LTC3868IGN-1 datasheet(21/38 Pages) LINER

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LTC3868-1

38681fb

applicaTions inForMaTion

linearrampingvoltageattheSSpin.TheLTC3868-1will

regulatetheVFBpin(andhenceVOUT)accordingtothe

voltageontheSSpin,allowingVOUTtorisesmoothlyfrom

0Vtoitsfinalregulatedvalue.Thetotalsoft-starttimewill

beapproximately:

µA

• .

0 8

INTVCC Regulators

TheLTC3868-1featurestwoseparateinternalP-channel

lowdropoutlinearregulators(LDO)thatsupplypower

attheINTVCCpinfromeithertheVINsupplypinorthe

EXTVCCpindependingontheconnectionoftheEXTVCC

pin. INTVCC powers the gate drivers and much of the

LTC3868-1’sinternalcircuitry.TheVINLDOandtheEXTVCC

LDOregulateINTVCCto5.1V.Eachofthesecansupplya

peakcurrentof50mAandmustbebypassedtoground

withaminimumof4.7µFlowESRcapacitor.Nomatter

whattypeofbulkcapacitorisused,anadditional1µFce-

ramiccapacitorplaceddirectlyadjacenttotheINTVCCand

PGNDICpinsishighlyrecommended.Goodbypassing

isneededtosupplythehightransientcurrentsrequired

bytheMOSFETgatedriversandtopreventinteraction

betweenthechannels.

HighinputvoltageapplicationsinwhichlargeMOSFETs

are being driven at high frequencies may cause the

maximumjunctiontemperatureratingfortheLTC3868-1

tobeexceeded.TheINTVCCcurrent,whichisdominated

bythegatechargecurrent,maybesuppliedbyeither

theVINLDOortheEXTVCCLDO.Whenthevoltageon

theEXTVCCpinislessthan4.7V,theVINLDOisenabled.

PowerdissipationfortheICinthiscaseishighestandis

equaltoVIN•IINTVCC.Thegatechargecurrentisdepen-

dentonoperatingfrequencyasdiscussedintheEfficiency

Considerationssection.Thejunctiontemperaturecanbe

estimatedbyusingtheequationsgiveninNote2ofthe

Electrical Characteristics. For example, the LTC3868-1

INTVCCcurrentislimitedtolessthan22mAfroma28V

supplywhennotusingtheEXTVCCsupplyat70°Cambient

temperatureintheSSOPpackage:

TJ=70°C+(22mA)(28V)(90°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

controlpowertobederivedfromoneoftheLTC3868-1’s

switchingregulatoroutputs(4.7V≤VOUT≤14V)during

normaloperationandfromtheVINLDOwhentheout-

putisoutofregulation(e.g.,start-up,short-circuit).If

morecurrentisrequiredthroughtheEXTVCCLDOthan

is specified, an external Schottky diode can be added

betweentheEXTVCCandINTVCCpins.Inthiscase,do

notapplymorethan6VtotheEXTVCCpinandmakesure

thatEXTVCC≤VIN.

Significantefficiencyandthermalgainscanberealized

bypoweringINTVCCfromtheoutput,sincetheVINcur-

rentresultingfromthedriverandcontrolcurrentswillbe

scaledbyafactorof(DutyCycle)/(SwitcherEfficiency).

For5Vto14Vregulatoroutputs,thismeansconnecting

theEXTVCCpindirectlytoVOUT.TyingtheEXTVCCpinto

a8.5Vsupplyreducesthejunctiontemperatureinthe

previousexamplefrom125°Cto:

TJ=70°C+(45mA)(8.5V)(90°C/W)=87°C

However,for3.3Vandotherlowvoltageoutputs,addi-

tionalcircuitryisrequiredtoderiveINTVCCpowerfrom

theoutput.

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