LTC3857
Low I , Dual, 2-Phase
Q
Synchronous Step-Down
Controller
FeaTures
DescripTion
Theꢀ LTC®3857ꢀ isꢀ aꢀ highꢀ performanceꢀ dualꢀ step-downꢀ
switchingꢀregulatorꢀcontrollerꢀthatꢀdrivesꢀallꢀN-channelꢀ
synchronousꢀpowerꢀMOSFETꢀstages.ꢀAꢀconstantꢀfrequencyꢀ
currentꢀmodeꢀarchitectureꢀallowsꢀaꢀphase-lockableꢀfre-
quencyꢀofꢀupꢀtoꢀ850kHz.ꢀPowerꢀlossꢀandꢀnoiseꢀdueꢀtoꢀtheꢀ
ESRꢀofꢀtheꢀinputꢀcapacitorꢀESRꢀareꢀminimizedꢀbyꢀoperatingꢀ
theꢀtwoꢀcontrollerꢀoutputꢀstagesꢀoutꢀofꢀphase.
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ꢀ Low Operating I : 50µA (One Channel On)
Q
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ꢀ Wide Output Voltage Range: 0.8V ≤ V
≤ 24V
OUT
ꢀ Wide V Range: 4V to 38V (40V Abs Max)
IN
ꢀ R
or DCR Current Sensing
SENSE
ꢀ Out-of-PhaseꢀControllersꢀReduceꢀRequiredꢀInputꢀ
CapacitanceꢀandꢀPowerꢀSupplyꢀInducedꢀNoise
®
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ꢀ OPTI-LOOP ꢀCompensationꢀMinimizesꢀC
OUT
ꢀ Phase-LockableꢀFrequencyꢀ(75kHz-850kHz)
ꢀ ProgrammableꢀFixedꢀFrequencyꢀ(50kHz-900kHz)
ꢀ SelectableꢀContinuous,ꢀPulse-SkippingꢀorꢀLowꢀRippleꢀ
BurstꢀMode®ꢀOperationꢀatꢀLightꢀLoads
Theꢀ50μAꢀno-loadꢀquiescentꢀcurrentꢀextendsꢀoperatingꢀrunꢀ
timeꢀinꢀbattery-poweredꢀsystems.ꢀTheꢀLTC3857ꢀfeaturesꢀaꢀ
precisionꢀ0.8Vꢀreferenceꢀandꢀpowerꢀgoodꢀoutputꢀindicators.ꢀAꢀ
wideꢀ4Vꢀtoꢀ38Vꢀinputꢀsupplyꢀrangeꢀencompassesꢀaꢀwideꢀrangeꢀ
ofꢀintermediateꢀbusꢀvoltagesꢀandꢀbatteryꢀchemistries.
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ꢀ SelectableꢀCurrentꢀLimit
ꢀ VeryꢀLowꢀDropoutꢀOperation:ꢀ99%ꢀDutyꢀCycle
ꢀ AdjustableꢀOutputꢀVoltageꢀSoft-StartꢀorꢀTracking
ꢀ PowerꢀGoodꢀOutputꢀVoltageꢀMonitors
ꢀ OutputꢀOvervoltageꢀProtection
IndependentꢀTRACK/SSꢀpinsꢀforꢀeachꢀcontrollerꢀrampꢀtheꢀ
outputꢀvoltagesꢀduringꢀstart-up.ꢀCurrentꢀfoldbackꢀlimitsꢀ
MOSFETꢀheatꢀdissipationꢀduringꢀshort-circuitꢀconditions.ꢀ
TheꢀPLLIN/MODEꢀpinꢀselectsꢀamongꢀBurstꢀModeꢀopera-
tion,ꢀpulse-skippingꢀmode,ꢀorꢀcontinuousꢀinductorꢀcurrentꢀ
modeꢀatꢀlightꢀloads.ꢀ
ꢀ LowꢀShutdownꢀI ꢀ:ꢀ<8µA
Q
ꢀ InternalꢀLDOꢀPowersꢀGateꢀDriveꢀfromꢀV ꢀorꢀEXTV
IN
CC
ꢀ NoꢀCurrentꢀFoldbackꢀDuringꢀStart-up
ꢀ SmallꢀLowꢀProfileꢀ(0.75mm)ꢀ5mmꢀ×ꢀ5mmꢀQFNꢀPackage
Forꢀaꢀleadedꢀ28-leadꢀSSOPꢀpackageꢀwithꢀaꢀfixedꢀcurrentꢀ
limitꢀandꢀoneꢀPGOODꢀoutput,ꢀwithoutꢀphaseꢀmodulationꢀ
orꢀaꢀclockꢀoutput,ꢀseeꢀtheꢀLTC3857-1ꢀdataꢀsheet.
applicaTions
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ꢀ AutomotiveꢀAlways-OnꢀSystems
L,ꢀLT,ꢀLTC,ꢀLTM,ꢀBurstꢀMode,ꢀOPTI-LOOP,ꢀPolyPhase,ꢀµModule,ꢀLinearꢀTechnologyꢀandꢀtheꢀ
LinearꢀlogoꢀareꢀregisteredꢀtrademarksꢀandꢀNoꢀR
ꢀandꢀUltraFastꢀareꢀtrademarksꢀofꢀLinearꢀ
SENSE
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ꢀ BatteryꢀOperatedꢀDigitalꢀDevices
TechnologyꢀCorporation.ꢀAllꢀotherꢀtrademarksꢀareꢀtheꢀpropertyꢀofꢀtheirꢀrespectiveꢀowners.ꢀ
ProtectedꢀbyꢀU.S.ꢀPatents,ꢀincludingꢀ5481178,ꢀ5929620,ꢀ6177787,ꢀ6144194,ꢀ5408150,ꢀ
6580258,ꢀ5705919,ꢀ6100678.
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ꢀ DistributedꢀDCꢀPowerꢀSystems
Typical applicaTion
Efficiency and Power Loss
High Efficiency Dual 3.3V/8.5V Step-Down Converter
V
IN
vs Output Current
9V TO 38V
22µF
50V
100
90
10000
1000
100
10
4.7µF
V
V
= 12V
IN
OUT
V
INTV
CC
IN
= 3.3V
TG1
TG2
FIGURE 13 CIRCUIT
80
0.1µF
0.1µF
BOOST1
SW1
BOOST2
SW2
3.3µH
7.2µH
70
60
50
BG1
BG2
LTC3857
PGND
40
30
20
10
0
+
+
SENSE1
SENSE2
0.010Ω
193k
0.007Ω
1
–
–
V
SENSE2
OUT2
SENSE1
FB1
V
OUT1
3.3V
5A
8.5V
3.5A
V
V
I
FB2
0.1
62.5k
I
0.000010.0001 0.001 0.01
0.1
1
10
TH1
TH2
150µF
680pF
15k
680pF
15k
150µF
OUTPUT CURRENT (A)
TRACK/SS1 SGND TRACK/SS2
0.1µF
20k
3857 TA01b
20k
0.1µF
3857 TA01
3857fa
ꢀ