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LTC3868EUHPBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC3868EUHPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 38 page  LTC3868 3868fb operaTion (Refer to the Functional Diagram) TheLTC3868usesaconstantfrequency,currentmode step-downarchitecturewiththetwocontrollerchannels operating180degreesoutofphase.Duringnormalop- eration,eachexternaltopMOSFETisturnedonwhenthe clockforthatchannelsetstheRSlatch,andisturnedoff whenthemaincurrentcomparator,ICMP,resetstheRS latch.ThepeakinductorcurrentatwhichICMPtripsand resetsthelatchiscontrolledbythevoltageontheITHpin, whichistheoutputoftheerroramplifier,EA.Theerror amplifiercomparestheoutputvoltagefeedbacksignalat theVFBpin(whichisgeneratedwithanexternalresistor divider connected across the output voltage, VOUT, to ground)totheinternal0.800Vreferencevoltage.Whenthe loadcurrentincreases,itcausesaslightdecreaseinVFB relativetothereference,whichcausestheEAtoincrease theITHvoltageuntiltheaverageinductorcurrentmatches thenewloadcurrent. AfterthetopMOSFETisturnedoffeachcycle,thebottom MOSFETisturnedonuntileithertheinductorcurrentstarts toreverse,asindicatedbythecurrentcomparatorIR,or thebeginningofthenextclockcycle. INTVCC/EXTVCC Power PowerforthetopandbottomMOSFETdriversandmost otherinternalcircuitryisderivedfromtheINTVCCpin.When theEXTVCCpinisleftopenortiedtoavoltagelessthan 4.7V,theVINLDO(lowdropoutlinearregulator)supplies 5.1VfromVINtoINTVCC.IfEXTVCCistakenabove4.7V, theVINLDOisturnedoffandanEXTVCCLDOisturnedon. Onceenabled,theEXTVCCLDOsupplies5.1VfromEXTVCC toINTVCC.UsingtheEXTVCCpinallowstheINTVCCpower tobederivedfromahighefficiencyexternalsourcesuch asoneoftheLTC3868switchingregulatoroutputs. EachtopMOSFETdriverisbiasedfromthefloatingboot- strapcapacitor,CB,whichnormallyrechargesduringeach cyclethroughanexternaldiodewhenthetopMOSFET turnsoff.Iftheinputvoltage,VIN,decreasestoavoltage closetoVOUT,theloopmayenterdropoutandattempt toturnonthetopMOSFETcontinuously.Thedropout detectordetectsthisandforcesthetopMOSFETofffor aboutone-twelfthoftheclockperiodeverytenthcycleto allowCBtorecharge. Shutdown and Start-Up (RUN1, RUN2 and SS1, SS2 Pins) ThetwochannelsoftheLTC3868canbeindependently shutdownusingtheRUN1andRUN2pins.Pullingeitherof thesepinsbelow1.26Vshutsdownthemaincontrolloop forthatcontroller.Pullingbothpinsbelow0.7Vdisables bothcontrollersandmostinternalcircuits,includingthe INTVCCLDOs.Inthisstate,theLTC3868drawsonly8µA ofquiescentcurrent. TheRUNpinmaybeexternallypulledupordrivendirectly bylogic.WhendrivingtheRUNpinwithalowimpedance source,donotexceedtheabsolutemaximumratingof 8V.TheRUNpinhasaninternal11Vvoltageclampthat allowstheRUNpintobeconnectedthrougharesistortoa highervoltage(forexample,VIN),solongasthemaximum currentintotheRUNpindoesnotexceed100µA. Thestart-upofeachcontroller’soutputvoltage,VOUT,is controlledbythevoltageontheSSpinforthatchannel. WhenthevoltageontheSSpinislessthanthe0.8Vinternal reference,theLTC3868regulatestheVFBvoltagetotheSS pinvoltageinsteadofthe0.8Vreference.Thisallowsthe SSpintobeusedtoprogramasoft-startbyconnecting anexternalcapacitorfromtheSSpintoSGND.Aninternal 1µApull-upcurrentchargesthiscapacitorcreatingavolt- agerampontheSSpin.AstheSSvoltageriseslinearly from0Vto0.8V(andbeyonduptotheabsolutemaximum ratingof6V),theoutputvoltageVOUTrisessmoothlyfrom zerotoitsfinalvalue. Short-Circuit Latchoff Afterthecontrollerhasbeenstartedandbeengivenad- equatetimetorampuptheoutputvoltage,theSScapaci- torisusedinashort-circuittimeoutcircuit.Specifically, oncethevoltageontheSSpinrisesabove2V(thearming threshold),theshort-circuittimeoutcircuitisenabled(see Figure1).Iftheoutputvoltagefallsbelow70%ofitsnomi- nalregulatedvoltage,theSScapacitorbeginsdischarg- ingwithanet9µApulldowncurrentontheassumption thattheoutputisinanovercurrentand/orshort-circuit condition.Iftheconditionlastslongenoughtoallowthe SSpinvoltagetofallbelow1.5V(thelatchoffthreshold), thecontrollerwillshutdown(latchoff)untiltheRUNpin voltageortheVINvoltageisrecycled. |
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