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LTC3857IUHTRPBF Datasheet(PDF) 8 Page - Linear Technology

Part # LTC3857IUHTRPBF
Description  Low IQ, Dual, 2-Phase Synchronous Step-Down Controller
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC3857IUHTRPBF Datasheet(HTML) 8 Page - Linear Technology

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LTC3857

3857fa
pin FuncTions
SENSE1, SENSE2(Pin 1, Pin 9):The(–)Inputtothe
Differential Current Comparators. When greater than
INTVCC–0.5V,theSENSEpinsuppliescurrenttothe
currentcomparator.
FREQ (Pin 2):Thefrequencycontrolpinfortheinternal
VCO.ConnectingthepintoGNDforcestheVCOtoafixed
lowfrequencyof350kHz.ConnectingthepintoINTVCC
forces the VCO to a fixed high frequency of 535kHz.
Other frequencies between 50kHz and 900kHz can be
programmedusingaresistorbetweenFREQandGND.
Aninternal20µApull-upcurrentdevelopsthevoltageto
beusedbytheVCOtocontrolthefrequency
PHASMD (Pin 3):ControlInputtoPhaseSelectorwhich
determines the phase relationships between control-
ler1, controller 2 and the CLKOUT signal. Pulling this
pintogroundforcesTG2andCLKOUTtobeoutofphase
180°and60°withrespecttoTG1.Connectingthispinto
INTVCCforcesTG2andCLKOUTtobeoutofphase240°
and120°withrespecttoTG1.FloatingthispinforcesTG2
andCLKOUTtobeoutofphase180°and90°withrespect
toTG1.RefertoTable1.
CLKOUT (Pin 4):Outputclocksignalavailabletodaisy-
chainothercontrollerICsforadditionalMOSFETdriver
stages/phases.TheoutputlevelsswingfromINTVCCto
ground.
PLLIN/MODE (Pin 5):ExternalSynchronizationInputto
PhaseDetectorandForcedContinuousModeInput.When
anexternalclockisappliedtothispin,thephase-locked
loopwillforcetherisingTG1signaltobesynchronized
with the rising edge of the external clock. When not
synchronizingtoanexternalclock,thisinput,whichacts
onbothcontrollers,determineshowtheLTC3857operates
at light loads. Pulling this pin to ground selects Burst
Modeoperation.Aninternal100kresistortogroundalso
invokesBurstModeoperationwhenthepinisfloated.
TyingthispintoINTVCCforcescontinuousinductorcurrent
operation.Tyingthispintoavoltagegreaterthan1.2Vand
lessthanINTVCC–1.3Vselectspulse-skippingoperation.
Thiscanbedonebyaddinga100kresistorbetweenthe
PLLIN/MODEpinandINTVCC.
SGND (Pin 6, Exposed Pad Pin 33):Small-signalground
commontobothcontrollers,mustberoutedseparately
fromhighcurrentgroundstothecommon(–)terminals
oftheCINcapacitors.Theexposedpadmustbesoldered
tothePCBforratedthermalperformance.
RUN1, RUN2 (Pin 7, Pin 8):DigitalRunControlInputsfor
EachController.Forcingeitherofthesepinsbelow1.26V
shutsdownthatcontroller.Forcingbothofthesepinsbelow
0.7VshutsdowntheentireLTC3857,reducingquiescent
currenttoapproximately8µA.Donotfloatthesepins.
INTVCC(Pin19):OutputoftheInternalLinearLowDropout
Regulator. The driver and control circuits are powered
fromthisvoltagesource.Mustbedecoupledtopower
groundwithaminimumof4.7µFceramicorotherlow
ESRcapacitor.DonotusetheINTVCCpinforanyother
purpose.
EXTVCC (Pin 20):ExternalPowerInputtoanInternalLDO
ConnectedtoINTVCC.ThisLDOsuppliesINTVCCpower,
bypassingtheinternalLDOpoweredfromVINwhenever
EXTVCCishigherthan4.7V.SeeEXTVCCConnectionin
theApplicationsInformationsection.Donotexceed14V
onthispin.
PGND (Pin 21):DriverPowerGround.Connectstothe
sourcesofbottom(synchronous)N-channelMOSFETs
andthe(–)terminal(s)ofCIN.
VIN (Pin 22):MainSupplyPin.Abypasscapacitorshould
betiedbetweenthispinandthesignalgroundpin.
BG1, BG2 (Pin 23, Pin 18): High Current Gate Drives
forBottom(Synchronous)N-ChannelMOSFETs.Voltage
swingatthesepinsisfromgroundtoINTVCC.
BOOST1,BOOST2(Pin24,Pin17):BootstrappedSupplies
totheTopsideFloatingDrivers.Capacitorsareconnected
betweentheBOOSTandSWpinsandSchottkydiodesare
tiedbetweentheBOOSTandINTVCCpins.Voltageswing
attheBOOSTpinsisfromINTVCCto(VIN+INTVCC).
SW1, SW2 (Pin 25, Pin 16):SwitchNodeConnections
toInductors.


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