元器件交易网

SGLS371–JANUARY2007

GENERALINFORMATION

Overview

TheTPS40200isanonsynchronouscontrollerwithabuilt-in200-mAdriver,designedtodrivehigh-speedP-channelFETSupto500kHz.Itssmallsizecombinedwithcompletefunctionalitymakesthepartbothversatileandeasytouse.

Thecontrollerusesalow-valuecurrent-sensingresistorinserieswiththeinputvoltageandthepowerFETsourceconnectiontodetectswitchingcurrent.Whenthevoltagedropacrossthisresistorexceeds100mV,thepartentersahiccupfaultmodeatapproximately2%oftheoperatingfrequency.

Thepartusesvoltagefeedbacktoanerroramplifierthatisbiasedbyaprecision700-mVreference.Feed-forwardcompensationfromtheinputkeepsthepulse-widthmodulator(PWM)gainconstantoverthefullinputvoltagerange,eliminatingtheneedtochangefrequencycompensationfordifferentinputvoltages.Thepartalsoincorporatesasoft-startfeaturewheretheoutputfollowsaslowlyrisingsoft-startvoltage,preventingoutput-voltageovershoot.

ProgrammingtheOperatingFrequency

Theoperatingfrequencyofthecontrollerisdeterminedbyanexternalresistor,RRC,thatisconnectedfromtheRCpintoVDDandacapacitorattachedfromtheRCpintoground.Thisconnection,andthetwooscillatorcomparatorsinsidetheIC,areshowninFigure23Theoscillatorfrequencycanbecalculatedfromthefollowingequation:

1

fSW=

RRC´CRC´0.105(1)Where:

fSW=Clockfrequency

RRC=Timingresistorvalue(in )CRC=Timingcapacitorvalue(inF)

RRCmustbekeptlargeenoughthatthecurrentthroughitdoesnotexceed750µAwhentheinternalswitch

(showninFigure23isdischargingthetimingcapacitor.Thisconditionmaybeexpressedby:VIN

£750mARRC(2)

SynchronizingtheOscillator

Figure23showsthefunctionaldiagramoftheTPS40200oscillator.WhensynchronizingtheoscillatortoanRCmustbepulledbelow150mVfor20nsormore.Theexternalclockfrequencymustbehigherthanthefree-runningfrequencyoftheconverteraswell.Whensynchronizingthecontroller,ifRCisheldlowforanexcessiveamountoftime,erraticoperationmayoccur.ThemaximumamountoftimethatRCshouldbeheldlowis50%ofanominaloutputpulse,or10%oftheperiodofthesynchronizationfrequency.

Undercircumstanceswheretheinputvoltageishighandthedutycycleislessthan50%,aSchottkydiodeconnectedfromRCtoanexternalclockmaybeusedtosynchronizetheoscillator.ThecathodeofthediodeisconnectedtoRC.Thetrippointoftheoscillatorissetbyaninternalvoltagedividertobe1/10oftheinputvoltage.Theclocksignalmusthaveanamplitudehigherthanthistrippoint.Whentheclockgoeslow,itallowstheresetcurrenttorestarttheRCramp,synchronizingtheoscillatortotheexternalclock.Thisprovidesasimple,single-componentmethodforclocksynchronization.

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