LTC3858 datasheet(21/38 Pages) LINER

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LTC3858

3858fa

applicaTions inForMaTion

Figure 7. Using the TRACK/SS Pin to Program Soft-Start

Soft-startisenabledbysimplyconnectingacapacitorfrom

theSSpintoground,asshowninFigure7.Aninternal1µA

currentsourcechargesthecapacitor,providingalinear

rampingvoltageattheSSpin.TheLTC3858willregulate

theVFBpin(andhenceVOUT)accordingtothevoltageon

theSSpin,allowingVOUTtorisesmoothlyfrom0Vto

itsfinalregulatedvalue.Thetotalsoft-starttimewillbe

approximately:

µA

•

0 8

as discussed in the Efficiency Considerations section.

Thejunctiontemperaturecanbeestimatedbyusingthe

equationsgiveninNote3oftheElectricalCharacteristics.

Forexample,theLTC3858INTVCCcurrentislimitedtoless

than32mAfroma40VsupplywhennotusingtheEXTVCC

supplyat70°Cambienttemperature:

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.

UsingtheEXTVCCLDOallowstheMOSFETdriverand

controlpowertobederivedfromoneoftheswitching

regulatoroutputs(4.7V≤VOUT≤14V)duringnormal

operationandfromtheVINLDOwhentheoutputisout

ofregulation(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.

1/2 LTC3858

CSS

SGND

3858 F07

INTVCC Regulators

TheLTC3858featurestwoseparateinternalP-channellow

dropoutlinearregulators(LDO)thatsupplypoweratthe

INTVCCpinfromeithertheVINsupplypinortheEXTVCC

pindependingontheconnectionoftheEXTVCCpin.INTVCC

powersthegatedriversandmuchoftheinternalcircuitry.

TheVINLDOandtheEXTVCCLDOregulateINTVCCto5.1V.

Eachofthesecansupplyapeakcurrentof50mAandmust

bebypassedtogroundwithaminimumof4.7µFlowESR

capacitor.Regardlessofwhattypeofbulkcapacitoris

used,anadditional1µFceramiccapacitorplaceddirectly

adjacenttotheINTVCCandPGNDpinsishighlyrecom-

mended.Goodbypassingisneededtosupplythehigh

transientcurrentsrequiredbytheMOSFETgatedrivers

andtopreventinteractionbetweenthechannels.

HighinputvoltageapplicationsinwhichlargeMOSFETsare

beingdrivenathighfrequenciesmaycausethemaximum

junctiontemperatureratingfortheLTC3858tobeexceeded.

TheINTVCCcurrent,whichisdominatedbythegatecharge

current,maybesuppliedbyeithertheVINLDOorthe

EXTVCCLDO.WhenthevoltageontheEXTVCCpinisless

than4.7V,theVINLDOisenabled.Powerdissipationforthe

ICinthiscaseishighestandisequaltoVIN•IINTVCC.The

gatechargecurrentisdependentonoperatingfrequency

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