Application of the MC34063 switching regulator

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4SLVA252B–September2006–RevisedNovember2007SubmitDocumentationFeedbackwww.ti.com2FunctionalDescriptionFunctionalDescriptionTheoscillatoriscomposedofacurrentsourceandsink,whichchargeanddischargetheexternaltimingcapacitor(CT)betweenanupperandlowerpresetthreshold.Thetypicalchargeanddischargecurrentsare35mAand200mA,respectively,yieldingapproximatelya6:1ratio.Thus,theramp-upperiodissixtimeslongerthanthatoftheramp-downperiod(seeFigure3).Theupperthresholdisequaltointernalreferencevoltageof1.25V,andthelowerthresholdisapproximatelyequalto0.75V.TheoscillatorrunscontinuouslyataratecontrolledbythevalueofCT.Duringtheramp-upportionofthecycle,alogic1ispresentattheAinputoftheANDgate.Iftheoutputvoltageoftheswitchingregulatorisbelownominal,alogic1isalsopresentattheBinput.ThisconditionsetsthelatchandcausestheQoutputtobealogic1,enablingthedriverandoutputswitchtoconduct.Whentheoscillatorreachesitsupperthreshold,CTstartstodischarge,andalogic0ispresentattheAinputoftheANDgate.Thislogiclevelisalsoconnectedtoaninverterwhoseoutputpresentsalogic1totheresetinputofthelatch.ThisconditioncausesQtogolow,disablingthedriverandoutputswitch.AlogictruthtableofthesefunctionalblocksisshowninTable1.Table1.LogicTruthTableofFunctionalBlocksANDGateInputsLatchInputsActiveConditionofOutputCommentsTimingCapacitor,CTSwitchABSRRegulatoroutputisgreaterthanorBeginrampup000equaltonominal(B=0).Nochange,becauseBwas0beforeBeginrampdown000CTrampdown.Nochangeeventhoughregulatoroutputlessthannominal.OutputRampingdown0010switchcannotbeinitiatedduringRTrampdown.Nochange,becauseoutputswitchRampingdown0010conditionwasterminatedwhenA=0.RegulatoroutputbecamelessthannominalduringCTrampup(whenBRampingup10changedto1).Partialoncycleforoutputswitch.Regulatoroutputbecamegreaterthanorequaltonominal(Bchangedto0)Rampingup101duringrampupofCT.Nochange,becauseBcannotresetthelatch.Completeoncycle,becauseB=1Beginrampup1beforeCTrampupstarted.OutputswitchconductionisalwaysBeginrampdown1terminatedwhenCTisrampingdown.TheoutputofthecomparatorcansetthelatchonlyduringtherampupofCTandcaninitiateapartialorfulloncycleofoutputswitchconduction.Oncethecomparatorhassetthelatch,itcannotresetit .The latchremainssetuntilCTbeginsrampingdown.Thus,thecomparatorcaninitiateoutputswitchconductionbutcannotterminateit,andthelatchisalwaysresetwhenCTbeginsrampingdown .The comparator’soutputisatalogic0whentheoutputvoltageoftheswitchingregulatorisabovenominal.Undertheseconditions,thecomparator’soutputcaninhibitaportionoftheoutputswitchoncycle,acompletecycle,acompletecycleplusaportionofonecycle,multiplecycle,ormultiplecyclesplusaportionofonecycle.SLVA252B–September2006–RevisedNovember2007ApplicationoftheMC34063SwitchingRegulator5SubmitDocumentationFeedbackwww.ti.com3BuckRegulatorRLCoutD1Q1CinVinGNDVoutL++3.1BuckConverterOperation3.2Time-OnandTime-OffCalculation3.3SwitchPeakCurrentCalculationBuckRegulatorFigure6showsthebasicbuckswitchingregulator.Q1interruptstheinputvoltageandprovidesavariableduty-cyclesquarewavetoanLCfilter.ThefilteraveragesthesquarewaveandproducesadcoutputvoltagethatcanbesettoanylevellessthantheinputbycontrollingthepercentconductiontimeofQ1tothatofthetotalswitchingcycletime.Vout=Vin(%ton)orVout=Vin(ton/(ton+toff))Figure6.BuckRegulatorAsanexample,supposethatthetransistorQ1isoff,theinductorcurrent(IL)iszero,andtheoutputvoltageisatitsnominalvalue.TheoutputvoltageacrosscapacitorCoutwillultimatelydecaybelowthenominaloutputlevel,becauseitistheonlysourceofsupplycurrenttoloadRL.ThisvoltagedeficiencyissensedbytheswitchingcontrolcircuitandcausesQ1toturnon.TheinductorcurrentstartstoflowfromVinthroughQ1andCoutinparallelwithRL,anditrisesatarateofΔI/Δt=V/L.ThevoltageacrosstheinductorisequaltoVin–Vsat–Vout,andtheinductorpeakcurrentatanyinstantiscalculatedasshownhere:IL=((Vin–Vsat–Vout)/L)tAttheendoftheonperiod,Q1isturnedoff.Asthemagneticfieldintheinductorstartstocollapse,itgeneratesareversevoltagethatforwardbiasesD1,andthepeakcurrentdecaysatarateofΔI/Δt=V/LasenergyissuppliedtoCoutandRL.ThevoltageacrosstheinductorduringthisperiodisequaltoVout+VFofD1.Thecurrentasafunctionoftimeiscalculatedasshownhere:IL=IL(pk)–((Vout+VF)/L)tWhereVFistheforwardvoltageofD1.Asanexample,supposethatduringquiescentoperation,theaverageoutputvoltageisconstant,andthesystemisoperatinginthediscontinuousmode.ThenIL(pk)attainedduringtonmustdecaytozeroduringtoff,andaratiooftontotoffcanbedetermined.((Vin–Vsat–Vout)/L)ton=((Vout+VF)/L)toff∴ton/toff=(Vout+VF)/(Vin–Vsat–Vout)Thevolt-timeproductoftonmustbeequaltothatoftoff,andtheinductancevalueisnotafactorwhendeterminingtheirratio.Iftheoutputvoltageinsideaswitchingperiodistoremainconstant,theaveragecurrentintotheinductormustbeequaltotheoutputcurrentforacompletecycle.Thepeakinductorcurrentwithrespecttooutputcurrentis:(IL(pk)/2)ton+(IL(pk)/2)toff=Ioutton+Iouttoff∴IL(pk)=2IoutApplicationoftheMC34063SwitchingRegulator ... 46BuckRegulator 67BuckSwitchingRegulatorWaveforms 88BoostSwitchingRegulator 99BoostSwitchingRegulatorWaveforms 1110SwitchingInverterRegulator 1111InverterSwitchingRegulatorWaveforms 12ListofTables1LogicTruthTableofFunctionalBlocks ... SLLLinearABSTRACTThisapplicationreportprovidesthefeaturesthatarenecessarytoimplementdc-to-dcfixed-frequencyschemeswithaminimumnumberofexternalcomponentsusingthe MC34063. Thisdevicerepresentssignificantadvancementsineaseofusewithhighlyefficientand,yet,simpleswitchingregulators.Theuseofswitchingregulatorisbecomingmorepronouncedoverthatoflinearregulators,becauseofthesizeandpower-efficiencyrequirementofnewequipmentdesigns.Theuseofswitchingregulatorsincreasesapplicationflexibilityandreducesthecost.Contents1MC34063Description...

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