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June24,2014 |DATASHEET|Rev3.6
18
IR3897
4AHighlyIntegratedSupIRBuckTM
Single‐InputVoltage,SynchronousBuckRegulator
‐
THEORYOFOPERATION
DESCRIPTION
TheIR3897usesaPWMvoltagemodecontrolschemewith
externalcompensationtoprovidegoodnoiseimmunity
andmaximumflexibilityinselectinginductorvaluesand
capacitortypes.
Theswitchingfrequencyisprogrammablefrom300kHz
to1.5MHzandprovidesthecapabilityofoptimizingthe
designintermsofsizeandperformance.
IR3897providespreciselyregulatedoutputvoltage
programmedviatwoexternalresistorsfrom0.5Vto
0.86*Vin.
TheIR3897operateswithaninternalbiassupply(LDO)
whichisconnectedtotheVcc/LDO_outpin.Thisallows
operationwithsinglesupply.Thebiasvoltageisvariable
accordingtoloadcondition.Iftheoutputloadcurrentis
lessthanhalfofthepeak‐to‐peakinductorcurrent,alower
biasvoltage,4.4V,isusedastheinternalgatedrive
voltage;otherwise,ahighervoltage,6.4V,isused.
Thisfeaturehelpstheconvertertoreducepowerlosses.
Thedevicecanalsobeoperatedwithanexternalsupply
from4.5to7.5V,allowinganextendedoperatinginput
voltage(PVin)rangefrom1.0Vto16V.Forusingthe
internalLDOsupply,theVinpinshouldbeconnectedto
PVinpin.Ifanexternalsupplyisused,itshouldbe
connectedtoVcc/LDO_OutpinandtheVinpinshouldbe
shortedtoVcc/LDO_Outpin.
Thedeviceutilizestheon‐resistanceofthelowside
MOSFET(synchronousMosfet)fortheovercurrent
protection.Thismethodenhancestheconverter’s
efficiencyandreducescostbyeliminatingtheneedfor
externalcurrentsenseresistor.
IR3897includestwolowRds(on)MOSFETsusingIR’sHEXFET
technology.Thesearespecificallydesignedforhigh
efficiencyapplications.
UNDER‐VOLTAGELOCKOUTANDPOR
Theunder‐voltagelockoutcircuitmonitorsthevoltageof
Vcc/LDO_OutpinandtheEnableinput.Itassuresthatthe
MOSFETdriveroutputsremainintheoffstatewhenever
eitherofthesetwosignalsdropbelowthesetthresholds.
NormaloperationresumesonceVcc/LDO_OutandEnable
riseabovetheirthresholds.
ThePOR(PowerOnReady)signalisgeneratedwhenall
thesesignalsreachthevalidlogiclevel(seesystemblock
diagram).WhenthePORisassertedthesoftstart
sequencestarts(seesoftstartsection).
ENABLE
TheEnablefeaturesanotherlevelofflexibilityforstartup.
TheEnablehasprecisethresholdwhichisinternally
monitoredbyUnder‐VoltageLockout(UVLO)circuit.
Therefore,theIR3897willturnononlywhenthevoltage
attheEnablepinexceedsthisthreshold,typically,1.2V.
IftheinputtotheEnablepinisderivedfromthebus
voltagebyasuitablyprogrammedresistivedivider,itcan
beensuredthattheIR3897doesnotturnonuntilthebus
voltagereachesthedesiredlevel(Fig.4).Onlyafterthebus
voltagereachesorexceedsthislevelandvoltageatthe
Enablepinexceedsitsthreshold,IR3897willbeenabled.
Therefore,inadditiontobeingalogicinputpintoenable
theIR3897,theEnablefeature,withitsprecisethreshold,
alsoallowstheusertoimplementanUnder‐Voltage
Lockoutforthebusvoltage(PVin).Thisisdesirable
particularlyforhighoutputvoltageapplications,wherewe
mightwanttheIR3897tobedisabledatleastuntilPVIN
exceedsthedesiredoutputvoltagelevel.
Pvin (12V)
Vcc
Enable
Intl_SS
10. 2 V
Enable Threshold = 1.2V
Figure4:NormalStartup,deviceturnson
whenthebusvoltagereaches10.2V
AresistordividerisusedatENpinfromPVintoturnonthe
deviceat10.2V.