LTC3857 datasheet(21/38 Pages) LINER

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LTC3857

3857fa

applicaTions inForMaTion

INTVCC Regulators

TheLTC3857featurestwoseparateinternalP-channellow

dropoutlinearregulators(LDO)thatsupplypoweratthe

INTVCCpinfromeithertheVINsupplypinortheEXTVCC

pin depending on the connection of the EXTVCC pin.

INTVCCpowersthegatedriversandmuchoftheLTC3857’s

internalcircuitry.TheVINLDOandtheEXTVCCLDOregulate

INTVCCto5.1V.Eachofthesecansupplyapeakcurrentof

50mAandmustbebypassedtogroundwithaminimum

of4.7µFceramiccapacitor.Nomatterwhattypeofbulk

capacitor is used, an additional 1µF ceramic capacitor

placeddirectlyadjacenttotheINTVCCandPGNDpinsis

highlyrecommended.Goodbypassingisneededtosupply

thehightransientcurrentsrequiredbytheMOSFETgate

driversandtopreventinteractionbetweenthechannels.

HighinputvoltageapplicationsinwhichlargeMOSFETs

are being driven at high frequencies may cause the

maximumjunctiontemperatureratingfortheLTC3857

tobeexceeded.TheINTVCCcurrent,whichisdominated

bythegatechargecurrent,maybesuppliedbyeitherthe

VINLDOortheEXTVCCLDO.Whenthevoltageonthe

EXTVCCpinislessthan4.7V,theVINLDOisenabled.Power

dissipationfortheICinthiscaseishighestandisequal

toVIN•IINTVCC.Thegatechargecurrentisdependent

on operating frequency as discussed in the Efficiency

Considerations section. The junction temperature can

beestimatedbyusingtheequationsgiveninNote3of

theElectricalCharacteristics.Forexample,theLTC3857

INTVCCcurrentislimitedtolessthan32mAfroma40V

supplywhennotusingtheEXTVCCsupplyat70°Cambi-

enttemperature:

TJ=70°C+(32mA)(40V)(43°C/W)=125°C

Topreventthemaximumjunctiontemperaturefrombe-

ingexceeded,theinputsupplycurrentmustbechecked

whileoperatinginforcedcontinuousmode(PLLIN/MODE

=INTVCC)atmaximumVIN.

WhenthevoltageappliedtoEXTVCCrisesabove4.7V,the

VINLDOisturnedoffandtheEXTVCCLDOisenabled.The

EXTVCCLDOremainsonaslongasthevoltageappliedto

EXTVCCremainsabove4.5V.TheEXTVCCLDOattempts

toregulatetheINTVCCvoltageto5.1V,sowhileEXTVCC

islessthan5.1V,theLDOisindropoutandtheINTVCC

voltageisapproximatelyequaltoEXTVCC.WhenEXTVCC

isgreaterthan5.1V,uptoanabsolutemaximumof14V,

INTVCCisregulatedto5.1V.

UsingtheEXTVCCLDOallowstheMOSFETdriverandcon-

trolpowertobederivedfromoneoftheLTC3857’sswitch-

ingregulatoroutputs(4.7V≤VOUT≤14V)duringnormal

operationandfromtheVINLDOwhentheoutputisoutof

regulation(e.g.,start-up,short-circuit).Ifmorecurrent

isrequiredthroughtheEXTVCCLDOthanisspecified,an

externalSchottkydiodecanbeaddedbetweentheEXTVCC

andINTVCCpins.Inthiscase,donotapplymorethan6V

totheEXTVCCpinandmakesurethatEXTVCC≤VIN.

Significantefficiencyandthermalgainscanberealized

bypoweringINTVCCfromtheoutput,sincetheVINcur-

rentresultingfromthedriverandcontrolcurrentswillbe

scaledbyafactorof(DutyCycle)/(SwitcherEfficiency).

For5Vto14Vregulatoroutputs,thismeansconnecting

theEXTVCCpindirectlytoVOUT.TyingtheEXTVCCpinto

an8.5Vsupplyreducesthejunctiontemperatureinthe

previousexamplefrom125°Cto:

TJ=70°C+(32mA)(8.5V)(43°C/W)=82°C

However,for3.3Vandotherlowvoltageoutputs,addi-

tionalcircuitryisrequiredtoderiveINTVCCpowerfrom

theoutput.

Thefollowinglistsummarizesthefourpossibleconnec-

tionsforEXTVCC:

1.EXTVCCLeftOpen(orGrounded).ThiswillcauseINTVCC

tobepoweredfromtheinternal5.1Vregulatorresult-

inginanefficiencypenaltyofupto10%athighinput

voltages.

2.EXTVCCConnectedDirectlytoVOUT.Thisisthenormal

connectionfora5Vto14Vregulatorandprovidesthe

highestefficiency.

3.EXTVCCConnectedtoanExternalSupply.Ifanexternal

supplyisavailableinthe5Vto14Vrange,itmaybe

usedtopowerEXTVCCprovidingitiscompatiblewiththe

MOSFETgatedriverequirements.EnsurethatEXTVCC

<VIN.

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