LTC3868 datasheet(21/38 Pages) LINER

21 / 38 page

LTC3868

3868fb

applicaTions inForMaTion

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

Soft-startisenabledbysimplyconnectingacapacitorfrom

theSSpintoground,asshowninFigure7.Aninternal1µA

currentsourcechargesthecapacitor,providingalinear

rampingvoltageattheSSpin.TheLTC3868willregulate

theVFBpin(andhenceVOUT)accordingtothevoltageon

theSSpin,allowingVOUTtorisesmoothlyfrom0Vto

itsfinalregulatedvalue.Thetotalsoft-starttimewillbe

approximately:

tSS = CSS •

0.8V

1µA

as discussed in the Efficiency Considerations section.

Thejunctiontemperaturecanbeestimatedbyusingthe

equationsgiveninNote3oftheElectricalCharacteristics.

Forexample,theLTC3868INTVCCcurrentislimitedtoless

than45mAfroma28VsupplywhennotusingtheEXTVCC

supplyat70°Cambienttemperature:

TJ=70°C+(45mA)(28V)(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

controlpowertobederivedfromoneoftheLTC3868’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

an8.5Vsupplyreducesthejunctiontemperatureinthe

previousexamplefrom125°Cto:

TJ=70°C+(45mA)(8.5V)(43°C/W)=86°C

However,for3.3Vandotherlowvoltageoutputs,addi-

tionalcircuitryisrequiredtoderiveINTVCCpowerfrom

theoutput.

1/2 LTC3868

CSS

SGND

3868 F07

INTVCC Regulators

The LTC3868 features two separate internal P-channel

lowdropoutlinearregulators(LDO)thatsupplypower

attheINTVCCpinfromeithertheVINsupplypinorthe

EXTVCCpindependingontheconnectionoftheEXTVCC

pin. INTVCC powers the gate drivers and much of the

LTC3868’sinternalcircuitry.TheVINLDOandtheEXTVCC

LDOregulateINTVCCto5.1V.Eachofthesecansupplya

peakcurrentof50mAandmustbebypassedtoground

withaminimumof4.7µFlowESRcapacitor.Nomatter

what type of bulk capacitor is used, an additional 1µF

ceramiccapacitorplaceddirectlyadjacenttotheINTVCC

andPGNDpinsishighlyrecommended.Goodbypassing

isneededtosupplythehightransientcurrentsrequired

bytheMOSFETgatedriversandtopreventinteraction

betweenthechannels.

HighinputvoltageapplicationsinwhichlargeMOSFETsare

beingdrivenathighfrequenciesmaycausethemaximum

junctiontemperatureratingfortheLTC3868tobeexceeded.

TheINTVCCcurrent,whichisdominatedbythegatecharge

current,maybesuppliedbyeithertheVINLDOorthe

EXTVCCLDO.WhenthevoltageontheEXTVCCpinisless

than4.7V,theVINLDOisenabled.Powerdissipationforthe

ICinthiscaseishighestandisequaltoVIN•IINTVCC.The

gatechargecurrentisdependentonoperatingfrequency

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