerꢀonꢀanotherꢀvoltageꢀsupplyꢀ
connectedꢀtoꢀthisꢀpinꢀallowsꢀtheꢀLTC3857-1ꢀoutputꢀtoꢀtrackꢀ
theꢀotherꢀsupplyꢀduringꢀstart-up.
TG1, TG2 (Pin 26, Pin 15):ꢀHighꢀCurrentꢀGateꢀDrivesꢀforꢀ
TopꢀN-ChannelꢀMOSFETs.ꢀTheseꢀareꢀtheꢀoutputsꢀofꢀfloat-
ingꢀdriversꢀwithꢀaꢀvoltageꢀswingꢀequalꢀtoꢀINTV ꢀ–ꢀ0.5Vꢀ
CC
superimposedꢀonꢀtheꢀswitchꢀnodeꢀvoltageꢀSW.
38571fa
ꢈ
LTC3857-1
FuncTional DiagraM
INTV
V
IN
CC
DUPLICATE FOR SECOND
CONTROLLER CHANNEL
BOOST
D
B
C
B
TG
DROP
OUT
DET
TOP
BOT
+
C
PGOOD1
0.88V
IN
D
BOT
–
SW
TOP ON
V
S
R
Q
FB1
+
INTV
CC
Q
–
SWITCH
LOGIC
0.72V
BG
SHDN
C
OUT
PGND
20µA
FREQ
V
OUT
VCO
CLK2
CLK1
+
–
R
SENSE
0.425V
SLEEP
L
ICMP
IR
–
+
+
–
PFD
C
LP
+
+
–
–
+
3mV
SENSE
SENSE
SYNC
DET
2.7V
0.55V
PLLIN/MODE
–
100k
SLOPE COMP
V
FB
R
B
+
V
IN
0.80V
TRACK/SS
EA
–
R
A
EXTV
CC
+
–
OV
C
C
0.88V
I
TH
5.1V
LDO
EN
5.1V
LDO
EN
0.5µA
11V
C
C2
R
C
SHDN
RST
FB
1µA
TRACK/SS
+
–
FOLDBACK
2(V
)
4.7V
C
SHDN
SS
SGND
INTV
RUN
CC
38571 FD
38571fa
ꢀ0
LTC3857-1
operaTion (Refer to the Functional Diagram)
Main Control Loop
toꢀturnꢀonꢀtheꢀtopꢀMOSFETꢀcontinuously.ꢀTheꢀdropoutꢀ
detectorꢀdetectsꢀthisꢀandꢀforcesꢀtheꢀtopꢀMOSFETꢀoffꢀforꢀ
aboutꢀone-twelfthꢀofꢀtheꢀclockꢀperiodꢀeveryꢀtenthꢀcycleꢀtoꢀ
TheꢀLTC3857-1ꢀusesꢀaꢀconstantꢀfrequency,ꢀ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)
TheꢀtwoꢀchannelsꢀofꢀtheꢀLTC3857-1ꢀcanꢀbeꢀindependentlyꢀ
shutꢀdownꢀusingꢀtheꢀRUN1ꢀandꢀRUN2ꢀpins.ꢀPullingꢀeitherꢀofꢀ
theseꢀpinsꢀbelowꢀ1.26Vꢀshutsꢀdownꢀtheꢀmainꢀcontrolꢀloopꢀ
forꢀthatꢀcontroller.ꢀPullingꢀbothꢀpinsꢀbelowꢀ0.7Vꢀdisablesꢀ
bothꢀcontrollersꢀandꢀmostꢀinternalꢀcircuits,ꢀincludingꢀtheꢀ
andꢀresetsꢀtheꢀlatchꢀisꢀcontrolledꢀbyꢀtheꢀvoltageꢀonꢀtheꢀI ꢀ
TH
pin,ꢀwhichꢀisꢀtheꢀoutputꢀofꢀtheꢀerrorꢀamplifier,ꢀEA.ꢀTheꢀerrorꢀ
amplifierꢀcomparesꢀtheꢀoutputꢀvoltageꢀfeedbackꢀsignalꢀatꢀ
theꢀV ꢀpin,ꢀ(whichꢀisꢀgeneratedꢀwithꢀanꢀexternalꢀresistorꢀ
FB
INTV ꢀLDOs.ꢀInꢀthisꢀstate,ꢀtheꢀLTC3857-1ꢀdrawsꢀonlyꢀ8µAꢀ
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,ꢀtheꢀLTC3857-1ꢀregulatesꢀ
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
asꢀoneꢀofꢀtheꢀLTC3857-1ꢀswitchingꢀregulatorꢀoutputs.
zeroꢀtoꢀitsꢀfinalꢀvalue.
Eachꢀ topꢀ MOSFETꢀ driverꢀ isꢀ biasedꢀ fromꢀ theꢀ floatingꢀ
AlternativelyꢀtheꢀTRACK/SSꢀpinꢀcanꢀbeꢀusedꢀtoꢀcauseꢀtheꢀ
bootstrapꢀcapacitorꢀC ,ꢀwhichꢀnormallyꢀrechargesꢀduringꢀ
start-upꢀofꢀV ꢀtoꢀtrackꢀꢀthatꢀofꢀanotherꢀsupply.ꢀTypically,ꢀ
B
OUT
eachꢀcycleꢀthroughꢀanꢀexternalꢀdiodeꢀwhenꢀtheꢀtopꢀMOSFETꢀ
thisꢀrequiresꢀconnectingꢀtoꢀtheꢀTRACK/SSꢀpinꢀanꢀexternalꢀ
resistorꢀ dividerꢀ fromꢀ theꢀ otherꢀ supplyꢀ toꢀ groundꢀ (seeꢀ
ApplicationsꢀInformationꢀsection).
turnsꢀoff.ꢀIfꢀtheꢀinputꢀvoltage,ꢀV ,ꢀdecreasesꢀtoꢀaꢀvoltageꢀ
IN
closeꢀtoꢀV ,ꢀtheꢀloopꢀmayꢀenterꢀdropoutꢀandꢀattemptꢀ
OUTꢀ
38571fa
ꢀꢀ
LTC3857-1
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
largeꢀtransientꢀconditions.ꢀTheꢀpeakꢀinductorꢀcurrentꢀisꢀ
TheꢀLTC3857-1ꢀcanꢀbeꢀenabledꢀtoꢀenterꢀhighꢀefficiencyꢀ
BurstꢀModeꢀoperation,ꢀconstantꢀfrequencyꢀpulse-skippingꢀ
mode,ꢀorꢀforcedꢀcontinuousꢀconductionꢀmodeꢀatꢀlowꢀloadꢀ
currents.ꢀToꢀselectꢀBurstꢀModeꢀoperation,ꢀtieꢀtheꢀPLLIN/ꢀ
MODEꢀpinꢀtoꢀGND.ꢀToꢀselectꢀforcedꢀcontinuousꢀoperation,ꢀ
determinedꢀbyꢀtheꢀvoltageꢀonꢀtheꢀI ꢀpin,ꢀjustꢀasꢀinꢀnormalꢀ
TH
operation.ꢀInꢀthisꢀmode,ꢀtheꢀefficiencyꢀatꢀlightꢀloadsꢀisꢀlowerꢀ
thanꢀinꢀBurstꢀModeꢀoperation.ꢀHowever,ꢀcontinuousꢀopera-
tionꢀhasꢀtheꢀadvantageꢀofꢀlowerꢀoutputꢀvoltageꢀrippleꢀandꢀ
lessꢀinterferenceꢀtoꢀaudioꢀcircuitry.ꢀInꢀforcedꢀcontinuousꢀ
mode,ꢀtheꢀoutputꢀrippleꢀisꢀindependentꢀofꢀloadꢀcurrent.
tieꢀtheꢀPLLIN/MODEꢀpinꢀtoꢀINTV .ꢀToꢀselectꢀpulse-skippingꢀ
CC
mode,ꢀtieꢀtheꢀPLLIN/MODEꢀpinꢀtoꢀaꢀDCꢀvoltageꢀgreaterꢀ
thanꢀ1.2VꢀandꢀlessꢀthanꢀINTV ꢀ–ꢀ1.3V.
CC
WhenꢀtheꢀPLLIN/MODEꢀpinꢀisꢀconnectedꢀforꢀpulse-skippingꢀ
mode,ꢀtheꢀLTC3857-1ꢀoperatesꢀinꢀPWMꢀpulse-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.
WhenꢀaꢀcontrollerꢀisꢀenabledꢀforꢀBurstꢀModeꢀoperation,ꢀ
theꢀminimumꢀpeakꢀcurrentꢀinꢀtheꢀinductorꢀisꢀsetꢀtoꢀap-
proximatelyꢀ15%ꢀofꢀtheꢀmaximumꢀsenseꢀvoltageꢀevenꢀ
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,ꢀ
reducingꢀtheꢀquiescentꢀcurrentꢀthatꢀtheꢀLTC3857-1ꢀdraws.ꢀ
Ifꢀoneꢀchannelꢀisꢀshutꢀdownꢀandꢀtheꢀotherꢀchannelꢀisꢀinꢀ
sleepꢀmode,ꢀtheꢀLTC3857-1ꢀdrawsꢀonlyꢀ50µAꢀofꢀquiescentꢀ
current.ꢀIfꢀbothꢀchannelsꢀareꢀinꢀsleepꢀmode,ꢀtheꢀLTC3857-1ꢀ
drawsꢀonlyꢀ80µAꢀofꢀquiescentꢀcurrent.ꢀInꢀsleepꢀmode,ꢀ
theꢀloadꢀcurrentꢀisꢀsuppliedꢀbyꢀtheꢀoutputꢀcapacitor.ꢀAsꢀ
theꢀoutputꢀvoltageꢀdecreases,ꢀtheꢀEA’sꢀoutputꢀbeginsꢀtoꢀ
Frequency Selection and Phase-Locked Loop
(FREQ and PLLIN/MODE Pins)
Theꢀselectionꢀofꢀswitchingꢀfrequencyꢀisꢀaꢀtradeoffꢀbetweenꢀ
efficiencyꢀ andꢀ componentꢀ size.ꢀ Lowꢀ frequencyꢀ opera-
tionꢀincreasesꢀefficiencyꢀbyꢀreducingꢀMOSFETꢀswitchingꢀ
losses,ꢀbutꢀrequiresꢀlargerꢀinductanceꢀand/orꢀcapacitanceꢀ
toꢀmaintainꢀlowꢀoutputꢀrippleꢀvoltage.
rise.ꢀWhenꢀtheꢀoutputꢀvoltageꢀdropsꢀenough,ꢀtheꢀI ꢀpinꢀ
TH
isꢀreconnectedꢀtoꢀtheꢀoutputꢀofꢀtheꢀEA,ꢀtheꢀsleepꢀsignalꢀ
goesꢀlow,ꢀandꢀtheꢀcontrollerꢀresumesꢀnormalꢀoperationꢀ
byꢀturningꢀonꢀtheꢀtopꢀexternalꢀMOSFETꢀonꢀtheꢀnextꢀcycleꢀ
ofꢀtheꢀinternalꢀoscillator.
TheꢀswitchingꢀfrequencyꢀofꢀtheꢀLTC3857-1’sꢀcontrollersꢀ
canꢀbeꢀselectedꢀusingꢀtheꢀFREQꢀpin.
WhenꢀaꢀcontrollerꢀisꢀenabledꢀforꢀBurstꢀModeꢀoperation,ꢀtheꢀ
inductorꢀcurrentꢀisꢀnotꢀallowedꢀtoꢀreverse.ꢀTheꢀreverseꢀcur-
rentꢀcomparator,ꢀIR,ꢀturnsꢀoffꢀtheꢀbottomꢀexternalꢀMOSFETꢀ
justꢀbeforeꢀtheꢀinductorꢀcurrentꢀreachesꢀzero,ꢀpreventingꢀ
itꢀfromꢀreversingꢀandꢀgoingꢀnegative.ꢀThus,ꢀtheꢀcontrollerꢀ
operatesꢀinꢀdiscontinuousꢀoperation.
IfꢀtheꢀPLLIN/MODEꢀpinꢀisꢀnotꢀbeingꢀdrivenꢀbyꢀanꢀexternalꢀ
clockꢀsource,ꢀtheꢀFREQꢀpinꢀcanꢀbeꢀtiedꢀtoꢀSGND,ꢀtiedꢀtoꢀ
INTVCCꢀorꢀprogrammedꢀthroughꢀanꢀexternalꢀresistor.ꢀTyingꢀ
FREQꢀtoꢀSGNDꢀselectsꢀ350kHzꢀwhileꢀtyingꢀFREQꢀtoꢀINTVCCꢀ
selectsꢀ535kHz.ꢀPlacingꢀaꢀresistorꢀbetweenꢀFREQꢀandꢀ
38571fa
ꢀꢁ
LTC3857-1
operaTion (Refer to the Functional Diagram)
SGNDꢀallowsꢀtheꢀfrequencyꢀtoꢀbeꢀprogrammedꢀbetweenꢀ Power Good (PGOOD1 Pin)
50kHzꢀandꢀ900kHz.
TheꢀPGOOD1ꢀpinꢀisꢀconnectedꢀtoꢀanꢀopenꢀdrainꢀofꢀanꢀinternalꢀ
Aꢀphase-lockedꢀloopꢀ(PLL)ꢀisꢀavailableꢀonꢀtheꢀLTC3857-1ꢀ N-channelꢀMOSFET.ꢀTheꢀMOSFETꢀturnsꢀonꢀandꢀpullsꢀtheꢀ
toꢀsynchronizeꢀtheꢀinternalꢀoscillatorꢀtoꢀanꢀexternalꢀclockꢀ PGOOD1ꢀpinꢀlowꢀwhenꢀtheꢀcorrespondingꢀV ꢀpinꢀvolt-
FB1
sourceꢀthatꢀisꢀconnectedꢀtoꢀtheꢀPLLIN/MODEꢀpin.ꢀTheꢀ ageꢀisꢀnotꢀwithinꢀ 10%ꢀofꢀtheꢀ0.8Vꢀreferenceꢀvoltage.ꢀTheꢀ
phaseꢀdetectorꢀadjustsꢀtheꢀvoltageꢀ(throughꢀanꢀinternalꢀ PGOOD1ꢀpinꢀisꢀalsoꢀpulledꢀlowꢀwhenꢀtheꢀcorrespondingꢀ
lowpassꢀfilter)ꢀofꢀtheꢀVCOꢀinputꢀtoꢀalignꢀtheꢀturn-onꢀofꢀ RUN1ꢀpinꢀisꢀlowꢀ(shutꢀdown).ꢀWhenꢀtheꢀV ꢀpinꢀvoltageꢀ
FB1
controllerꢀ1’sꢀexternalꢀtopꢀMOSFETꢀtoꢀtheꢀrisingꢀedgeꢀofꢀ isꢀwithinꢀtheꢀ 10%ꢀrequirement,ꢀtheꢀMOSFETꢀisꢀturnedꢀ
theꢀsynchronizingꢀsignal.ꢀThus,ꢀtheꢀturn-onꢀofꢀcontrollerꢀ offꢀandꢀtheꢀpinꢀisꢀallowedꢀtoꢀbeꢀpulledꢀupꢀbyꢀanꢀexternalꢀ
2’sꢀexternalꢀtopꢀMOSFETꢀisꢀ180ꢀdegreesꢀoutꢀofꢀphaseꢀtoꢀ resistorꢀtoꢀaꢀsourceꢀnoꢀgreaterꢀthanꢀ6V.
theꢀrisingꢀedgeꢀofꢀtheꢀexternalꢀclockꢀsource.
Foldback Current
Theꢀ VCOꢀ inputꢀ voltageꢀ isꢀ prebiasedꢀ toꢀ theꢀ operatingꢀ
Whenꢀtheꢀoutputꢀvoltageꢀfallsꢀtoꢀlessꢀthanꢀ70%ꢀofꢀitsꢀ
nominalꢀlevel,ꢀfoldbackꢀcurrentꢀlimitingꢀisꢀactivated,ꢀpro-
gressivelyꢀloweringꢀtheꢀpeakꢀcurrentꢀlimitꢀinꢀproportionꢀtoꢀ
theꢀseverityꢀofꢀtheꢀovercurrentꢀorꢀshort-circuitꢀcondition.ꢀ
Foldbackꢀcurrentꢀlimitingꢀisꢀdisabledꢀduringꢀtheꢀsoft-startꢀ
frequencyꢀsetꢀbyꢀtheꢀFREQꢀpinꢀbeforeꢀtheꢀexternalꢀclockꢀ
isꢀapplied.ꢀIfꢀprebiasedꢀnearꢀtheꢀexternalꢀclockꢀfrequeny,ꢀ
theꢀPLLꢀloopꢀonlyꢀneedsꢀtoꢀmakeꢀslightꢀchangesꢀtoꢀtheꢀ
VCOꢀinputꢀinꢀorderꢀtoꢀsynchronizeꢀtheꢀrisingꢀedgeꢀofꢀtheꢀ
externalꢀclock’sꢀtoꢀtheꢀrisingꢀedgeꢀofꢀTG1.ꢀTheꢀabilityꢀtoꢀ
prebiasꢀtheꢀloopꢀfilterꢀallowsꢀtheꢀPLLꢀtoꢀlock-inꢀrapidlyꢀ
withoutꢀdeviatingꢀfarꢀfromꢀtheꢀdesiredꢀfrequency.
intervalꢀ(asꢀlongꢀasꢀtheꢀV ꢀvoltageꢀisꢀkeepingꢀupꢀwithꢀtheꢀ
FB
TRACK/SSꢀvoltage).
Theꢀtypicalꢀcaptureꢀrangeꢀofꢀtheꢀphase-lockedꢀloopꢀisꢀfromꢀ
approximatelyꢀ55kHzꢀtoꢀ1MHz,ꢀwithꢀaꢀguaranteeꢀoverꢀallꢀ
manufacturingꢀvariationsꢀtoꢀbeꢀbetweenꢀ75kHzꢀandꢀ850kHz.ꢀ
Inꢀotherꢀwords,ꢀtheꢀLTC3857-1’sꢀPLLꢀisꢀguaranteedꢀtoꢀlockꢀ
toꢀanꢀexternalꢀclockꢀsourceꢀwhoseꢀfrequencyꢀisꢀbetweenꢀ
75kHzꢀandꢀ850kHz.
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
pulsesꢀincreasedꢀtheꢀtotalꢀRMSꢀcurrentꢀflowingꢀfromꢀtheꢀ
inputꢀcapacitor,ꢀrequiringꢀtheꢀuseꢀofꢀmoreꢀexpensiveꢀinputꢀ
capacitorsꢀandꢀincreasingꢀbothꢀEMIꢀandꢀlossesꢀinꢀtheꢀinputꢀ
TheꢀtypicalꢀinputꢀclockꢀthresholdsꢀonꢀtheꢀPLLIN/MODEꢀ
pinꢀareꢀ1.6Vꢀ(rising)ꢀandꢀ1.1Vꢀ(falling).
Output Overvoltage Protection
Anꢀovervoltageꢀcomparatorꢀguardsꢀagainstꢀtransientꢀover-
shootsꢀasꢀwellꢀasꢀotherꢀmoreꢀseriousꢀconditionsꢀthatꢀmayꢀ capacitorꢀandꢀbattery.
overvoltageꢀtheꢀoutput.ꢀWhenꢀtheꢀV ꢀpinꢀrisesꢀbyꢀmoreꢀ
FB
Withꢀ 2-phaseꢀ operation,ꢀ theꢀ twoꢀ channelsꢀ ofꢀ theꢀ dualꢀ
switchingꢀregulatorꢀareꢀoperatedꢀ180ꢀdegreesꢀoutꢀofꢀphase.ꢀ
Thisꢀeffectivelyꢀinterleavesꢀtheꢀcurrentꢀpulsesꢀdrawnꢀbyꢀtheꢀ
switches,ꢀgreatlyꢀreducingꢀtheꢀoverlapꢀtimeꢀwhereꢀtheyꢀaddꢀ
thanꢀ10%ꢀaboveꢀitsꢀregulationꢀpointꢀofꢀ0.800V,ꢀtheꢀtopꢀ
MOSFETꢀisꢀturnedꢀoffꢀandꢀtheꢀbottomꢀMOSFETꢀisꢀturnedꢀ
onꢀuntilꢀtheꢀovervoltageꢀconditionꢀisꢀcleared.
38571fa
ꢀꢂ
LTC3857-1
operaTion (Refer to the Functional Diagram)
together.ꢀTheꢀresultꢀisꢀaꢀsignificantꢀreductionꢀinꢀtotalꢀRMSꢀ theꢀRMSꢀinputꢀcurrentꢀvariesꢀforꢀsingleꢀphaseꢀandꢀ2-phaseꢀ
inputꢀcurrent,ꢀwhichꢀinꢀturnꢀallowsꢀlessꢀexpensiveꢀinputꢀ operationꢀforꢀ3.3Vꢀandꢀ5Vꢀregulatorsꢀoverꢀaꢀwideꢀinputꢀ
capacitorsꢀtoꢀbeꢀused,ꢀreducesꢀshieldingꢀrequirementsꢀforꢀ voltageꢀrange.
EMIꢀandꢀimprovesꢀrealꢀworldꢀoperatingꢀefficiency.
Itꢀcanꢀreadilyꢀbeꢀseenꢀthatꢀtheꢀadvantagesꢀofꢀ2-phaseꢀop-
Figureꢀ1ꢀcomparesꢀtheꢀinputꢀwaveformsꢀforꢀaꢀsingle-phaseꢀ erationꢀareꢀnotꢀjustꢀlimitedꢀtoꢀaꢀnarrowꢀoperatingꢀrange,ꢀ
dualꢀ switchingꢀ regulatorꢀ toꢀ aꢀ 2-phaseꢀ dualꢀ switchingꢀ forꢀmostꢀapplicationsꢀisꢀthatꢀ2-phaseꢀoperationꢀwillꢀreduceꢀ
regulator.ꢀAnꢀactualꢀmeasurementꢀofꢀtheꢀRMSꢀinputꢀcur- theꢀinputꢀcapacitorꢀrequirementꢀtoꢀthatꢀforꢀjustꢀoneꢀchannelꢀ
rentꢀunderꢀtheseꢀconditionsꢀshowsꢀthatꢀ2-phaseꢀoperationꢀ operatingꢀatꢀmaximumꢀcurrentꢀandꢀ50%ꢀdutyꢀcycle.
droppedꢀtheꢀinputꢀcurrentꢀfromꢀ2.53A
ꢀtoꢀ1.55A
.ꢀ
RMS
RMS
3.0
2.5
2.0
1.5
1.0
0.5
0
Whileꢀthisꢀisꢀanꢀimpressiveꢀreductionꢀinꢀitself,ꢀrememberꢀ
2
SINGLE PHASE
thatꢀtheꢀpowerꢀlossesꢀareꢀproportionalꢀtoꢀI
,ꢀmeaningꢀ
RMS
DUAL CONTROLLER
thatꢀtheꢀactualꢀpowerꢀwastedꢀisꢀreducedꢀbyꢀaꢀfactorꢀofꢀ2.66.ꢀ
Theꢀreducedꢀinputꢀrippleꢀvoltageꢀalsoꢀmeansꢀlessꢀpowerꢀisꢀ
lostꢀinꢀtheꢀinputꢀpowerꢀpath,ꢀwhichꢀcouldꢀincludeꢀbatter-
ies,ꢀswitches,ꢀtrace/connectorꢀresistancesꢀandꢀprotectionꢀ
circuitry.ꢀImprovementsꢀinꢀbothꢀconductedꢀandꢀradiatedꢀ
EMIꢀalsoꢀdirectlyꢀaccrueꢀasꢀaꢀresultꢀofꢀtheꢀreducedꢀRMSꢀ
inputꢀcurrentꢀandꢀvoltage.
2-PHASE
DUAL CONTROLLER
V
O1
V
O2
= 5V/3A
= 3.3V/3A
Ofꢀcourse,ꢀtheꢀimprovementꢀaffordedꢀbyꢀ2-phaseꢀopera-
tionꢀisꢀaꢀfunctionꢀofꢀtheꢀdualꢀswitchingꢀregulator’sꢀrelativeꢀ
dutyꢀcyclesꢀwhich,ꢀinꢀturn,ꢀareꢀdependentꢀuponꢀtheꢀinputꢀ
voltageꢀV ꢀ(DutyꢀCycleꢀ=ꢀV /V ).ꢀFigureꢀ2ꢀshowsꢀhowꢀ
0
10
20
30
40
INPUT VOLTAGE (V)
38571 F02
Figure 2. RMS Input Current Comparison
IN
OUT IN
5V SWITCH
20V/DIV
3.3V SWITCH
20V/DIV
INPUT CURRENT
5A/DIV
INPUT VOLTAGE
500mV/DIV
38571 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
38571fa
ꢀꢃ
LTC3857-1
applicaTions inForMaTion
Theꢀ Typicalꢀ Applicationꢀ onꢀ theꢀ firstꢀ pageꢀ isꢀ aꢀ basicꢀ
LTC3857-1ꢀapplicationꢀcircuit.ꢀLTC3857-1ꢀcanꢀbeꢀconfiguredꢀ
toꢀuseꢀeitherꢀDCRꢀ(inductorꢀresistance)ꢀsensingꢀorꢀlowꢀ
valueꢀresistorꢀsensing.ꢀTheꢀchoiceꢀbetweenꢀtheꢀtwoꢀcurrentꢀ
sensingꢀschemesꢀisꢀlargelyꢀaꢀdesignꢀtrade-offꢀbetweenꢀ
cost,ꢀpowerꢀconsumption,ꢀandꢀaccuracy.ꢀDCRꢀsensingꢀ
isꢀbecomingꢀpopularꢀbecauseꢀitꢀsavesꢀexpensiveꢀcurrentꢀ
sensingꢀresistorsꢀandꢀisꢀmoreꢀpowerꢀefficient,ꢀespeciallyꢀ
inꢀ highꢀ currentꢀ applications.ꢀ However,ꢀ currentꢀ sensingꢀ
resistorsꢀprovideꢀtheꢀmostꢀaccurateꢀcurrentꢀlimitsꢀforꢀtheꢀ
controller.ꢀOtherꢀexternalꢀcomponentꢀselectionꢀisꢀdrivenꢀ
byꢀtheꢀloadꢀrequirement,ꢀandꢀbeginsꢀwithꢀtheꢀselectionꢀofꢀ
programmedꢀcurrentꢀlimitꢀunpredictable.ꢀIfꢀinductorꢀDCRꢀ
sensingꢀisꢀusedꢀ(Figureꢀ4b),ꢀresistorꢀR1ꢀshouldꢀbeꢀplacedꢀ
closeꢀtoꢀtheꢀswitchingꢀnode,ꢀtoꢀpreventꢀnoiseꢀfromꢀcouplingꢀ
intoꢀsensitiveꢀsmall-signalꢀnodes.
TO SENSE FILTER,
NEXT TO THE CONTROLLER
C
OUT
38571 F03
INDUCTOR OR R
SENSE
Figure 3. Sense Lines Placement with Inductor or Sense Resistor
R
ꢀ(ifꢀR
ꢀisꢀused)ꢀandꢀinductorꢀvalue.ꢀNext,ꢀtheꢀ
SENSE
SENSE
V
V
IN
powerꢀMOSFETsꢀandꢀSchottkyꢀdiodesꢀareꢀselected.ꢀFinally,ꢀ
inputꢀandꢀoutputꢀcapacitorsꢀareꢀselected.
IN
INTV
CC
BOOST
TG
+
–
SENSE and SENSE Pins
R
SENSE
SW
V
OUT
+
–
TheꢀSENSE ꢀandꢀSENSE ꢀpinsꢀareꢀtheꢀinputsꢀtoꢀtheꢀcurrentꢀ
comparators.ꢀTheꢀcommonꢀmodeꢀvoltageꢀrangeꢀonꢀtheseꢀ
pinsꢀisꢀ0Vꢀtoꢀ24Vꢀ(absꢀmax),ꢀenablingꢀtheꢀLTC3857-1ꢀtoꢀ
regulateꢀoutputꢀvoltagesꢀupꢀtoꢀaꢀnominalꢀ24Vꢀ(allowingꢀ
marginꢀforꢀtolerancesꢀandꢀtransients).ꢀ
LTC3857-1
BG
+
SENSE
PLACE CAPACITOR NEAR
SENSE PINS
–
SENSE
SGND
+
TheꢀSENSE ꢀpinꢀisꢀhighꢀimpedanceꢀoverꢀtheꢀfullꢀcommonꢀ
38571 F04a
modeꢀrange,ꢀdrawingꢀatꢀmostꢀ 1µA.ꢀThisꢀhighꢀimpedanceꢀ
allowsꢀtheꢀcurrentꢀcomparatorsꢀtoꢀbeꢀusedꢀinꢀinductorꢀ
DCRꢀsensing.
(4a) Using a Resistor to Sense Current
V
INTV
V
IN
IN
–
TheꢀimpedanceꢀofꢀtheꢀSENSE ꢀpinꢀchangesꢀdependingꢀonꢀ
CC
–
theꢀcommonꢀmodeꢀvoltage.ꢀWhenꢀSENSE ꢀisꢀlessꢀthanꢀ
INDUCTOR
DCR
BOOST
TG
INTV ꢀ–ꢀ0.5V,ꢀaꢀsmallꢀcurrentꢀofꢀlessꢀthanꢀ1µAꢀflowsꢀoutꢀ
CC
L
–
ofꢀtheꢀpin.ꢀWhenꢀSENSE ꢀisꢀaboveꢀINTV ꢀ+ꢀ0.5V,ꢀaꢀhigherꢀ
SW
V
OUT
CC
LTC3857-1
currentꢀ(~550µA)ꢀflowsꢀintoꢀtheꢀpin.ꢀBetweenꢀINTV ꢀ–ꢀ0.5Vꢀ
CC
BG
andꢀINTV ꢀ+ꢀ0.5V,ꢀtheꢀcurrentꢀtransitionsꢀfromꢀtheꢀsmallerꢀ
CC
R1
C1* R2
currentꢀtoꢀtheꢀhigherꢀcurrent.
+
SENSE
Filterꢀcomponentsꢀmutualꢀtoꢀtheꢀsenseꢀlinesꢀshouldꢀbeꢀ
placedꢀcloseꢀtoꢀtheꢀLTC3857-1,ꢀandꢀtheꢀsenseꢀlinesꢀshouldꢀ
runꢀcloseꢀtogetherꢀtoꢀaꢀKelvinꢀconnectionꢀunderneathꢀtheꢀ
currentꢀsenseꢀelementꢀ(shownꢀinꢀFigureꢀ3).ꢀSensingꢀcur-
rentꢀelsewhereꢀcanꢀeffectivelyꢀaddꢀparasiticꢀinductanceꢀ
andꢀcapacitanceꢀtoꢀtheꢀcurrentꢀsenseꢀelement,ꢀdegradingꢀ
theꢀinformationꢀatꢀtheꢀsenseꢀterminalsꢀandꢀmakingꢀtheꢀ
–
SENSE
SGND
38571 F04b
R2
R1 + R2
L
DCR
||
(R1 R2) • C1 =
*PLACE C1 NEAR
SENSE PINS
R
= DCR
SENSE(EQ)
(4b) Using the Inductor DCR to Sense Current
Figure 4. Current Sensing Methods
38571fa
ꢀꢄ
LTC3857-1
applicaTions inForMaTion
Low Value Resistor Current Sensing
acrossꢀtheꢀexternalꢀcapacitorꢀisꢀequalꢀtoꢀtheꢀdropꢀacrossꢀ
theꢀinductorꢀDCRꢀmultipliedꢀbyꢀR2/(R1ꢀ+ꢀR2).ꢀR2ꢀscalesꢀtheꢀ
voltageꢀacrossꢀtheꢀsenseꢀterminalsꢀforꢀapplicationsꢀwhereꢀ
theꢀDCRꢀisꢀgreaterꢀthanꢀtheꢀtargetꢀsenseꢀresistorꢀvalue.ꢀ
Toꢀproperlyꢀdimensionꢀtheꢀexternalꢀfilterꢀcomponents,ꢀtheꢀ
DCRꢀofꢀtheꢀinductorꢀmustꢀbeꢀknown.ꢀItꢀcanꢀbeꢀmeasuredꢀ
usingꢀaꢀgoodꢀRLCꢀmeter,ꢀbutꢀtheꢀDCRꢀtoleranceꢀisꢀnotꢀ
alwaysꢀtheꢀsameꢀandꢀvariesꢀwithꢀtemperature;ꢀconsultꢀtheꢀ
manufacturers’ꢀdataꢀsheetsꢀforꢀdetailedꢀinformation.
Aꢀtypicalꢀsensingꢀcircuitꢀusingꢀaꢀdiscreteꢀresistorꢀisꢀshownꢀ
inꢀ Figureꢀ 4a.ꢀ R
outputꢀcurrent.
ꢀ isꢀ chosenꢀ basedꢀ onꢀ theꢀ requiredꢀ
SENSE
Theꢀ currentꢀ comparatorꢀ hasꢀ aꢀ maximumꢀ thresholdꢀ
.ꢀTheꢀcurrentꢀcomparatorꢀthresholdꢀvoltageꢀ
V
SENSE(MAX)
setsꢀtheꢀpeakꢀofꢀtheꢀinductorꢀcurrent,ꢀyieldingꢀaꢀmaximumꢀ
averageꢀoutputꢀcurrent,ꢀI
,ꢀequalꢀtoꢀtheꢀpeakꢀvalueꢀlessꢀ
MAX
halfꢀtheꢀpeak-to-peakꢀrippleꢀcurrent,ꢀ∆I .ꢀToꢀcalculateꢀtheꢀ
L
UsingꢀtheꢀinductorꢀrippleꢀcurrentꢀvalueꢀfromꢀtheꢀInductorꢀ
ValueꢀCalculationꢀsection,ꢀtheꢀtargetꢀsenseꢀresistorꢀvalueꢀ
is:
senseꢀresistorꢀvalue,ꢀuseꢀtheꢀequation:
VSENSE(MAX)
RSENSE
=
∆IL
VSENSE(MAX)
IMAX
+
RSENSE(EQUIV)
=
2
ꢀ
∆IL
IMAX
+
Whenꢀusingꢀtheꢀcontrollerꢀinꢀveryꢀlowꢀdropoutꢀconditions,ꢀ
theꢀmaximumꢀoutputꢀcurrentꢀlevelꢀwillꢀbeꢀreducedꢀdueꢀtoꢀtheꢀ
internalꢀcompensationꢀrequiredꢀtoꢀmeetꢀstabilityꢀcriterionꢀ
forꢀbuckꢀregulatorsꢀoperatingꢀatꢀgreaterꢀthanꢀ50%ꢀdutyꢀ
factor.ꢀAꢀcurveꢀisꢀprovidedꢀinꢀtheꢀTypicalꢀPerformanceꢀChar-
acteristicsꢀsectionꢀtoꢀestimateꢀthisꢀreductionꢀinꢀpeakꢀoutputꢀ
currentꢀdependingꢀuponꢀtheꢀoperatingꢀdutyꢀfactor.
ꢀ
2
Toꢀensureꢀthatꢀtheꢀapplicationꢀwillꢀdeliverꢀfullꢀloadꢀcurrentꢀ
overꢀ theꢀ fullꢀ operatingꢀ temperatureꢀ range,ꢀ chooseꢀ theꢀ
minimumꢀvalueꢀforꢀtheꢀmaximumꢀcurrentꢀsenseꢀthresholdꢀ
voltageꢀ(V
).
SENSE(MAX)
Next,ꢀdetermineꢀtheꢀDCRꢀofꢀtheꢀinductor.ꢀWhenꢀprovided,ꢀ
useꢀtheꢀmanufacturer’sꢀmaximumꢀvalue,ꢀusuallyꢀgivenꢀatꢀ
20°C.ꢀIncreaseꢀthisꢀvalueꢀtoꢀaccountꢀforꢀtheꢀtemperatureꢀ
coefficientꢀofꢀcopperꢀresistance,ꢀwhichꢀisꢀapproximatelyꢀ
Inductor DCR Sensing
Forꢀapplicationsꢀrequiringꢀtheꢀhighestꢀpossibleꢀefficiencyꢀ
atꢀhighꢀloadꢀcurrents,ꢀtheꢀLTC3857-1ꢀisꢀcapableꢀofꢀsensingꢀ
theꢀvoltageꢀdropꢀacrossꢀtheꢀinductorꢀDCR,ꢀasꢀshownꢀinꢀ
Figureꢀ4b.ꢀTheꢀDCRꢀofꢀtheꢀinductorꢀrepresentsꢀtheꢀsmallꢀ
amountꢀofꢀDCꢀresistanceꢀofꢀtheꢀcopperꢀwire,ꢀwhichꢀcanꢀbeꢀ
lessꢀthanꢀ1mΩꢀforꢀtoday’sꢀlowꢀvalue,ꢀhighꢀcurrentꢀinductors.ꢀ
Inꢀaꢀhighꢀcurrentꢀapplicationꢀrequiringꢀsuchꢀanꢀinductor,ꢀ
powerꢀlossꢀthroughꢀaꢀsenseꢀresistorꢀwouldꢀcostꢀseveralꢀ
pointsꢀofꢀefficiencyꢀcomparedꢀtoꢀinductorꢀDCRꢀsensing.
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
IfꢀtheꢀexternalꢀR1||R2ꢀ•ꢀC1ꢀtimeꢀconstantꢀisꢀchosenꢀtoꢀbeꢀ
exactlyꢀequalꢀtoꢀtheꢀL/DCRꢀtimeꢀconstant,ꢀtheꢀvoltageꢀdropꢀ
+
haveꢀbeenꢀcausedꢀbyꢀtheꢀSENSE ꢀpin’sꢀ 1µAꢀcurrent.
38571fa
ꢀꢅ
LTC3857-1
applicaTions inForMaTion
TheꢀequivalentꢀresistanceꢀR1||ꢀR2ꢀisꢀscaledꢀtoꢀtheꢀroomꢀ
temperatureꢀinductanceꢀandꢀmaximumꢀDCR:
Acceptingꢀlargerꢀvaluesꢀofꢀ∆I ꢀallowsꢀtheꢀuseꢀofꢀlowꢀin-
L
ductances,ꢀbutꢀresultsꢀinꢀhigherꢀoutputꢀvoltageꢀrippleꢀandꢀ
greaterꢀcoreꢀlosses.ꢀAꢀreasonableꢀstartingꢀpointꢀforꢀsettingꢀ
L
R1||R2 =
rippleꢀcurrentꢀisꢀ∆I ꢀ=0.3(I
).ꢀTheꢀmaximumꢀ∆I ꢀoccursꢀ
L
L
MAX
DCR at 20°C •C1
atꢀtheꢀmaximumꢀinputꢀvoltage.
ꢀ
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
controllerꢀinꢀtheꢀLTC3857-1:ꢀoneꢀN-channelꢀMOSFETꢀforꢀ
theꢀtopꢀ(main)ꢀswitch,ꢀandꢀoneꢀN-channelꢀMOSFETꢀforꢀtheꢀ
bottomꢀ(synchronous)ꢀswitch.
Theꢀinductorꢀvalueꢀhasꢀaꢀdirectꢀeffectꢀonꢀrippleꢀcurrent.ꢀ
Theꢀinductorꢀrippleꢀcurrent,ꢀ∆I ,ꢀdecreasesꢀwithꢀhigherꢀ
L
inductanceꢀorꢀhigherꢀfrequencyꢀandꢀincreasesꢀwithꢀhigherꢀ
V :
IN
VOUT
1
∆IL =
VOUT 1–
V
f L
IN
ꢀ
38571fa
ꢀꢆ
LTC3857-1
applicaTions inForMaTion
Theꢀpeak-to-peakꢀdriveꢀlevelsꢀareꢀsetꢀbyꢀtheꢀINTV ꢀvoltage.ꢀ
whereꢀδꢀisꢀtheꢀtemperatureꢀdependencyꢀofꢀR
DR
ꢀandꢀ
CC
DS(ON)
Thisꢀvoltageꢀisꢀtypicallyꢀ5.1Vꢀduringꢀstart-upꢀ(seeꢀEXTV ꢀ
R ꢀ(approximatelyꢀ2Ω)ꢀisꢀtheꢀeffectiveꢀdriverꢀresistanceꢀ
CC
Pinꢀ Connection).ꢀ Consequently,ꢀ logic-levelꢀ thresholdꢀ
atꢀtheꢀMOSFET’sꢀMillerꢀthresholdꢀvoltage.ꢀV
ꢀisꢀtheꢀ
THMIN
MOSFETsꢀmustꢀbeꢀusedꢀinꢀmostꢀapplications.ꢀTheꢀonlyꢀ
typicalꢀMOSFETꢀminimumꢀthresholdꢀvoltage.
exceptionꢀisꢀifꢀlowꢀinputꢀvoltageꢀisꢀexpectedꢀ(V ꢀ<ꢀ4V);ꢀ
IN
GS(TH)
2
BothꢀMOSFETsꢀhaveꢀI RꢀlossesꢀwhileꢀtheꢀtopsideꢀN-channelꢀ
equationꢀincludesꢀanꢀadditionalꢀtermꢀforꢀtransitionꢀlosses,ꢀ
then,ꢀsub-logicꢀlevelꢀthresholdꢀMOSFETsꢀ(V
ꢀ<ꢀ3V)ꢀ
shouldꢀbeꢀused.ꢀPayꢀcloseꢀattentionꢀtoꢀtheꢀBV ꢀspeci-
DSS
whichꢀareꢀhighestꢀatꢀhighꢀinputꢀvoltages.ꢀForꢀV ꢀ<ꢀ20Vꢀ
IN
ficationꢀforꢀtheꢀMOSFETsꢀasꢀwell;ꢀmanyꢀofꢀtheꢀlogic-levelꢀ
theꢀhighꢀcurrentꢀefficiencyꢀgenerallyꢀimprovesꢀwithꢀlargerꢀ
MOSFETsꢀareꢀlimitedꢀtoꢀ30Vꢀorꢀless.
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).ꢀItꢀcanꢀbeꢀ
shownꢀthatꢀtheꢀworst-caseꢀcapacitorꢀRMSꢀcurrentꢀoccursꢀ
whenꢀonlyꢀoneꢀcontrollerꢀisꢀoperating.ꢀTheꢀcontrollerꢀwithꢀ
1
1
+
f
( )
VINTVCC – VTHMIN VTHMIN
V – VOUT
2
theꢀhighestꢀ(V )(I )ꢀproductꢀneedsꢀtoꢀbeꢀusedꢀinꢀtheꢀ
IN
OUT OUT
PSYNC
I
1+ δ R
(
MAX) (
)
DS(ON)
formulaꢀshownꢀinꢀEquationꢀ1ꢀtoꢀdetermineꢀtheꢀmaximumꢀ
V
IN
ꢀ
38571fa
ꢀꢇ
LTC3857-1
applicaTions inForMaTion
RMSꢀcapacitorꢀcurrentꢀrequirement.ꢀIncreasingꢀtheꢀout-
putꢀcurrentꢀdrawnꢀfromꢀtheꢀotherꢀcontrollerꢀwillꢀactuallyꢀ
decreaseꢀtheꢀinputꢀRMSꢀrippleꢀcurrentꢀfromꢀitsꢀmaximumꢀ
value.ꢀTheꢀout-of-phaseꢀtechniqueꢀtypicallyꢀreducesꢀtheꢀ
inputꢀcapacitor’sꢀRMSꢀrippleꢀcurrentꢀbyꢀaꢀfactorꢀofꢀ30%ꢀ
toꢀ70%ꢀwhenꢀcomparedꢀtoꢀaꢀsingleꢀphaseꢀpowerꢀsupplyꢀ
solution.
1cmꢀofꢀeachꢀotherꢀandꢀshareꢀaꢀcommonꢀC (s).ꢀSeparatingꢀ
IN
theꢀdrainsꢀandꢀC ꢀmayꢀproduceꢀundesirableꢀvoltageꢀandꢀ
IN
currentꢀresonancesꢀatꢀV .
IN
Aꢀsmallꢀ(0.1µFꢀtoꢀ1µF)ꢀbypassꢀcapacitorꢀbetweenꢀtheꢀchipꢀ
V ꢀpinꢀandꢀground,ꢀplacedꢀcloseꢀtoꢀtheꢀLTC3857-1,ꢀisꢀ
IN
alsoꢀsuggested.ꢀAꢀ10ΩꢀresistorꢀplacedꢀbetweenꢀC ꢀ(C1)ꢀ
IN
andꢀtheꢀV ꢀpinꢀprovidesꢀfurtherꢀisolationꢀbetweenꢀtheꢀ
IN
Inꢀcontinuousꢀmode,ꢀtheꢀsourceꢀcurrentꢀofꢀtheꢀtopꢀMOSFETꢀ
twoꢀchannels.
isꢀaꢀsquareꢀwaveꢀofꢀdutyꢀcycleꢀ(V )/(V ).ꢀToꢀpreventꢀ
OUT
IN
TheꢀselectionꢀofꢀC ꢀisꢀdrivenꢀbyꢀtheꢀeffectiveꢀseriesꢀ
OUT
largeꢀvoltageꢀtransients,ꢀaꢀlowꢀESRꢀcapacitorꢀsizedꢀforꢀtheꢀ
maximumꢀRMSꢀcurrentꢀofꢀoneꢀchannelꢀmustꢀbeꢀused.ꢀTheꢀ
maximumꢀRMSꢀcapacitorꢀcurrentꢀisꢀgivenꢀby:
resistanceꢀ(ESR).ꢀTypically,ꢀonceꢀtheꢀESRꢀrequirementꢀ
isꢀsatisfied,ꢀtheꢀcapacitanceꢀisꢀadequateꢀforꢀfiltering.ꢀTheꢀ
outputꢀrippleꢀ(∆V )ꢀisꢀapproximatedꢀby:
OUT
IMAX
1/2
(1)
ꢀ
CIN Required IRMS
≈
V
OUT )(
V – V
(
)
1
IN
OUT
V
∆VOUT ≈ ∆IL ESR +
IN
8 • f •C
OUT
ꢀ
ThisꢀformulaꢀhasꢀaꢀmaximumꢀatꢀV ꢀ=ꢀ2V ,ꢀwhereꢀI
ꢀ
IN
OUTꢀ
RMS
whereꢀfꢀisꢀtheꢀoperatingꢀfrequency,ꢀC ꢀisꢀtheꢀoutputꢀ
OUT
=ꢀI /2.ꢀThisꢀsimpleꢀworst-caseꢀconditionꢀisꢀcommonlyꢀ
OUT
capacitanceꢀandꢀ∆I ꢀisꢀtheꢀrippleꢀcurrentꢀinꢀtheꢀinductor.ꢀ
L
usedꢀforꢀdesignꢀbecauseꢀevenꢀsignificantꢀdeviationsꢀdoꢀnotꢀ
offerꢀmuchꢀrelief.ꢀNoteꢀthatꢀcapacitorꢀmanufacturers’ꢀrippleꢀ
currentꢀratingsꢀareꢀoftenꢀbasedꢀonꢀonlyꢀ2000ꢀhoursꢀofꢀlife.ꢀ
Thisꢀmakesꢀitꢀadvisableꢀtoꢀfurtherꢀderateꢀtheꢀcapacitor,ꢀorꢀ
toꢀchooseꢀaꢀcapacitorꢀratedꢀatꢀaꢀhigherꢀtemperatureꢀthanꢀ
required.ꢀSeveralꢀcapacitorsꢀmayꢀbeꢀparalleledꢀtoꢀmeetꢀ
sizeꢀorꢀheightꢀrequirementsꢀinꢀtheꢀdesign.ꢀDueꢀtoꢀtheꢀhighꢀ
operatingꢀfrequencyꢀofꢀtheꢀLTC3857-1,ꢀceramicꢀcapacitorsꢀ
Theꢀoutputꢀrippleꢀisꢀhighestꢀatꢀmaximumꢀinputꢀvoltageꢀ
sinceꢀ∆I ꢀincreasesꢀwithꢀinputꢀvoltage.
L
Setting Output Voltage
TheꢀLTC3857-1ꢀoutputꢀvoltagesꢀareꢀeachꢀsetꢀbyꢀanꢀexter-
nalꢀfeedbackꢀresistorꢀdividerꢀcarefullyꢀplacedꢀacrossꢀtheꢀ
output,ꢀasꢀshownꢀinꢀFigureꢀ5.ꢀTheꢀregulatedꢀoutputꢀvoltageꢀ
isꢀdeterminedꢀby:
canꢀalsoꢀbeꢀusedꢀforꢀC .ꢀAlwaysꢀconsultꢀtheꢀmanufacturerꢀ
IN
ifꢀthereꢀisꢀanyꢀquestion.
RB
R
VOUT = 0.8V 1+
TheꢀbenefitꢀofꢀtheꢀLTC3857-1ꢀ2-phaseꢀoperationꢀcanꢀbeꢀ
calculatedꢀbyꢀusingꢀEquationꢀ1ꢀforꢀtheꢀhigherꢀpowerꢀcontrol-
lerꢀandꢀthenꢀcalculatingꢀtheꢀlossꢀthatꢀwouldꢀhaveꢀresultedꢀ
ifꢀbothꢀcontrollerꢀchannelsꢀswitchedꢀonꢀatꢀtheꢀsameꢀtime.ꢀ
TheꢀtotalꢀRMSꢀpowerꢀlostꢀisꢀlowerꢀwhenꢀbothꢀcontrollersꢀ
areꢀoperatingꢀdueꢀtoꢀtheꢀreducedꢀoverlapꢀofꢀcurrentꢀpulsesꢀ
requiredꢀthroughꢀtheꢀinputꢀcapacitor’sꢀESR.ꢀThisꢀisꢀwhyꢀ
theꢀinputꢀcapacitor’sꢀrequirementꢀcalculatedꢀaboveꢀforꢀtheꢀ
worst-caseꢀcontrollerꢀisꢀadequateꢀforꢀtheꢀdualꢀcontrollerꢀ
design.ꢀAlso,ꢀtheꢀinputꢀprotectionꢀfuseꢀresistance,ꢀbatteryꢀ
resistance,ꢀandꢀPCꢀboardꢀtraceꢀresistanceꢀlossesꢀareꢀalsoꢀ
reducedꢀdueꢀtoꢀtheꢀreducedꢀpeakꢀcurrentsꢀinꢀaꢀ2-phaseꢀ
system.ꢀTheꢀoverallꢀbenefitꢀofꢀaꢀmultiphaseꢀdesignꢀwillꢀ
onlyꢀbeꢀfullyꢀrealizedꢀwhenꢀtheꢀsourceꢀimpedanceꢀofꢀtheꢀ
powerꢀsupply/batteryꢀisꢀincludedꢀinꢀtheꢀefficiencyꢀtesting.ꢀ
TheꢀdrainsꢀofꢀtheꢀtopꢀMOSFETsꢀshouldꢀbeꢀplacedꢀwithinꢀ
A
ꢀ
Toꢀimproveꢀtheꢀfrequencyꢀresponse,ꢀaꢀfeedforwardꢀca-
pacitor,ꢀC ,ꢀmayꢀbeꢀused.ꢀGreatꢀcareꢀshouldꢀbeꢀtakenꢀtoꢀ
FFꢀ
routeꢀtheꢀV ꢀlineꢀawayꢀfromꢀnoiseꢀsources,ꢀsuchꢀasꢀtheꢀ
FB
inductorꢀorꢀtheꢀSWꢀline.
V
OUT
R
B
C
FF
1/2 LTC3857-1
V
FB
R
A
38571 F05
Figure 5. Setting Output Voltage
38571fa
ꢀꢈ
LTC3857-1
applicaTions inForMaTion
Tracking and Soft-Start (TRACK/SS Pins)
V
V
X(MASTER)
Theꢀstart-upꢀofꢀeachꢀV ꢀisꢀcontrolledꢀbyꢀtheꢀvoltageꢀonꢀ
OUT
theꢀrespectiveꢀTRACK/SSꢀpin.ꢀWhenꢀtheꢀvoltageꢀonꢀtheꢀ
TRACK/SSꢀpinꢀisꢀlessꢀthanꢀtheꢀinternalꢀ0.8Vꢀreference,ꢀtheꢀ
OUT(SLAVE)
LTC3857-1ꢀregulatesꢀtheꢀV ꢀpinꢀvoltageꢀtoꢀtheꢀvoltageꢀonꢀ
FB
theꢀTRACK/SSꢀpinꢀinsteadꢀofꢀ0.8V.ꢀTheꢀTRACK/SSꢀpinꢀcanꢀ
beꢀusedꢀtoꢀprogramꢀanꢀexternalꢀsoft-startꢀfunctionꢀorꢀtoꢀ
allowꢀV ꢀtoꢀtrackꢀanotherꢀsupplyꢀduringꢀstart-up.
OUT
38571 F07a
TIME
Soft-startꢀisꢀenabledꢀbyꢀsimplyꢀconnectingꢀaꢀcapacitorꢀ
fromꢀtheꢀTRACK/SSꢀpinꢀtoꢀground,ꢀasꢀshownꢀinꢀFigureꢀ6.ꢀ
Anꢀ internalꢀ 1µAꢀ currentꢀ sourceꢀ chargesꢀ theꢀ capacitor,ꢀ
providingꢀaꢀlinearꢀrampingꢀvoltageꢀatꢀtheꢀTRACK/SSꢀpin.ꢀ
(7a) Coincident Tracking
V
V
X(MASTER)
OUT(SLAVE)
TheꢀLTC3857-1ꢀwillꢀregulateꢀtheꢀV ꢀpinꢀ(andꢀhenceꢀV )ꢀ
FB
OUT
accordingꢀtoꢀtheꢀvoltageꢀonꢀtheꢀTRACK/SSꢀpin,ꢀallowingꢀ
V
ꢀtoꢀriseꢀsmoothlyꢀfromꢀ0Vꢀtoꢀitsꢀfinalꢀregulatedꢀvalue.ꢀ
OUT
Theꢀtotalꢀsoft-startꢀtimeꢀwillꢀbeꢀapproximately:
0.8V
1µA
tSS = CSS
•
ꢀ
38571 F07b
TIME
1/2 LTC3857-1
TRACK/SS
(7b) Ratiometric Tracking
C
SS
Figure 7. Two Different Modes of Output Voltage Tracking
SGND
38571 F06
V
V
OUT
x
Figure 6. Using the TRACK/SS Pin to Program Soft-Start
1/2 LTC3857-1
R
B
V
FB
Alternatively,ꢀtheꢀTRACK/SSꢀpinꢀcanꢀbeꢀusedꢀtoꢀtrackꢀtwoꢀ
(orꢀmore)ꢀsuppliesꢀduringꢀstart-up,ꢀasꢀshownꢀqualitativelyꢀ
inꢀFiguresꢀ7aꢀandꢀ7b.ꢀToꢀdoꢀthis,ꢀaꢀresistorꢀdividerꢀshouldꢀ
R
A
R
R
TRACKB
TRACK/SS
38571 F08
beꢀconnectedꢀfromꢀtheꢀmasterꢀsupplyꢀ(V )ꢀtoꢀtheꢀTRACK/
TRACKA
X
SSꢀpinꢀofꢀtheꢀslaveꢀsupplyꢀ(V ),ꢀasꢀshownꢀinꢀFigureꢀ8.ꢀ
OUT
Duringꢀstart-upꢀV ꢀwillꢀtrackꢀV ꢀaccordingꢀtoꢀtheꢀratioꢀ
OUT
X
Figure 8. Using the TRACK/SS Pin for Tracking
setꢀbyꢀtheꢀresistorꢀdivider:
INTV Regulators
RTRACKA +RTRACKB
RA +RB
VX
RA
CC
=
•
VOUT RTRACKA
TheꢀLTC3857-1ꢀfeaturesꢀtwoꢀseparateꢀinternalꢀP-channelꢀ
lowꢀdropoutꢀlinearꢀregulatorsꢀ(LDO)ꢀthatꢀsupplyꢀpowerꢀ
ꢀ
Forꢀcoincidentꢀtrackingꢀ(V ꢀ=ꢀV ꢀduringꢀstart-up):
OUT
X
atꢀtheꢀINTV ꢀpinꢀfromꢀeitherꢀtheꢀV ꢀsupplyꢀpinꢀorꢀtheꢀ
CC
IN
EXTV ꢀpinꢀdependingꢀonꢀtheꢀconnectionꢀofꢀtheꢀEXTV ꢀ
ꢀ R ꢀ=ꢀR
CC
CC
A
TRACKA
TRACKB
pin.ꢀ INTV ꢀ powersꢀ theꢀ gateꢀ driversꢀ andꢀ muchꢀ ofꢀ theꢀ
CC
ꢀ R ꢀ=ꢀR
B
LTC3857-1’sꢀinternalꢀcircuitry.ꢀTheꢀV ꢀLDOꢀandꢀtheꢀEXTV ꢀ
IN
CC
38571fa
ꢁ0
2.ꢀ
EXTV ꢀConnectedꢀdirectlyꢀtoꢀV .ꢀThisꢀisꢀtheꢀnormalꢀ
CC OUTꢀ
connectionꢀforꢀaꢀ5Vꢀtoꢀ14Vꢀregulatorꢀandꢀprovidesꢀtheꢀ
highestꢀefficiency.
4.ꢀ
EXTV ꢀConnectedꢀtoꢀanꢀOutput-DerivedꢀBoostꢀNetwork.ꢀ
1.ꢀ
EXTV ꢀLeftꢀOpenꢀ(orꢀGrounded).ꢀThisꢀwillꢀcauseꢀINTV ꢀ
CC CC
LTC3857-1
applicaTions inForMaTion
LDOꢀregulateꢀINTV ꢀtoꢀ5.1V.ꢀEachꢀofꢀtheseꢀcanꢀsupplyꢀaꢀ
switchingꢀregulatorꢀoutputsꢀ(4.7Vꢀ≤ꢀVOUTꢀ≤ꢀ14V)ꢀduringꢀ
normalꢀoperationꢀandꢀfromꢀtheꢀVINꢀLDOꢀwhenꢀtheꢀout-
putꢀisꢀoutꢀofꢀregulationꢀ(e.g.,ꢀstart-up,ꢀshort-circuit).ꢀIfꢀ
moreꢀcurrentꢀisꢀrequiredꢀthroughꢀtheꢀEXTVCCꢀLDOꢀthanꢀ
isꢀspecified,ꢀanꢀexternalꢀSchottkyꢀdiodeꢀcanꢀbeꢀaddedꢀ
betweenꢀtheꢀEXTVCCꢀandꢀINTVCCꢀpins.ꢀInꢀthisꢀcase,ꢀdoꢀ
notꢀapplyꢀmoreꢀthanꢀ6VꢀtoꢀtheꢀEXTVCCꢀpinꢀandꢀmakeꢀsureꢀ
thatꢀEXTVCCꢀ≤ꢀVIN.
CC
peakꢀcurrentꢀofꢀ50mAꢀandꢀmustꢀbeꢀbypassedꢀtoꢀgroundꢀ
withꢀaꢀminimumꢀofꢀ4.7µFꢀceramicꢀcapacitor.ꢀNoꢀmatterꢀ
whatꢀtypeꢀofꢀbulkꢀcapacitorꢀisꢀused,ꢀanꢀadditionalꢀ1µFꢀ
ceramicꢀcapacitorꢀplacedꢀdirectlyꢀadjacentꢀtoꢀtheꢀINTV ꢀ
CC
andꢀPGNDꢀpinsꢀisꢀhighlyꢀrecommended.ꢀGoodꢀbypassingꢀ
isꢀneededꢀtoꢀsupplyꢀtheꢀhighꢀtransientꢀcurrentsꢀrequiredꢀ
byꢀtheꢀMOSFETꢀgateꢀdriversꢀandꢀtoꢀpreventꢀinteractionꢀ
betweenꢀtheꢀchannels.
Significantꢀefficiencyꢀandꢀthermalꢀgainsꢀcanꢀbeꢀrealizedꢀ
HighꢀinputꢀvoltageꢀapplicationsꢀinꢀwhichꢀlargeꢀMOSFETsꢀ
areꢀbeingꢀdrivenꢀatꢀhighꢀfrequenciesꢀmayꢀcauseꢀtheꢀmaxi-
mumꢀjunctionꢀtemperatureꢀratingꢀforꢀtheꢀLTC3857-1ꢀtoꢀbeꢀ
byꢀpoweringꢀINTV ꢀfromꢀtheꢀoutput,ꢀsinceꢀtheꢀV ꢀcur-
CC
IN
rentꢀresultingꢀfromꢀtheꢀdriverꢀandꢀcontrolꢀcurrentsꢀwillꢀbeꢀ
scaledꢀbyꢀaꢀfactorꢀofꢀ(DutyꢀCycle)/(SwitcherꢀEfficiency).ꢀ
Forꢀ5Vꢀtoꢀ14Vꢀregulatorꢀoutputs,ꢀthisꢀmeansꢀconnectingꢀ
theꢀEXTV ꢀpinꢀdirectlyꢀtoꢀV .ꢀTyingꢀtheꢀEXTV ꢀpinꢀtoꢀ
exceeded.ꢀTheꢀINTV ꢀcurrent,ꢀwhichꢀisꢀdominatedꢀbyꢀtheꢀ
CC
gateꢀchargeꢀcurrent,ꢀmayꢀbeꢀsuppliedꢀbyꢀeitherꢀtheꢀV ꢀLDOꢀ
IN
CC
OUTꢀ
CC
orꢀtheꢀEXTV ꢀLDO.ꢀWhenꢀtheꢀvoltageꢀonꢀtheꢀEXTV ꢀpinꢀ
anꢀ8.5Vꢀsupplyꢀreducesꢀtheꢀjunctionꢀtemperatureꢀinꢀtheꢀ
previousꢀexampleꢀfromꢀ125°Cꢀto:
CC
CC
isꢀlessꢀthanꢀ4.7V,ꢀtheꢀV ꢀLDOꢀisꢀenabled.ꢀPowerꢀdissipa-
IN
tionꢀforꢀtheꢀICꢀinꢀthisꢀcaseꢀisꢀhighestꢀandꢀisꢀequalꢀtoꢀV ꢀ•ꢀ
IN
ꢀ T ꢀ=ꢀ70°Cꢀ+ꢀ(15mA)(8.5V)(90°C/W)ꢀ=ꢀ82°C
J
I
.ꢀTheꢀgateꢀchargeꢀcurrentꢀisꢀdependentꢀonꢀoperatingꢀ
INTVCC
However,ꢀforꢀ3.3Vꢀandꢀotherꢀlowꢀvoltageꢀoutputs,ꢀaddi-
frequencyꢀasꢀdiscussedꢀinꢀtheꢀEfficiencyꢀConsiderationsꢀ
section.ꢀTheꢀjunctionꢀtemperatureꢀcanꢀbeꢀestimatedꢀbyꢀusingꢀ
theꢀequationsꢀgivenꢀinꢀNoteꢀ3ꢀofꢀtheꢀElectricalꢀCharacteris-
tionalꢀcircuitryꢀisꢀrequiredꢀtoꢀderiveꢀINTV ꢀpowerꢀfromꢀ
CC
theꢀoutput.
tics.ꢀForꢀexample,ꢀtheꢀLTC3857-1ꢀINTV ꢀcurrentꢀisꢀlimitedꢀ
CC
Theꢀfollowingꢀlistꢀsummarizesꢀtheꢀfourꢀpossibleꢀconnec-
toꢀlessꢀthanꢀ15mAꢀfromꢀaꢀ40Vꢀsupplyꢀwhenꢀnotꢀusingꢀtheꢀ
tionsꢀforꢀEXTV :
CC
EXTV ꢀsupplyꢀatꢀaꢀ70°Cꢀambientꢀtemperature:
CC
ꢀ T ꢀ=ꢀ70°Cꢀ+ꢀ(15mA)(40V)(90°C/W)ꢀ=ꢀ125°C
J
toꢀbeꢀpoweredꢀfromꢀtheꢀinternalꢀ5.1Vꢀregulatorꢀresult-
ingꢀinꢀanꢀefficiencyꢀpenaltyꢀofꢀupꢀtoꢀ10%ꢀatꢀhighꢀinputꢀ
voltages.
Toꢀpreventꢀtheꢀmaximumꢀjunctionꢀtemperatureꢀfromꢀbe-
ingꢀexceeded,ꢀtheꢀinputꢀsupplyꢀcurrentꢀmustꢀbeꢀcheckedꢀ
whileꢀoperatingꢀinꢀforcedꢀcontinuousꢀmodeꢀ(PLLIN/MODEꢀ
=ꢀINTV )ꢀatꢀmaximumꢀV .
CC
IN
WhenꢀtheꢀvoltageꢀappliedꢀtoꢀEXTV ꢀrisesꢀaboveꢀ4.7V,ꢀtheꢀ
CC
V ꢀLDOꢀisꢀturnedꢀoffꢀandꢀtheꢀEXTV ꢀLDOꢀisꢀenabled.ꢀTheꢀ
IN
CC
3.ꢀꢀEXTV ꢀConnectedꢀtoꢀanꢀExternalꢀsupply.ꢀIfꢀanꢀexternalꢀ
CC
EXTV ꢀLDOꢀremainsꢀonꢀasꢀlongꢀasꢀtheꢀvoltageꢀappliedꢀtoꢀ
CC
supplyꢀisꢀavailableꢀinꢀtheꢀ5Vꢀtoꢀ14Vꢀrange,ꢀitꢀmayꢀbeꢀ
EXTV ꢀremainsꢀaboveꢀ4.5V.ꢀTheꢀEXTV ꢀLDOꢀattemptsꢀ
CC
CC
usedꢀtoꢀpowerꢀEXTV .ꢀEnsureꢀthatꢀEXTV ꢀ<ꢀV .
CC
CC
IN
toꢀregulateꢀtheꢀINTV ꢀvoltageꢀtoꢀ5.1V,ꢀsoꢀwhileꢀEXTV ꢀ
CC
CC
CC
isꢀlessꢀthanꢀ5.1V,ꢀtheꢀLDOꢀisꢀinꢀdropoutꢀandꢀtheꢀINTV ꢀ
CC
Forꢀ3.3Vꢀandꢀotherꢀlowꢀvoltageꢀregulators,ꢀefficiencyꢀ
voltageꢀisꢀapproximatelyꢀequalꢀtoꢀEXTV .ꢀWhenꢀEXTV ꢀ
CC
CC
gainsꢀcanꢀstillꢀbeꢀrealizedꢀbyꢀconnectingꢀEXTV ꢀtoꢀanꢀ
CC
isꢀgreaterꢀthanꢀ5.1V,ꢀupꢀtoꢀanꢀabsoluteꢀmaximumꢀofꢀ14V,ꢀ
output-derivedꢀvoltageꢀthatꢀhasꢀbeenꢀboostedꢀtoꢀgreaterꢀ
INTV ꢀisꢀregulatedꢀtoꢀ5.1V.
CC
thanꢀ4.7V.ꢀThisꢀcanꢀbeꢀdoneꢀwithꢀtheꢀcapacitiveꢀchargeꢀ
UsingꢀtheꢀEXTVCCꢀLDOꢀallowsꢀtheꢀMOSFETꢀdriverꢀandꢀ
controlꢀpowerꢀtoꢀbeꢀderivedꢀfromꢀoneꢀofꢀtheꢀLTC3857-1’sꢀ
pumpꢀshownꢀinꢀFigureꢀ9.ꢀEnsureꢀthatꢀEXTV ꢀ<ꢀV .
CC
IN
38571fa
ꢁꢀ
LTC3857-1
applicaTions inForMaTion
voltageꢀfallsꢀbelowꢀ70%ꢀofꢀitsꢀnominalꢀoutputꢀlevel,ꢀthenꢀ
theꢀmaximumꢀsenseꢀvoltageꢀisꢀprogressivelyꢀloweredꢀtoꢀ
aboutꢀhalfꢀofꢀitsꢀmaximumꢀselectedꢀvalue.ꢀUnderꢀshort-
circuitꢀconditionsꢀwithꢀveryꢀlowꢀdutyꢀcycles,ꢀtheꢀLTC3857-1ꢀ
willꢀbeginꢀcycleꢀskippingꢀinꢀorderꢀtoꢀlimitꢀtheꢀshort-circuitꢀ
current.ꢀInꢀthisꢀsituationꢀtheꢀbottomꢀMOSFETꢀwillꢀbeꢀdis-
sipatingꢀmostꢀofꢀtheꢀpowerꢀbutꢀlessꢀthanꢀinꢀnormalꢀopera-
tion.ꢀTheꢀshort-circuitꢀrippleꢀcurrentꢀisꢀdeterminedꢀbyꢀtheꢀ
C
IN
BAT85
BAT85
BAT85
V
IN
MTOP
MBOT
VN2222LL
TG1
1/2 LTC3857-1
L
R
SENSE
V
EXTV
SW
OUT
CC
minimumꢀon-time.ꢀt
,ꢀofꢀtheꢀLTC3857-1ꢀ(≈90ns),ꢀ
ON(MIN)
C
OUT
D
BG1
theꢀinputꢀvoltageꢀandꢀinductorꢀvalue:
38571 F09
PGND
IN
V
∆IL(SC) = tON(MIN)
L
ꢀ
Figure 9. Capacitive Charge Pump for EXTVCC
Theꢀresultingꢀaverageꢀshort-circuitꢀcurrentꢀis:
Topside MOSFET Driver Supply (C , D )
B
B
1
ISC = 50% •ILIM(MAX) – ∆IL(SC)
Externalꢀbootstrapꢀcapacitors,ꢀC ,ꢀconnectedꢀtoꢀtheꢀBOOSTꢀ
B
2
ꢀ
pinsꢀsupplyꢀtheꢀgateꢀdriveꢀvoltagesꢀforꢀtheꢀtopsideꢀMOSFETs.ꢀ
CapacitorꢀC ꢀinꢀtheꢀFunctionalꢀDiagramꢀisꢀchargedꢀthoughꢀ
B
Fault Conditions: Overvoltage Protection (Crowbar)
externalꢀdiodeꢀD ꢀfromꢀINTV ꢀwhenꢀtheꢀSWꢀpinꢀisꢀlow.ꢀ
B
CC
Theꢀovervoltageꢀcrowbarꢀisꢀdesignedꢀtoꢀblowꢀaꢀsystemꢀ
inputꢀfuseꢀwhenꢀtheꢀoutputꢀvoltageꢀofꢀtheꢀregulatorꢀrisesꢀ
muchꢀhigherꢀthanꢀnominalꢀlevels.ꢀTheꢀcrowbarꢀcausesꢀhugeꢀ
currentsꢀtoꢀflow,ꢀthatꢀblowꢀtheꢀfuseꢀtoꢀprotectꢀagainstꢀaꢀ
shortedꢀtopꢀMOSFETꢀifꢀtheꢀshortꢀoccursꢀwhileꢀtheꢀcontrol-
lerꢀisꢀoperating.
WhenꢀoneꢀofꢀtheꢀtopsideꢀMOSFETsꢀisꢀtoꢀbeꢀturnedꢀon,ꢀtheꢀ
driverꢀplacesꢀtheꢀC ꢀvoltageꢀacrossꢀtheꢀgate-sourceꢀofꢀtheꢀ
B
desiredꢀMOSFET.ꢀThisꢀenhancesꢀtheꢀMOSFETꢀandꢀturnsꢀonꢀ
theꢀtopsideꢀswitch.ꢀTheꢀswitchꢀnodeꢀvoltage,ꢀSW,ꢀrisesꢀtoꢀ
V ꢀandꢀtheꢀBOOSTꢀpinꢀfollows.ꢀWithꢀtheꢀtopsideꢀMOSFETꢀ
IN
on,ꢀtheꢀboostꢀvoltageꢀisꢀaboveꢀtheꢀinputꢀsupply:ꢀV
ꢀ=ꢀ
BOOST
B
V ꢀ+ꢀV
.ꢀTheꢀvalueꢀofꢀtheꢀboostꢀcapacitor,ꢀC ,ꢀneedsꢀ
IN
INTVCC
Aꢀcomparatorꢀmonitorsꢀtheꢀoutputꢀforꢀovervoltageꢀcondi-
tions.ꢀTheꢀcomparatorꢀdetectsꢀfaultsꢀgreaterꢀthanꢀ10%ꢀ
aboveꢀtheꢀnominalꢀoutputꢀvoltage.ꢀWhenꢀthisꢀconditionꢀ
isꢀsensed,ꢀtheꢀtopꢀMOSFETꢀisꢀturnedꢀoffꢀandꢀtheꢀbottomꢀ
MOSFETꢀisꢀturnedꢀonꢀuntilꢀtheꢀovervoltageꢀconditionꢀisꢀ
cleared.ꢀTheꢀbottomꢀMOSFETꢀremainsꢀonꢀcontinuouslyꢀ
forꢀasꢀlongꢀasꢀtheꢀovervoltageꢀconditionꢀpersists;ꢀifꢀV
returnsꢀtoꢀaꢀsafeꢀlevel,ꢀnormalꢀoperationꢀautomaticallyꢀ
resumes.ꢀ
toꢀbeꢀ100ꢀtimesꢀthatꢀofꢀtheꢀtotalꢀinputꢀcapacitanceꢀofꢀtheꢀ
topsideꢀMOSFET(s).ꢀTheꢀreverseꢀbreakdownꢀofꢀtheꢀexternalꢀ
SchottkyꢀdiodeꢀmustꢀbeꢀgreaterꢀthanꢀV
.ꢀ
IN(MAX)
Whenꢀadjustingꢀtheꢀgateꢀdriveꢀlevel,ꢀtheꢀfinalꢀarbiterꢀisꢀtheꢀ
totalꢀinputꢀcurrentꢀforꢀtheꢀregulator.ꢀIfꢀaꢀchangeꢀisꢀmadeꢀ
andꢀtheꢀinputꢀcurrentꢀdecreases,ꢀthenꢀtheꢀefficiencyꢀhasꢀ
improved.ꢀIfꢀthereꢀisꢀnoꢀchangeꢀinꢀinputꢀcurrent,ꢀthenꢀthereꢀ
isꢀnoꢀchangeꢀinꢀefficiency.
ꢀ
OUT
AꢀshortedꢀtopꢀMOSFETꢀwillꢀresultꢀinꢀaꢀhighꢀcurrentꢀconditionꢀ
whichꢀwillꢀopenꢀtheꢀsystemꢀfuse.ꢀTheꢀswitchingꢀregulatorꢀ
Fault Conditions: Current Limit and Current Foldback
TheꢀLTC3857-1ꢀincludesꢀcurrentꢀfoldbackꢀtoꢀhelpꢀlimitꢀloadꢀ willꢀregulateꢀproperlyꢀwithꢀaꢀleakyꢀtopꢀMOSFETꢀbyꢀalteringꢀ
currentꢀwhenꢀtheꢀoutputꢀisꢀshortedꢀtoꢀground.ꢀIfꢀtheꢀoutputꢀ theꢀdutyꢀcycleꢀtoꢀaccommodateꢀtheꢀleakage.
38571fa
ꢁꢁ
LTC3857-1
applicaTions inForMaTion
Phase-Locked Loop and Frequency Synchronization
1000
900
800
700
600
500
400
300
200
100
0
TheꢀLTC3857-1ꢀhasꢀanꢀinternalꢀphase-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)
38571 F10
Figure 10. Relationship Between Oscillator Frequency
and Resistor Value at the FREQ Pin
Ifꢀtheꢀexternalꢀclockꢀfrequencyꢀisꢀgreaterꢀthanꢀtheꢀinternalꢀ
oscillator’sꢀfrequency,ꢀf ,ꢀthenꢀcurrentꢀisꢀsourcedꢀcontinu-
OSC
ouslyꢀfromꢀtheꢀphaseꢀdetectorꢀoutput,ꢀpullingꢀupꢀtheꢀVCOꢀ
input.ꢀWhenꢀtheꢀexternalꢀclockꢀfrequencyꢀisꢀlessꢀthanꢀfOSC,ꢀ
currentꢀisꢀsunkꢀcontinuously,ꢀpullingꢀdownꢀtheꢀVCOꢀinput.ꢀ
Ifꢀtheꢀexternalꢀandꢀinternalꢀfrequenciesꢀareꢀtheꢀsameꢀbutꢀ
exhibitꢀaꢀphaseꢀdifference,ꢀtheꢀcurrentꢀsourcesꢀturnꢀonꢀforꢀ
anꢀamountꢀofꢀtimeꢀcorrespondingꢀtoꢀtheꢀphaseꢀdifference.ꢀ
TheꢀvoltageꢀatꢀtheꢀVCOꢀinputꢀisꢀadjustedꢀuntilꢀtheꢀphaseꢀ
andꢀfrequencyꢀofꢀtheꢀinternalꢀandꢀexternalꢀoscillatorsꢀareꢀ
identical.ꢀAtꢀtheꢀstableꢀoperatingꢀpoint,ꢀtheꢀphaseꢀdetectorꢀ
outputꢀisꢀhighꢀimpedanceꢀandꢀtheꢀinternalꢀfilterꢀcapacitor,ꢀ
CLP,ꢀholdsꢀtheꢀvoltageꢀatꢀtheꢀVCOꢀinput.
prebiasedꢀatꢀaꢀfrequencyꢀcorrespondingꢀtoꢀtheꢀfrequencyꢀ
setꢀbyꢀtheꢀFREQꢀpin.ꢀOnceꢀprebiased,ꢀtheꢀPLLꢀonlyꢀneedsꢀ
toꢀadjustꢀtheꢀfrequencyꢀslightlyꢀtoꢀachieveꢀphaseꢀlockꢀ
andꢀsynchronization.ꢀAlthoughꢀitꢀisꢀnotꢀrequiredꢀthatꢀtheꢀ
free-runningꢀfrequencyꢀbeꢀnearꢀexternalꢀclockꢀfrequency,ꢀ
doingꢀsoꢀwillꢀpreventꢀtheꢀoperatingꢀfrequencyꢀfromꢀpassingꢀ
throughꢀaꢀlargeꢀrangeꢀofꢀfrequenciesꢀasꢀtheꢀPLLꢀlocks.
Tableꢀ2ꢀsummarizesꢀtheꢀdifferentꢀstatesꢀinꢀwhichꢀtheꢀFREQꢀ
pinꢀcanꢀbeꢀused.
Table 2
NoteꢀthatꢀtheꢀLTC3857-1ꢀcanꢀonlyꢀbeꢀsynchronizedꢀtoꢀanꢀ
externalꢀ clockꢀ whoseꢀ frequencyꢀ isꢀ withinꢀ rangeꢀ ofꢀ theꢀ
LTC3857-1’sꢀ internalꢀ VCO,ꢀ whichꢀ isꢀ nominallyꢀ 55kHzꢀ
toꢀ1MHz.ꢀThisꢀisꢀguaranteedꢀtoꢀbeꢀbetweenꢀ75kHzꢀandꢀ
850kHz.ꢀ
FREQ PIN
PLLIN/MODE PIN
DCꢀVoltage
FREQUENCY
350kHz
0V
INTV
DCꢀVoltage
535kHz
CC
Resistor
DCꢀVoltage
50kHz–900kHz
AnyꢀofꢀtheꢀAbove
ExternalꢀClock
Phaseꢀ–Lockedꢀtoꢀ
ExternalꢀClock
Typically,ꢀ theꢀ externalꢀ clockꢀ (onꢀ theꢀ PLLIN/MODEꢀ pin)ꢀ
inputꢀhighꢀthresholdꢀisꢀ1.6V,ꢀwhileꢀtheꢀinputꢀlowꢀthresholdꢀ
isꢀ1.1V.
Minimum On-Time Considerations
Minimumꢀon-time,ꢀt ,ꢀisꢀtheꢀsmallestꢀtimeꢀdura-
RapidꢀphaseꢀlockingꢀcanꢀbeꢀachievedꢀbyꢀusingꢀtheꢀFREQꢀ
pinꢀ toꢀ setꢀ aꢀ free-runningꢀ frequencyꢀ nearꢀ theꢀ desiredꢀ
synchronizationꢀfrequency.ꢀTheꢀVCO’sꢀinputꢀvoltageꢀisꢀ
ON(MIN)
tionꢀthatꢀtheꢀLTC3857-1ꢀisꢀcapableꢀofꢀturningꢀonꢀtheꢀtopꢀ
MOSFET.ꢀItꢀisꢀdeterminedꢀbyꢀinternalꢀtimingꢀdelaysꢀandꢀtheꢀ
38571fa
ꢁꢂ
3.ꢀ
I RꢀlossesꢀareꢀpredictedꢀfromꢀtheꢀDCꢀresistancesꢀofꢀtheꢀ
1.ꢀ
TheꢀV ꢀcurrentꢀisꢀtheꢀDCꢀinputꢀsupplyꢀcurrentꢀgivenꢀ
gateꢀchargeꢀrequiredꢀtoꢀturnꢀonꢀtheꢀtopꢀMOSFET.ꢀLowꢀdutyꢀ 2.ꢀ
cycleꢀapplicationsꢀmayꢀapproachꢀthisꢀminimumꢀon-timeꢀ
limitꢀandꢀcareꢀshouldꢀbeꢀtakenꢀtoꢀensureꢀthat
INTV ꢀcurrentꢀisꢀtheꢀsumꢀofꢀtheꢀMOSFETꢀdriverꢀandꢀ
LTC3857-1
applicaTions inForMaTion
CC
controlꢀcurrents.ꢀTheꢀMOSFETꢀdriverꢀcurrentꢀresultsꢀ
fromꢀ switchingꢀ theꢀ gateꢀ capacitanceꢀ ofꢀ theꢀ powerꢀ
MOSFETs.ꢀEachꢀtimeꢀaꢀMOSFETꢀgateꢀisꢀswitchedꢀfromꢀ
lowꢀtoꢀhighꢀtoꢀlowꢀagain,ꢀaꢀpacketꢀofꢀcharge,ꢀdQ,ꢀmovesꢀ
VOUT
tON(MIN)
<
V
f
IN
fromꢀINTV ꢀtoꢀground.ꢀTheꢀresultingꢀdQ/dtꢀisꢀaꢀcurrentꢀ
CC
ꢀ
outꢀofꢀINTV ꢀthatꢀisꢀtypicallyꢀmuchꢀlargerꢀthanꢀtheꢀ
CC
Ifꢀtheꢀdutyꢀcycleꢀfallsꢀbelowꢀwhatꢀcanꢀbeꢀaccommodatedꢀ
byꢀtheꢀminimumꢀon-time,ꢀtheꢀcontrollerꢀwillꢀbeginꢀtoꢀskipꢀ
cycles.ꢀTheꢀoutputꢀvoltageꢀwillꢀcontinueꢀtoꢀbeꢀregulated,ꢀ
butꢀtheꢀrippleꢀvoltageꢀandꢀcurrentꢀwillꢀincrease.
controlꢀcircuitꢀcurrent.ꢀInꢀcontinuousꢀmode,ꢀI
ꢀ
GATECHG
=ꢀf(Q ꢀ+ꢀQ ),ꢀwhereꢀQ ꢀandꢀQ ꢀareꢀtheꢀgateꢀchargesꢀofꢀ
T
B
T
B
theꢀtopsideꢀandꢀbottomꢀsideꢀMOSFETs.
ꢀ SupplyingꢀINTV ꢀfromꢀanꢀoutput-derivedꢀpowerꢀsourceꢀ
CC
Theꢀminimumꢀon-timeꢀforꢀtheꢀLTC3857-1ꢀisꢀapproximatelyꢀ
95ns.ꢀHowever,ꢀasꢀtheꢀpeakꢀsenseꢀvoltageꢀdecreasesꢀtheꢀ
minimumꢀon-timeꢀgraduallyꢀincreasesꢀupꢀtoꢀaboutꢀ130ns.ꢀ
Thisꢀisꢀofꢀparticularꢀconcernꢀinꢀforcedꢀcontinuousꢀapplica-
tionsꢀwithꢀlowꢀrippleꢀcurrentꢀatꢀlightꢀloads.ꢀIfꢀtheꢀdutyꢀcycleꢀ
dropsꢀbelowꢀtheꢀminimumꢀon-timeꢀlimitꢀinꢀthisꢀsituation,ꢀ
aꢀsignificantꢀamountꢀofꢀcycleꢀskippingꢀcanꢀoccurꢀwithꢀcor-
respondinglyꢀlargerꢀcurrentꢀandꢀvoltageꢀripple.
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
fuseꢀ(ifꢀused),ꢀMOSFET,ꢀinductor,ꢀcurrentꢀsenseꢀresis-
tor,ꢀandꢀinputꢀandꢀoutputꢀcapacitorꢀESR.ꢀInꢀcontinuousꢀ
modeꢀtheꢀaverageꢀoutputꢀcurrentꢀflowsꢀthroughꢀLꢀandꢀ
Efficiency Considerations
Theꢀpercentꢀefficiencyꢀofꢀaꢀswitchingꢀregulatorꢀisꢀequalꢀtoꢀ
theꢀoutputꢀpowerꢀdividedꢀbyꢀtheꢀinputꢀpowerꢀtimesꢀ100%.ꢀ
Itꢀisꢀoftenꢀusefulꢀtoꢀanalyzeꢀindividualꢀlossesꢀtoꢀdetermineꢀ
whatꢀisꢀlimitingꢀtheꢀefficiencyꢀandꢀwhichꢀchangeꢀwouldꢀ
produceꢀtheꢀmostꢀimprovement.ꢀPercentꢀefficiencyꢀcanꢀ
beꢀexpressedꢀas:
R
,ꢀbutꢀisꢀchoppedꢀbetweenꢀtheꢀtopsideꢀMOSFETꢀ
SENSE
andꢀtheꢀsynchronousꢀMOSFET.ꢀIfꢀtheꢀtwoꢀMOSFETsꢀhaveꢀ
approximatelyꢀtheꢀsameꢀR
,ꢀthenꢀtheꢀresistanceꢀ
DS(ON)
ofꢀoneꢀMOSFETꢀcanꢀsimplyꢀbeꢀsummedꢀwithꢀtheꢀresis-
2
tancesꢀofꢀL,ꢀR
ꢀandꢀESRꢀtoꢀobtainꢀI Rꢀlosses.ꢀForꢀ
DS(ON)
SENSE
example,ꢀifꢀeachꢀR
ꢀ=ꢀ30mΩ,ꢀR ꢀ=ꢀ50mΩ,ꢀR
ꢀ
ꢀ %Efficiencyꢀ=ꢀ100%ꢀ–ꢀ(L1ꢀ+ꢀL2ꢀ+ꢀL3ꢀ+ꢀ...)
L
SENSE
=ꢀ10mΩꢀandꢀR ꢀ=ꢀ40mΩꢀ(sumꢀofꢀbothꢀinputꢀandꢀ
ESR
whereꢀL1,ꢀL2,ꢀetc.ꢀareꢀtheꢀindividualꢀlossesꢀasꢀaꢀpercent-
ageꢀofꢀinputꢀpower.
outputꢀcapacitanceꢀlosses),ꢀthenꢀtheꢀtotalꢀresistanceꢀ
isꢀ130mΩ.ꢀThisꢀresultsꢀinꢀlossesꢀrangingꢀfromꢀ3%ꢀtoꢀ
13%ꢀasꢀtheꢀoutputꢀcurrentꢀincreasesꢀfromꢀ1Aꢀtoꢀ5Aꢀforꢀ
aꢀ5Vꢀoutput,ꢀorꢀaꢀ4%ꢀtoꢀ20%ꢀlossꢀforꢀaꢀ3.3Vꢀoutput.ꢀ
Althoughꢀallꢀdissipativeꢀelementsꢀinꢀtheꢀcircuitꢀproduceꢀ
losses,ꢀfourꢀmainꢀsourcesꢀusuallyꢀaccountꢀforꢀmostꢀofꢀtheꢀ
lossesꢀinꢀLTC3857-1ꢀcircuits:ꢀ1)ꢀICꢀVINꢀcurrent,ꢀ2)ꢀINTVCCꢀ
regulatorꢀ current,ꢀ 3)ꢀ I2Rꢀ losses,ꢀ 4)ꢀ topsideꢀ MOSFETꢀ
EfficiencyꢀvariesꢀasꢀtheꢀinverseꢀsquareꢀofꢀV ꢀforꢀtheꢀ
OUT
sameꢀexternalꢀcomponentsꢀandꢀoutputꢀpowerꢀlevel.ꢀTheꢀ
combinedꢀeffectsꢀofꢀincreasinglyꢀlowerꢀoutputꢀvoltagesꢀ
andꢀhigherꢀcurrentsꢀrequiredꢀbyꢀhighꢀperformanceꢀdigitalꢀ
systemsꢀisꢀnotꢀdoublingꢀbutꢀquadruplingꢀtheꢀimportanceꢀ
ofꢀlossꢀtermsꢀinꢀtheꢀswitchingꢀregulatorꢀsystem!
transitionꢀlosses
.
IN
inꢀtheꢀElectricalꢀCharacteristicsꢀtable,ꢀwhichꢀexcludesꢀ
MOSFETꢀdriverꢀandꢀcontrolꢀcurrents.ꢀV ꢀcurrentꢀtypi-
callyꢀresultsꢀinꢀaꢀsmallꢀ(<0.1%)ꢀloss.
IN
38571fa
ꢁꢃ
LTC3857-1
applicaTions inForMaTion
4.ꢀ
TransitionꢀlossesꢀapplyꢀonlyꢀtoꢀtheꢀtopsideꢀMOSFET(s),ꢀ
canꢀalsoꢀbeꢀestimatedꢀbyꢀexaminingꢀtheꢀriseꢀtimeꢀatꢀtheꢀ
pin.ꢀTheꢀITHꢀexternalꢀcomponentsꢀshownꢀinꢀFigureꢀ13ꢀ
circuitꢀwillꢀprovideꢀanꢀadequateꢀstartingꢀpointꢀforꢀmostꢀ
applications.
andꢀbecomeꢀsignificantꢀonlyꢀwhenꢀoperatingꢀatꢀhighꢀ
inputꢀ voltagesꢀ (t
ypicallyꢀ 15Vꢀ orꢀ greater).ꢀ Transitionꢀ
lossesꢀcanꢀbeꢀestimatedꢀfrom:
ꢀ ꢀ TransitionꢀLossꢀ=ꢀ(1.7)ꢀ•ꢀV ꢀ•ꢀ2ꢀ•ꢀI
ꢀ•ꢀC ꢀ•ꢀf
RSS
TheꢀI ꢀseriesꢀR -C ꢀfilterꢀsetsꢀtheꢀdominantꢀpole-zeroꢀ
IN
O(MAX)
TH
C
C
loopꢀcompensation.ꢀTheꢀvaluesꢀcanꢀbeꢀmodifiedꢀslightlyꢀ
(fromꢀ0.5ꢀtoꢀ2ꢀtimesꢀtheirꢀsuggestedꢀvalues)ꢀtoꢀoptimizeꢀ
transientꢀresponseꢀonceꢀtheꢀfinalꢀPCꢀlayoutꢀisꢀdoneꢀandꢀ
theꢀparticularꢀoutputꢀcapacitorꢀtypeꢀandꢀvalueꢀhaveꢀbeenꢀ
determined.ꢀTheꢀoutputꢀcapacitorsꢀneedꢀtoꢀbeꢀselectedꢀ
becauseꢀtheꢀvariousꢀtypesꢀandꢀvaluesꢀdetermineꢀtheꢀloopꢀ
gainꢀandꢀphase.ꢀAnꢀoutputꢀcurrentꢀpulseꢀofꢀ20%ꢀtoꢀ80%ꢀ
ofꢀfull-loadꢀcurrentꢀhavingꢀaꢀriseꢀtimeꢀofꢀ1µsꢀtoꢀ10µsꢀwillꢀ
ꢀ Otherꢀhiddenꢀlossesꢀsuchꢀasꢀcopperꢀtraceꢀandꢀinternalꢀ
batteryꢀresistancesꢀcanꢀaccountꢀforꢀanꢀadditionalꢀ5%ꢀ
toꢀ10%ꢀefficiencyꢀdegradationꢀinꢀportableꢀsystems.ꢀItꢀ
isꢀveryꢀimportantꢀtoꢀincludeꢀtheseꢀsystemꢀlevelꢀlossesꢀ
duringꢀtheꢀdesignꢀphase.ꢀTheꢀinternalꢀbatteryꢀandꢀfuseꢀ
resistanceꢀlossesꢀcanꢀbeꢀminimizedꢀbyꢀmakingꢀsureꢀthatꢀ
C ꢀhasꢀadequateꢀchargeꢀstorageꢀandꢀveryꢀlowꢀESRꢀatꢀ
IN
theꢀswitchingꢀfrequency.ꢀAꢀ25Wꢀsupplyꢀwillꢀtypicallyꢀ
requireꢀ aꢀ minimumꢀ ofꢀ 20µFꢀ toꢀ 40µFꢀ ofꢀ capacitanceꢀ
havingꢀaꢀmaximumꢀofꢀ20mΩꢀtoꢀ50mΩꢀofꢀESR.ꢀTheꢀ
LTC3857-1ꢀ2-phaseꢀarchitectureꢀtypicallyꢀhalvesꢀthisꢀ
inputꢀcapacitanceꢀrequirementꢀoverꢀcompetingꢀsolu-
tions.ꢀOtherꢀlossesꢀincludingꢀSchottkyꢀconductionꢀlossesꢀ
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)ꢀ
loadꢀcurrent.ꢀWhenꢀaꢀloadꢀstepꢀoccurs,ꢀVOUTꢀshiftsꢀbyꢀ
anꢀamountꢀequalꢀtoꢀ∆ILOADꢀ(ESR),ꢀwhereꢀESRꢀisꢀtheꢀef-
fectiveꢀseriesꢀresistanceꢀofꢀCOUTꢀ.ꢀ∆ILOADꢀalsoꢀbeginsꢀtoꢀ
chargeꢀorꢀdischargeꢀCOUTꢀgeneratingꢀtheꢀfeedbackꢀerrorꢀ
signalꢀthatꢀforcesꢀtheꢀregulatorꢀtoꢀadaptꢀtoꢀtheꢀcurrentꢀ
changeꢀandꢀreturnꢀVOUTꢀtoꢀitsꢀsteady-stateꢀvalue.ꢀDuringꢀ
thisꢀrecoveryꢀtimeꢀVOUTꢀcanꢀbeꢀmonitoredꢀforꢀexcessiveꢀ
overshootꢀorꢀ ringing,ꢀ whichꢀ wouldꢀindicateꢀ aꢀ stabilityꢀ
problem.ꢀOPTI-LOOPꢀcompensationꢀallowsꢀtheꢀtransientꢀ
responseꢀtoꢀbeꢀoptimizedꢀoverꢀaꢀwideꢀrangeꢀofꢀoutputꢀ
capacitanceꢀandꢀESRꢀvalues.ꢀThe availability of the ITH pin
not only allows optimization of control loop behavior, but
it also provides a DC coupled and AC filtered closed-loop
response test point. The DC step, rise time and settling
at this test point truly reflects the closed-loop response.ꢀ
Assumingꢀaꢀpredominantlyꢀsecondꢀorderꢀsystem,ꢀphaseꢀ
marginꢀand/orꢀdampingꢀfactorꢀcanꢀbeꢀestimatedꢀusingꢀtheꢀ
percentageꢀofꢀovershootꢀseenꢀatꢀthisꢀpin.ꢀTheꢀbandwidthꢀ
isꢀwhyꢀitꢀisꢀbetterꢀtoꢀlookꢀatꢀtheꢀI ꢀpinꢀsignalꢀwhichꢀisꢀinꢀ
TH
theꢀfeedbackꢀloopꢀandꢀisꢀtheꢀfilteredꢀandꢀcompensatedꢀ
controlꢀloopꢀresponse.ꢀ
TheꢀgainꢀofꢀtheꢀloopꢀwillꢀbeꢀincreasedꢀbyꢀincreasingꢀR ꢀ
C
andꢀtheꢀbandwidthꢀofꢀtheꢀloopꢀwillꢀbeꢀincreasedꢀbyꢀde-
creasingꢀC .ꢀIfꢀR ꢀisꢀincreasedꢀbyꢀtheꢀsameꢀfactorꢀthatꢀC ꢀ
C
C
C
isꢀdecreased,ꢀtheꢀzeroꢀfrequencyꢀwillꢀbeꢀkeptꢀtheꢀsame,ꢀ
therebyꢀkeepingꢀtheꢀphaseꢀshiftꢀtheꢀsameꢀinꢀtheꢀmostꢀ
criticalꢀfrequencyꢀrangeꢀofꢀtheꢀfeedbackꢀloop.ꢀTheꢀoutputꢀ
voltageꢀsettlingꢀbehaviorꢀisꢀrelatedꢀtoꢀtheꢀstabilityꢀofꢀtheꢀ
closed-loopꢀsystemꢀandꢀwillꢀdemonstrateꢀtheꢀactualꢀoverallꢀ
supplyꢀperformance.
Aꢀsecond,ꢀmoreꢀsevereꢀtransientꢀisꢀcausedꢀbyꢀswitchingꢀ
inꢀloadsꢀwithꢀlargeꢀ(>1µF)ꢀsupplyꢀbypassꢀcapacitors.ꢀTheꢀ
dischargedꢀbypassꢀcapacitorsꢀareꢀeffectivelyꢀputꢀinꢀparallelꢀ
withꢀC ,ꢀcausingꢀaꢀrapidꢀdropꢀinꢀV .ꢀNoꢀregulatorꢀcanꢀ
OUTꢀ
OUTꢀ
alterꢀitsꢀdeliveryꢀofꢀcurrentꢀquicklyꢀenoughꢀtoꢀpreventꢀthisꢀ
suddenꢀstepꢀchangeꢀinꢀoutputꢀvoltageꢀifꢀtheꢀloadꢀswitchꢀ
resistanceꢀisꢀlowꢀandꢀitꢀisꢀdrivenꢀquickly.ꢀIfꢀtheꢀratioꢀofꢀ
38571fa
ꢁꢄ
LTC3857-1
applicaTions inForMaTion
C
ꢀtoꢀC ꢀisꢀgreaterꢀthanꢀ1:50,ꢀtheꢀswitchꢀriseꢀtimeꢀ
TheꢀpowerꢀdissipationꢀonꢀtheꢀtopsideꢀMOSFETꢀcanꢀbeꢀeasilyꢀ
estimated.ꢀChoosingꢀaꢀFairchildꢀFDS6982SꢀdualꢀMOSFETꢀ
LOAD
OUT
shouldꢀbeꢀcontrolledꢀsoꢀthatꢀtheꢀloadꢀriseꢀtimeꢀisꢀlimitedꢀ
toꢀapproximatelyꢀ25ꢀ•ꢀC .ꢀThusꢀaꢀ10µFꢀcapacitorꢀwouldꢀ
resultsꢀin:ꢀR
ꢀ=ꢀ0.035Ω/0.022Ω,ꢀC
ꢀ=ꢀ215pF.ꢀAtꢀ
LOAD
DS(ON)
MILLER
requireꢀaꢀ250µsꢀriseꢀtime,ꢀlimitingꢀtheꢀchargingꢀcurrentꢀ
toꢀaboutꢀ200mA.
maximumꢀinputꢀvoltageꢀwithꢀT(estimated)ꢀ=ꢀ50°C:
2
3.3V
22V
PMAIN
=
6A 1+ 0.005 50°C – 25°C
(
)
(
)(
)
Design Example
2 6A
Asꢀ aꢀ designꢀ exampleꢀ forꢀ oneꢀ channel,ꢀ assumeꢀ V ꢀ =ꢀ
0.035Ω + 22V
2.5Ω 215pF •
IN
(
) (
)
1
(
)(
)
2
12V(nominal),ꢀV ꢀ=ꢀ22Vꢀ(max),ꢀV ꢀ=ꢀ3.3V,ꢀI ꢀ=ꢀ6A,ꢀ
IN
OUT
MAX
1
V
ꢀ=ꢀ50mVꢀandꢀfꢀ=ꢀ350kHz.
SENSE(MAX)
+
350kHz = 433mW
(
)
5V – 2.3V 2.3V
Theꢀinductanceꢀvalueꢀisꢀchosenꢀfirstꢀbasedꢀonꢀaꢀ30%ꢀrippleꢀ
currentꢀassumption.ꢀTheꢀhighestꢀvalueꢀofꢀrippleꢀcurrentꢀ
occursꢀatꢀtheꢀmaximumꢀinputꢀvoltage.ꢀTieꢀtheꢀFREQꢀpinꢀ
toꢀ GND,ꢀ generatingꢀ 350kHzꢀ operation.ꢀ Theꢀ minimumꢀ
inductanceꢀforꢀ30%ꢀrippleꢀcurrentꢀis:
ꢀ
Aꢀshort-circuitꢀtoꢀgroundꢀwillꢀresultꢀinꢀaꢀfoldedꢀbackꢀcur-
rentꢀof:
95ns 22V
(
)
25mV
0.006Ω 2
1
ISC =
–
= 3.9A
3.9µH
VOUT
ƒ •L
VOUT
ꢀ
∆IL(NOM)
=
1–
V
IN(NOM)
withꢀaꢀtypicalꢀvalueꢀofꢀR
ꢀandꢀδꢀ=ꢀ(0.005/°C)(25°C)ꢀ
ꢀ
DS(ON)
=ꢀ0.125.ꢀTheꢀresultingꢀpowerꢀdissipatedꢀinꢀtheꢀbottomꢀ
Aꢀ3.9µHꢀinductorꢀwillꢀproduceꢀ29%ꢀrippleꢀcurrent.ꢀTheꢀ
peakꢀinductorꢀcurrentꢀwillꢀbeꢀtheꢀmaximumꢀDCꢀvalueꢀplusꢀ
oneꢀhalfꢀtheꢀrippleꢀcurrent,ꢀorꢀ6.88A.ꢀIncreasingꢀtheꢀrippleꢀ
currentꢀwillꢀalsoꢀhelpꢀensureꢀthatꢀtheꢀminimumꢀon-timeꢀ
ofꢀ95nsꢀisꢀnotꢀviolated.ꢀTheꢀminimumꢀon-timeꢀoccursꢀatꢀ
MOSFETꢀis:
2
PSYNC = 3.9A 1.125 0.022Ω = 376mW
ꢀ
whichꢀisꢀlessꢀthanꢀunderꢀfull-loadꢀconditions.
maximumꢀV :
IN
C ꢀisꢀchosenꢀforꢀanꢀRMSꢀcurrentꢀratingꢀofꢀatꢀleastꢀ3Aꢀatꢀ
IN
VOUT
IN(MAX)ƒ
3.3V
temperatureꢀassumingꢀonlyꢀthisꢀchannelꢀisꢀon.ꢀC ꢀisꢀ
OUT
tON(MIN)
=
=
= 429ns
V
chosenꢀwithꢀanꢀESRꢀofꢀ0.02Ωꢀforꢀlowꢀoutputꢀripple.ꢀTheꢀ
outputꢀrippleꢀinꢀcontinuousꢀmodeꢀwillꢀbeꢀhighestꢀatꢀtheꢀ
maximumꢀinputꢀvoltage.ꢀTheꢀoutputꢀvoltageꢀrippleꢀdueꢀtoꢀ
ESRꢀisꢀapproximately:
22V 350kHz
ꢀ
TheꢀequivalentꢀR
ꢀresistorꢀvalueꢀcanꢀbeꢀcalculatedꢀbyꢀ
SENSE
usingꢀtheꢀminimumꢀvalueꢀforꢀtheꢀmaximumꢀcurrentꢀsenseꢀ
thresholdꢀ(43mV):
ꢀ V ꢀ=ꢀR ꢀ(∆I )ꢀ=ꢀ0.02Ω(1.75A)ꢀ=ꢀ35mV
ORIPPLE ESR L P-P
43mV
6.88A
RSENSE
≤
= 0.006Ω
ꢀ
Choosingꢀ1%ꢀresistors:ꢀR ꢀ=ꢀ25kꢀandꢀR ꢀ=ꢀ80.6kꢀyieldsꢀ
A
B
anꢀoutputꢀvoltageꢀofꢀ3.33V.
38571fa
ꢁꢅ
1.ꢀ
2.ꢀ
AreꢀtheꢀtopꢀN-channelꢀMOSFETsꢀMTOP1ꢀandꢀMTOP2ꢀ
locatedꢀwithinꢀ1cmꢀofꢀeachꢀotherꢀwithꢀaꢀcommonꢀdrainꢀ
Areꢀtheꢀsignalꢀandꢀpowerꢀgroundsꢀkeptꢀseparate?ꢀTheꢀ
combinedꢀICꢀsignalꢀgroundꢀpinꢀandꢀtheꢀgroundꢀreturnꢀ
3.ꢀ
DoꢀtheꢀLTC3857-1ꢀV ꢀpins’ꢀresistiveꢀdividersꢀconnectꢀtoꢀ
4.ꢀ
5.ꢀ
AreꢀtheꢀSENSE ꢀandꢀSENSE ꢀleadsꢀroutedꢀtogetherꢀwithꢀ
minimumꢀPCꢀtraceꢀspacing?ꢀTheꢀfilterꢀcapacitorꢀbetweenꢀ
IsꢀtheꢀINTV ꢀdecouplingꢀcapacitorꢀconnectedꢀcloseꢀ
CC
6.ꢀ
Keepꢀtheꢀswitchingꢀnodesꢀ(SW1,ꢀSW2),ꢀtopꢀgateꢀnodesꢀ
(TG1,ꢀTG2),ꢀandꢀboostꢀnodesꢀ(BOOST1,ꢀBOOST2)ꢀawayꢀ
fromꢀ sensitiveꢀ small-signalꢀ nodes,ꢀ especiallyꢀ fromꢀ
theꢀoppositesꢀchannel’sꢀvoltageꢀandꢀcurrentꢀsensingꢀ
feedbackꢀpins.ꢀAllꢀofꢀtheseꢀnodesꢀhaveꢀveryꢀlargeꢀandꢀ
fastꢀmovingꢀsignalsꢀandꢀthereforeꢀshouldꢀbeꢀkeptꢀonꢀ
theꢀoutput sideꢀofꢀtheꢀLTC3857-1ꢀandꢀoccupyꢀminimumꢀ
PCꢀtraceꢀarea.
LTC3857-1
applicaTions inForMaTion
PC Board Layout Checklist
Whenꢀlayingꢀoutꢀtheꢀprintedꢀcircuitꢀboard,ꢀtheꢀfollowingꢀ
checklistꢀshouldꢀbeꢀusedꢀtoꢀensureꢀproperꢀoperationꢀofꢀ
theꢀIC.ꢀTheseꢀitemsꢀareꢀalsoꢀillustratedꢀgraphicallyꢀinꢀtheꢀ
layoutꢀdiagramꢀofꢀFigureꢀ11.ꢀFigureꢀ12ꢀillustratesꢀtheꢀcurrentꢀ
waveformsꢀpresentꢀinꢀtheꢀvariousꢀbranchesꢀofꢀtheꢀ2-phaseꢀ
synchronousꢀregulatorsꢀoperatingꢀinꢀtheꢀcontinuousꢀmode.ꢀ
Checkꢀtheꢀfollowingꢀinꢀyourꢀlayout:
7.ꢀUseꢀaꢀmodifiedꢀstargroundꢀꢀtechnique:ꢀaꢀlowꢀimpedance,ꢀ
largeꢀcopperꢀareaꢀcentralꢀgroundingꢀpointꢀonꢀtheꢀsameꢀ
sideꢀofꢀtheꢀPCꢀboardꢀasꢀtheꢀinputꢀandꢀoutputꢀcapacitorsꢀ
connectionꢀatꢀC ?ꢀDoꢀnotꢀattemptꢀtoꢀsplitꢀtheꢀinputꢀ
IN
withꢀtie-insꢀforꢀtheꢀbottomꢀofꢀtheꢀINTV ꢀdecouplingꢀ
CC
decouplingꢀforꢀtheꢀtwoꢀchannelsꢀasꢀitꢀcanꢀcauseꢀaꢀlargeꢀ
resonantꢀloop.
capacitor,ꢀtheꢀbottomꢀofꢀtheꢀvoltageꢀfeedbackꢀresistiveꢀ
dividerꢀandꢀtheꢀSGNDꢀpinꢀofꢀtheꢀIC.
PC Board Layout Debugging
ofꢀC
ꢀmustꢀreturnꢀtoꢀtheꢀcombinedꢀC ꢀ(–)ꢀter-
INTVCC
OUT
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.
minals.ꢀTheꢀpathꢀformedꢀbyꢀtheꢀtopꢀN-channelꢀMOSFET,ꢀ
SchottkyꢀdiodeꢀandꢀtheꢀC ꢀcapacitorꢀshouldꢀhaveꢀshortꢀ
IN
leadsꢀandꢀPCꢀtraceꢀlengths.ꢀTheꢀoutputꢀcapacitorꢀ(–)ꢀ
terminalsꢀshouldꢀbeꢀconnectedꢀasꢀcloseꢀasꢀpossibleꢀ
toꢀtheꢀ(–)ꢀterminalsꢀofꢀtheꢀinputꢀcapacitorꢀbyꢀplacingꢀ
theꢀcapacitorsꢀnextꢀtoꢀeachꢀotherꢀandꢀawayꢀfromꢀtheꢀ
Schottkyꢀloopꢀdescribedꢀabove.
FB
theꢀ(+)ꢀterminalsꢀofꢀC ?ꢀTheꢀresistiveꢀdividerꢀmustꢀbeꢀ
OUT
connectedꢀbetweenꢀtheꢀ(+)ꢀterminalꢀofꢀC ꢀandꢀsignalꢀ
OUT
ground.ꢀTheꢀfeedbackꢀresistorꢀconnectionsꢀshouldꢀnotꢀ
beꢀalongꢀtheꢀhighꢀcurrentꢀinputꢀfeedsꢀfromꢀtheꢀinputꢀ
capacitor(s).
Theꢀdutyꢀcycleꢀpercentageꢀshouldꢀbeꢀmaintainedꢀfromꢀcycleꢀ
toꢀcycleꢀinꢀaꢀwell-designed,ꢀlowꢀnoiseꢀPCBꢀimplementation.ꢀ
Variationꢀinꢀtheꢀdutyꢀcycleꢀatꢀaꢀsubharmonicꢀrateꢀcanꢀsug-
gestꢀnoiseꢀpickupꢀatꢀtheꢀcurrentꢀorꢀvoltageꢀsensingꢀinputsꢀ
orꢀinadequateꢀloopꢀcompensation.ꢀOvercompensationꢀofꢀ
theꢀloopꢀcanꢀbeꢀusedꢀtoꢀtameꢀaꢀpoorꢀPCꢀlayoutꢀifꢀregula-
torꢀ bandwidthꢀ optimizationꢀ isꢀ notꢀ required.ꢀ Onlyꢀ afterꢀ
eachꢀcontrollerꢀisꢀcheckedꢀforꢀitsꢀindividualꢀperformanceꢀ
shouldꢀbothꢀcontrollersꢀbeꢀturnedꢀonꢀatꢀtheꢀsameꢀtime.ꢀ
Aꢀparticularlyꢀdifficultꢀregionꢀofꢀoperationꢀisꢀwhenꢀoneꢀ
controllerꢀchannelꢀisꢀnearingꢀitsꢀcurrentꢀcomparatorꢀtripꢀ
pointꢀwhenꢀtheꢀotherꢀchannelꢀisꢀturningꢀonꢀitsꢀtopꢀMOSFET.ꢀ
Thisꢀoccursꢀaroundꢀ50%ꢀdutyꢀcycleꢀonꢀeitherꢀchannelꢀdueꢀ
toꢀtheꢀphasingꢀofꢀtheꢀinternalꢀclocksꢀandꢀmayꢀcauseꢀminorꢀ
dutyꢀcycleꢀjitter.
–
+
+
–
SENSE ꢀandꢀSENSE ꢀshouldꢀbeꢀasꢀcloseꢀasꢀpossibleꢀ
toꢀtheꢀIC.ꢀEnsureꢀaccurateꢀcurrentꢀsensingꢀwithꢀKelvinꢀ
connectionsꢀatꢀtheꢀSENSEꢀresistor.
toꢀtheꢀIC,ꢀbetweenꢀtheꢀINTV ꢀandꢀtheꢀpowerꢀgroundꢀ
CC
pins?ꢀThisꢀcapacitorꢀcarriesꢀtheꢀMOSFETꢀdrivers’ꢀcur-
rentꢀpeaks.ꢀAnꢀadditionalꢀ1µFꢀceramicꢀcapacitorꢀplacedꢀ
immediatelyꢀnextꢀtoꢀtheꢀINTV ꢀandꢀPGNDꢀpinsꢀcanꢀhelpꢀ
CC
improveꢀnoiseꢀperformanceꢀsubstantially.
38571fa
ꢁꢆ
LTC3857-1
applicaTions inForMaTion
I
TRACK/SS1
PGOOD1
TG1
R
PU1
TH1
V
PULL-UP
V
PGOOD1
FB1
L1
R
SENSE
+
–
V
SENSE1
OUT1
SW1
SENSE1
LTC3857-1
BOOST1
C
B1
M1
M2
D1
BG1
FREQ
R
IN
C
C
OUT1
V
f
IN
1µF
IN
PLLIN/MODE
RUN1
+
C
CERAMIC
VIN
PGND
GND
RUN2
+
EXTV
CC
CC
C
+
IN
V
C
SGND
IN
INTVCC
–
INTV
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
38571 F11
Figure 11. Recommended Printed Circuit Layout Diagram
Reduceꢀ V ꢀ fromꢀ itsꢀ nominalꢀ levelꢀ toꢀ verifyꢀ operationꢀ
andꢀpossiblyꢀBGꢀconnectionsꢀandꢀtheꢀsensitiveꢀvoltageꢀ
andꢀcurrentꢀpins.ꢀTheꢀcapacitorꢀplacedꢀacrossꢀtheꢀcurrentꢀ
sensingꢀpinsꢀneedsꢀtoꢀbeꢀplacedꢀimmediatelyꢀadjacentꢀtoꢀ
theꢀpinsꢀofꢀtheꢀIC.ꢀThisꢀcapacitorꢀhelpsꢀtoꢀminimizeꢀtheꢀ
effectsꢀofꢀdifferentialꢀnoiseꢀinjectionꢀdueꢀtoꢀhighꢀfrequencyꢀ
capacitiveꢀ coupling.ꢀ Ifꢀ problemsꢀ areꢀ encounteredꢀ withꢀ
highꢀcurrentꢀoutputꢀloadingꢀatꢀlowerꢀinputꢀvoltages,ꢀlookꢀ
IN
ofꢀtheꢀregulatorꢀinꢀdropout.ꢀCheckꢀtheꢀoperationꢀofꢀtheꢀ
undervoltageꢀlockoutꢀcircuitꢀbyꢀfurtherꢀloweringꢀV ꢀwhileꢀ
IN
monitoringꢀtheꢀoutputsꢀtoꢀverifyꢀoperation.
Investigateꢀwhetherꢀanyꢀproblemsꢀexistꢀonlyꢀatꢀhigherꢀout-
putꢀcurrentsꢀorꢀonlyꢀatꢀhigherꢀinputꢀvoltages.ꢀIfꢀproblemsꢀ
coincideꢀwithꢀhighꢀinputꢀvoltagesꢀandꢀlowꢀoutputꢀcurrents,ꢀ
lookꢀforꢀcapacitiveꢀcouplingꢀbetweenꢀtheꢀBOOST,ꢀSW,ꢀTG,ꢀ
forꢀinductiveꢀcouplingꢀbetweenꢀC ,ꢀSchottkyꢀandꢀtheꢀtopꢀ
IN
38571fa
ꢁꢇ
LTC3857-1
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
Theꢀoutputꢀvoltageꢀunderꢀthisꢀimproperꢀhookupꢀwillꢀstillꢀ
beꢀmaintainedꢀbutꢀtheꢀadvantagesꢀofꢀcurrentꢀmodeꢀcontrolꢀ
willꢀnotꢀbeꢀrealized.ꢀCompensationꢀofꢀtheꢀvoltageꢀloopꢀwillꢀ
beꢀ muchꢀ moreꢀ sensitiveꢀ toꢀ componentꢀ selection.ꢀ Thisꢀ
behaviorꢀcanꢀbeꢀinvestigatedꢀbyꢀtemporarilyꢀshortingꢀoutꢀ
theꢀcurrentꢀsensingꢀresistor—don’tꢀworry,ꢀtheꢀregulatorꢀ
willꢀstillꢀmaintainꢀcontrolꢀofꢀtheꢀoutputꢀvoltage.
MOSFETꢀcomponentsꢀtoꢀtheꢀsensitiveꢀcurrentꢀandꢀvoltageꢀ
sensingꢀtraces.ꢀInꢀaddition,ꢀinvestigateꢀcommonꢀgroundꢀ
pathꢀvoltageꢀpickupꢀbetweenꢀtheseꢀcomponentsꢀandꢀtheꢀ
SGNDꢀpinꢀofꢀtheꢀIC.
Anꢀembarrassingꢀproblem,ꢀwhichꢀcanꢀbeꢀmissedꢀinꢀanꢀ
otherwiseꢀproperlyꢀworkingꢀswitchingꢀregulator,ꢀresultsꢀ
whenꢀtheꢀcurrentꢀsensingꢀleadsꢀareꢀhookedꢀupꢀbackwards.ꢀ
38571fa
ꢁꢈ
LTC3857-1
Typical applicaTions
R
B1
215k
LTC3857-1
+
C
SENSE1
F1
INTV
CC
C1
1nF
15pF
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
15k
SS1
ITH1
I
TH1
D1
D2
C
820pF
ITH1
C
0.1µF
V
IN
V
IN
9V TO 38V
C
IN
TRACK/SS1
22µF
INTV
CC
C
4.7µF
INT
PGND
PLLIN/MODE
SGND
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
38581 F12
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
38571fa
ꢂ0
LTC3857-1
Typical applicaTions
High Efficiency Dual 2.5V/3.3V Step-Down Converter
R
B1
144k
LTC3857-1
+
C
SENSE1
INTV
CC
F1
C1
1nF
22pF
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
22k
ITH1
I
TH1
D1
D2
C
820pF
ITH1
C
SS1
0.01µF
V
IN
V
IN
4V TO 38V
C
IN
TRACK/SS1
22µF
INTV
CC
C
INT
4.7µF
PGND
PLLIN/MODE
SGND
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
38571 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
38571fa
ꢂꢀ
LTC3857-1
Typical applicaTions
High Efficiency Dual 12V/5V Step-Down Converter
R
B1
475k
+
C
SENSE1
INTV
F1
CC
C1
1nF
33pF
100k
–
R
A1
SENSE1
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
C
SS1
0.01µF
LTC3857-1
C
680pF
ITH1
V
IN
V
TRACK/SS1
IN
12.5V TO 38V
C
IN
INTV
CC
C
22µF
INT
4.7µF
PGND
PLLIN/MODE
SGND
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
OUT2
5V
SW2
BG2
TRACK/SS2
5.5A
C
C
680pF
OUT2
ITH2
R
17k
150µF
ITH2
I
TH2
C
100pF
C2
ITH2A
V
FB2
C
: KEMET T525D476M016E035
: SANYO 10TPD150M
R
OUT1
OUT2
A2
–
+
SENSE2
C
75k
L1: SUMIDA CDEP105-5R7M
L2: SUMIDA CDEP105-4R3M
MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP
C
1nF
F2
15pF
SENSE2
R
B2
392k
38571 TA03
38571fa
ꢂꢁ
LTC3857-1
Typical applicaTions
High Efficiency Dual 24V/5V Step-Down Converter
R
B1
487k
+
C
SENSE1
INTV
F1
CC
C1
1nF
18pF
100k
–
R
A1
SENSE1
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
25V
s2
0.47µF
R
46k
ITH1
I
TH1
D1
D2
C
0.01µF
SS1
CERAMIC
LTC3857-1
C
680pF
ITH1
V
IN
V
TRACK/SS1
IN
28V TO 38V
C
IN
INTV
CC
C
4.7µF
22µF
INT
PGND
PLLIN/MODE
SGND
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
392k
38571 TA04
38571fa
ꢂꢂ
LTC3857-1
Typical applicaTions
High Efficiency Dual 1V/1.2V Step-Down Converter
R
B1
28.7k
+
C
SENSE1
F1
INTV
CC
C1
1nF
56pF
100k
–
R
A1
SENSE1
PGOOD1
BG1
115k
L1
0.47µH
MBOT1
MTOP1
V
FB1
V
OUT1
C
220pF
ITH1A
1V
SW1
C
R
OUT1 8A
C
SENSE1
B1
BOOST1
TG1
220µF
3.5mΩ
0.47µF
R
ITH1
3.93k
s2
I
TH1
D1
D2
LTC3857-1
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
60k
0.47µF
L2
0.47µH
R
SENSE2
3.5mΩ
C
0.01µF
SS2
V
OUT2
1.2V
SW2
BG2
TRACK/SS2
C
OUT2 8A
C
1000pF
ITH2
220µF
R
3.43k
ITH2
s2
I
TH2
C
220pF
C2
ITH2A
V
FB2
R
C
, C
: SANYO 2R5TPE220M
A2
OUT1 OUT2
–
+
SENSE2
115k
L1: SUMIDA CDEP105-3R2M
L2: SUMIDA CDEP105-7R2M
MTOP1, MTOP2: RENESAS RJK0305
MBOT1, MBOT2: RENESAS RJK0328
C
1nF
F2
56pF
SENSE2
R
B2
38571 TA05
57.6k
38571fa
ꢂꢃ
LTC3857-1
Typical applicaTions
High Efficiency Dual 1V/1.2V Step-Down Converter with Inductor DCR Current Sensing
R
R
S1
1.18k
B1
28.7k
+
C
SENSE1
F1
INTV
CC
C1
0.1µF
56pF
100k
–
R
A1
SENSE1
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
LTC3857-1
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
3.43k
ITH2
s2
I
TH2
C
220pF
C2
ITH2A
V
FB2
R
A2
–
+
SENSE2
115k
C
, C
: SANYO 2R5TPE220M
OUT1 OUT2
L1, L2: SUMIDA IHL ERR47M06
MTOP1, MTOP2: RENESAS RJK0305
MBOT1, MBOT2: RENESAS RJK0328
C
0.1µF
F2
56pF
SENSE2
R
S2
1.18k
R
B2
57.6k
38571 TA06
38571fa
ꢂꢄ
LTC3857-1
package DescripTion
GN Package
28-Lead Plastic SSOP (Narrow .150 Inch)
(ReferenceꢀLTCꢀDWGꢀ#ꢀ05-08-1641)
.386 – .393*
(9.804 – 9.982)
.045 p.005
.033
(0.838)
REF
28 27 26 25 24 23 22 21 20 19 18 17 1615
.254 MIN
.150 – .165
.229 – .244
.150 – .157**
(5.817 – 6.198)
(3.810 – 3.988)
.0165 p.0015
.0250 BSC
1
2
3
4
5
6
7
8
9 10 11 12 13 14
RECOMMENDED SOLDER PAD LAYOUT
.015 p .004
(0.38 p 0.10)
.0532 – .0688
(1.35 – 1.75)
s 45o
.004 – .0098
(0.102 – 0.249)
.0075 – .0098
(0.19 – 0.25)
0o – 8o TYP
.016 – .050
(0.406 – 1.270)
.008 – .012
.0250
(0.635)
BSC
GN28 (SSOP) 0204
(0.203 – 0.305)
TYP
NOTE:
1. CONTROLLING DIMENSION: INCHES
INCHES
2. DIMENSIONS ARE IN
(MILLIMETERS)
3. DRAWING NOT TO SCALE
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
38571fa
ꢂꢅ
LTC3857-1
revision hisTory
REV
DATE
DESCRIPTION
PAGE NUMBER
A
12/09 ChangeꢀtoꢀAbsoluteꢀMaximumꢀRatings
ChangesꢀtoꢀElectricalꢀCharacteristics
ChangeꢀtoꢀTypicalꢀPerformanceꢀCharacteristics
ChangeꢀtoꢀPinꢀFunctions
2
3,ꢀ4
6
8,ꢀ9
TextꢀChangesꢀtoꢀOperationꢀSection
TextꢀChangesꢀtoꢀApplicationsꢀInformationꢀSection
ChangeꢀtoꢀTableꢀ2
11,ꢀ12,ꢀ13
21,ꢀ22,ꢀ23,ꢀ26
23
28
38
ChangeꢀtoꢀFigureꢀ11
ChangesꢀtoꢀRelatedꢀParts
38571fa
InformationꢀfurnishedꢀbyꢀLinearꢀTechnologyꢀCorporationꢀisꢀbelievedꢀtoꢀbeꢀaccurateꢀandꢀreliable.ꢀ
However,ꢀnoꢀresponsibilityꢀisꢀassumedꢀforꢀitsꢀuse.ꢀLinearꢀTechnologyꢀCorporationꢀmakesꢀnoꢀrepresenta-
tionꢀthatꢀtheꢀinterconnectionꢀofꢀitsꢀcircuitsꢀasꢀdescribedꢀhereinꢀwillꢀnotꢀinfringeꢀonꢀexistingꢀpatentꢀrights.
ꢂꢆ
LTC3857-1
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-LockableꢀFixedꢀOperatingꢀFrequencyꢀ250kHzꢀtoꢀ770kHz,ꢀ
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
38571fa
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
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