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
ꢀ