Giới thiệu về cấu trúc, cách hoạt động cùng một số ứng dụng cơ bản của IC nguồn xung MC34063
ApplicationReport SLVA252B–September2006–RevisedNovember2007 ApplicationoftheMC34063SwitchingRegulator ShafiSekanderandMahmoudHarmouch SLLLinear ABSTRACT Thisapplicationreportprovidesthefeaturesthatarenecessarytoimplementdc-to-dc fixed-frequencyschemeswithaminimumnumberofexternalcomponentsusingthe MC34063.Thisdevicerepresentssignificantadvancementsineaseofusewithhighly efficientand,yet,simpleswitchingregulators.Theuseofswitchingregulatoris becomingmorepronouncedoverthatoflinearregulators,becauseofthesizeand power-efficiencyrequirementofnewequipmentdesigns.Theuseofswitching regulatorsincreasesapplicationflexibilityandreducesthecost. Contents 1MC34063Description 2 2FunctionalDescription 5 3BuckRegulator 6 4BoostSwitchingRegulator 9 5InvertingSwitchingRegulator 11 6SelectingtheRightInductor 13 ListofFigures 1FunctionalBlockDiagram 2 2ReferenceVoltageCircuit 2 3OscillatorVoltageThresholds 3 4TimingCapacitorChargeCurrentvsCurrent-LimitSenseVoltage 3 5TypicalOperationWaveforms 4 6BuckRegulator 6 7BuckSwitchingRegulatorWaveforms 8 8BoostSwitchingRegulator 9 9BoostSwitchingRegulatorWaveforms 11 10SwitchingInverterRegulator 11 11InverterSwitchingRegulatorWaveforms 12 ListofTables 1LogicTruthTableofFunctionalBlocks 5 SLVA252B–September2006–RevisedNovember2007ApplicationoftheMC34063SwitchingRegulator1 SubmitDocumentationFeedback www.ti.com 1MC34063Description - + QS 1.25-V Reference Regulator R C T I pk Oscillator Q2 Q1 Switch Collector 4 Switch Emitter Timing Capacitor GND 3 2 18 7 6 5 Comparator Inverting Input V CC I pk Sense Drive Collector 100 W Comparator Latch 1.1ReferenceVoltage Comparator Inverting Input Output R2 R1 V = 1.25(R2/R1 + 1) out MC34063Description TheMC34063isamonolithiccontrolcircuitcontainingalltheactivefunctionsrequiredforswitching dc-to-dcconverters(seeFigure1).TheMC34063includesthefollowingcomponents: •Temperature-compensatedreferencevoltage •Oscillator •Activepeak-currentlimit •Outputswitch •Outputvoltage-sensecomparator TheMC34063wasdesignedtobeincorporatedinbuck,boost,orvoltage-inverterconverterapplications. Allthesefunctionsarecontainedinan8-pinDIPorSOICpackage. Figure1.FunctionalBlockDiagram Thereferencevoltageissetat1.25Vandisusedtosettheoutputvoltageoftheconverter. Figure2.ReferenceVoltageCircuit ApplicationoftheMC34063SwitchingRegulator 2SLVA252B–September2006–RevisedNovember2007 SubmitDocumentationFeedback www.ti.com 1.2Oscillator Upper Threshold (1.25 V Typical) Lower Threshold (0.75 V Typical) Discharge Time V Charge 6t t 1.3CurrentLimit V – Current-Limit Sense Voltage – V CLS I – Charging Current – mA chg 0 0.2 0.4 0.6 0.8 1 0.03 0.1 0.3 1 3 10 30 V = 5V CC T = 25°C A V = 40V CC I = I chg dischg MC34063Description Theoscillatoriscomposedofacurrentsourceandacurrentsinkthatchargeanddischargetheexternal timingcapacitor(C T )betweenanupperandlowerpresetthreshold.Thetypicalchargecurrentis35µA, andthetypicaldischargecurrentis200µA,yieldingapproximatelya6:1ratio.Thus,theramp-upperiodis sixtimeslongerthanthatoftheramp-downperiod(seeFigure3). Theupperthresholdis1.25V,whichissameastheinternalreferencevoltage,andthelowerthresholdis 0.75V.Theoscillatorrunsconstantly,atapacecontrolledbythevalueofC T . Figure3.OscillatorVoltageThresholds Currentlimitisaccomplishedbymonitoringthevoltagedropacrossanexternalsenseresistorlocatedin serieswithV CC andtheoutputswitch.Thevoltagedropdevelopedacrossthesenseresistorismonitored bythecurrent-sensepin,I pk .Whenthevoltagedropacrossthesenseresistorbecomesgreaterthanthe presetvalueof330mV,thecurrent-limitcircuitryprovidesanadditionalcurrentpathtochargethetiming capacitor(C T )rapidly,toreachtheupperoscillatorthresholdand,thus,limitingtheamountofenergy storedintheinductor.Theminimumsenseresistoris0.2Ω.Figure4showsthetimingcapacitorcharge currentversuscurrent-limitsensevoltage.Tosetthepeakcurrent,I pk =330mV/R sense . Figure4.TimingCapacitorChargeCurrentvsCurrent-LimitSenseVoltage SLVA252B–September2006–RevisedNovember2007ApplicationoftheMC34063SwitchingRegulator3 SubmitDocumentationFeedback www.ti.com 1.4OutputSwitch Comparator Output Timing Capacitor, C T Output Switch Nominal Output Voltage Output Voltage 1 0 On Off Startup Quiescent Operation MC34063Description TheoutputswitchisanNPNDarlingtontransistor.Thecollectoroftheoutputtransistoristiedtopin1, andtheemitteristiedtopin2.ThisallowsthedesignertousetheMC34063inbuck,boost,orinverter configurations.Themaximumcollector-emittersaturationvoltageat1.5A(peak)is1.3V,andthe maximumpeakcurrentoftheoutputswitchis1.5A.Forhigherpeakoutputcurrent,anexternaltransistor canbeused.Figure5showsthetypicaloperationwaveforms. Figure5.TypicalOperationWaveforms ApplicationoftheMC34063SwitchingRegulator 4SLVA252B–September2006–RevisedNovember2007 SubmitDocumentationFeedback www.ti.com 2FunctionalDescription FunctionalDescription Theoscillatoriscomposedofacurrentsourceandsink,whichchargeanddischargetheexternaltiming capacitor(C T )betweenanupperandlowerpresetthreshold.Thetypicalchargeanddischargecurrents are35mAand200mA,respectively,yieldingapproximatelya6:1ratio.Thus,theramp-upperiodissix timeslongerthanthatoftheramp-downperiod(seeFigure3).Theupperthresholdisequaltointernal referencevoltageof1.25V,andthelowerthresholdisapproximatelyequalto0.75V.Theoscillatorruns continuouslyataratecontrolledbythevalueofC T . Duringtheramp-upportionofthecycle,alogic1ispresentattheAinputoftheANDgate.Iftheoutput voltageoftheswitchingregulatorisbelownominal,alogic1isalsopresentattheBinput.Thiscondition setsthelatchandcausestheQoutputtobealogic1,enablingthedriverandoutputswitchtoconduct. Whentheoscillatorreachesitsupperthreshold,C T startstodischarge,andalogic0ispresentattheA inputoftheANDgate.Thislogiclevelisalsoconnectedtoaninverterwhoseoutputpresentsalogic1to theresetinputofthelatch.ThisconditioncausesQtogolow,disablingthedriverandoutputswitch.A logictruthtableofthesefunctionalblocksisshowninTable1. Table1.LogicTruthTableofFunctionalBlocks ANDGateInputsLatchInputs ActiveConditionofOutput Comments TimingCapacitor,C T Switch ABSR Regulatoroutputisgreaterthanor Beginrampup000 equaltonominal(B=0). Nochange,becauseBwas0before Beginrampdown000 C T rampdown. Nochangeeventhoughregulator outputlessthannominal.Output Rampingdown0010 switchcannotbeinitiatedduringR T rampdown. Nochange,becauseoutputswitch Rampingdown0010 conditionwasterminatedwhenA=0. Regulatoroutputbecamelessthan nominalduringC T rampup(whenB Rampingup10 changedto1).Partialoncyclefor outputswitch. Regulatoroutputbecamegreaterthan orequaltonominal(Bchangedto0) Rampingup101 duringrampupofC T .Nochange, becauseBcannotresetthelatch. Completeoncycle,becauseB=1 Beginrampup1 beforeC T rampupstarted. Outputswitchconductionisalways Beginrampdown1 terminatedwhenC T isrampingdown. TheoutputofthecomparatorcansetthelatchonlyduringtherampupofC T andcaninitiateapartialor fulloncycleofoutputswitchconduction.Oncethecomparatorhassetthelatch,itcannotresetit.The latchremainssetuntilC T beginsrampingdown.Thus,thecomparatorcaninitiateoutputswitch conductionbutcannotterminateit,andthelatchisalwaysresetwhenC T beginsrampingdown.The comparator’soutputisatalogic0whentheoutputvoltageoftheswitchingregulatorisabovenominal. Undertheseconditions,thecomparator’soutputcaninhibitaportionoftheoutputswitchoncycle,a completecycle,acompletecycleplusaportionofonecycle,multiplecycle,ormultiplecyclesplusa portionofonecycle. SLVA252B–September2006–RevisedNovember2007ApplicationoftheMC34063SwitchingRegulator5 SubmitDocumentationFeedback www.ti.com 3BuckRegulator R L C out D1 Q1 C in V in GND V out L + + 3.1BuckConverterOperation 3.2Time-OnandTime-OffCalculation 3.3SwitchPeakCurrentCalculation BuckRegulator Figure6showsthebasicbuckswitchingregulator.Q1interruptstheinputvoltageandprovidesavariable duty-cyclesquarewavetoanLCfilter.Thefilteraveragesthesquarewaveandproducesadcoutput voltagethatcanbesettoanylevellessthantheinputbycontrollingthepercentconductiontimeofQ1to thatofthetotalswitchingcycletime. V out =V in (%t on ) or V out =V in (t on /(t on +t off )) Figure6.BuckRegulator Asanexample,supposethatthetransistorQ1isoff,theinductorcurrent(I L )iszero,andtheoutput voltageisatitsnominalvalue.TheoutputvoltageacrosscapacitorC out willultimatelydecaybelowthe nominaloutputlevel,becauseitistheonlysourceofsupplycurrenttoloadR L .Thisvoltagedeficiencyis sensedbytheswitchingcontrolcircuitandcausesQ1toturnon.Theinductorcurrentstartstoflowfrom V in throughQ1andC out inparallelwithR L ,anditrisesatarateofΔI/Δt=V/L.Thevoltageacrossthe inductorisequaltoV in –V sat –V out ,andtheinductorpeakcurrentatanyinstantiscalculatedasshown here: I L =((V in –V sat –V out )/L)t Attheendoftheonperiod,Q1isturnedoff.Asthemagneticfieldintheinductorstartstocollapse,it generatesareversevoltagethatforwardbiasesD1,andthepeakcurrentdecaysatarateofΔI/Δt=V/L asenergyissuppliedtoC out andR L .Thevoltageacrosstheinductorduringthisperiodisequalto V out +V F ofD1.Thecurrentasafunctionoftimeiscalculatedasshownhere: I L =I L(pk) –((V out +V F )/L)t WhereV F istheforwardvoltageofD1. Asanexample,supposethatduringquiescentoperation,theaverageoutputvoltageisconstant,andthe systemisoperatinginthediscontinuousmode.ThenI L(pk) attainedduringt on mustdecaytozeroduring t off ,andaratiooft on tot off canbedetermined. ((V in –V sat –V out )/L)t on =((V out +V F )/L)t off ∴t on /t off =(V out +V F )/(V in –V sat –V out ) Thevolt-timeproductoft on mustbeequaltothatoft off ,andtheinductancevalueisnotafactorwhen determiningtheirratio.Iftheoutputvoltageinsideaswitchingperiodistoremainconstant,theaverage currentintotheinductormustbeequaltotheoutputcurrentforacompletecycle.Thepeakinductor currentwithrespecttooutputcurrentis: (I L(pk) /2)t on +(I L(pk) /2)t off =I out t on +I out t off ∴I L(pk) =2I out ApplicationoftheMC34063SwitchingRegulator 6SLVA252B–September2006–RevisedNovember2007 SubmitDocumentationFeedback www.ti.com 3.4TimingCapacitorCalculation 3.5InductanceCalculation 3.6OutputVoltageRipple BuckRegulator Thepeakinductorcurrentisalsoequaltothepeakswitchcurrent,sincethetwoareinseries.Theontime (t on )isthemaximumpossibleswitchconductiontime.ItisequaltothetimerequiredforC T torampup fromitslowertoupperthreshold.TherequiredvalueforC T canbedeterminedbyusingtheminimum oscillatorchargingcurrentandthetypicalvalueforthepeak-to-peakoscillatorvoltageswing,bothtaken fromthedatasheet. C T =I chg(min) (Δt/ΔV) C T =20×10 -6 (t on /0.5) C T =4.0×10 -5 (t on ) TheofftimeisthetimethatdiodeD1isinconductionanditisdeterminedbythetimerequiredforthe inductorcurrenttoreturntozero.Theofftimeisnotrelatedtotheramp-downtimeofCT.Thecycletime oftheLCnetworkisequaltot on(max) +t off ,andtheminimumoperationfrequencyiscalculatedasshown here: f min =1/(t on(max) +t off ) Theminimumvalueofinductance(L)cannowbecalculated.TheV-knownquantitiesarethevoltage acrosstheinductorandtherequiredpeakcurrentfortheselectedswitchconductiontime: L min =((V in –V sat –V out )/I pk(switch) )t on Theminimumvalueofinductanceiscalculatedassumingtheonsetofcontinuousconductionoperation withafixedinputvoltage,maximumoutputcurrent,andaminimumcharge-currentoscillator. Thenetchargepercycledeliveredtooutputfiltercapacitor(C out )mustbezero(Q+=Q–)iftheoutput voltageistoremainconstant. Theripplevoltagecanbecalculatedfromtheknownvaluesofontime,offtime,peakinductorcurrent,and outputcapacitorvalue: Duringt on ic(t)=I pk /t on ×t,positiveslope V(t)=1/C out ∫I pk /t on ×tdt =I pk /(C out ×t on )×t 2 /2+constant Theaxisoftheparabolapasswaschosenbyitsminimum,soconstant=0. =I pk /(C out ×t on )×t 2 /2 V(t on /2)=I pk /(C out ×t on )×(t on /2) 2 /2 =I pk /C out ×t on /8 Duringt off ic(t)=–I pk /t off ×t,negativeslope V(t)=–1/C out ∫I pk /t off ×tdt =–I pk /(C out ×t off )×t 2 /2+constant Theaxisoftheparabolapasswaschosenbyitsminimum,soconstant=0. =–I pk /(C out ×t off )×t 2 /2 V(t off /2)=–I pk /(C out ×t off )×(t off /2) 2 /2 =–I pk /C out ×t off /8 V ripple(C) =|V(t on /2)|+|V(t off /2)| =(I pk /C out )×(t on /8)+(I pk /C out )×(t off /8) SLVA252B–September2006–RevisedNovember2007ApplicationoftheMC34063SwitchingRegulator7 SubmitDocumentationFeedback www.ti.com Voltage Across Switch Q1 V CE Diode D1 Voltage V KA Switch Q1 Current Diode D1 Current Inductor Current Capacitor C Current out Capacitor C Ripple Voltage out V + V Fin V in V sat 0 V – V satin 0 V F I pk 0 0 0 0 I pk I D(AVG) I pk V + V pkout V out I out pk C(AVG) D(AVG) = I /2 = I + I +I /2 pk I = I in C(AVG) V in –I /2 pk V – V pkout t off/2 t on/2 V ripple(p-p) ½I p/2 Q+ Q– t 0 t 1 t 2 BuckRegulator V ripple(C) =(I pk /C out )×(t on +t off )/8 V ripple(ESR) =I pk ×ESR V ripple(p-p) =I pk /C out ×(t on +t off )+I pk ×ESR V ripple(p-p) =I pk ×[(1/8C)×(t on +t off )+ESR] Figure7showsagraphicalderivationofthepeak-to-peakripplevoltagethatwasobtainedfromthe capacitorcurrentandvoltagewaveforms. Thecalculationsshownaboveaccountfortheripplevoltagecontributedbytheripplecurrentintoanideal capacitor. Inpractice,thecalculatedvalueshouldbeincreasedduetotheinternalequivalentseriesresistance (ESR)ofthecapacitor.TheadditionalripplevoltageisequaltoI pk(ESR) .Increasingthevalueofthefilter capacitorreducestheoutputripplevoltage.However,apointofdiminishingreturnisreached,because thecomparatorrequiresafinitevoltagedifferenceacrossitsinputstocontrolthelatch.Thevoltage differencerequiredtocompletelychangethelatchstatesisabout1.5mV,andtheminimumachievable rippleattheoutputisthefeedbackdividerratiomultipliedby1.5mV: V ripple(p-p) (min)=(V out /V ref )(1.5×10 -3 ) Figure7.BuckSwitchingRegulatorWaveforms ApplicationoftheMC34063SwitchingRegulator 8SLVA252B–September2006–RevisedNovember2007 SubmitDocumentationFeedback www.ti.com 4BoostSwitchingRegulator R L C out D1 Q1 C in V in GND V out L + + 4.1OperationofMC34063asBoostConverter 4.2Time-OnandTime-OffCalculation 4.3PeakCurrentCalculation BoostSwitchingRegulator Figure8showsabasicswitchingregulator.Energyisstoredintheinductorduringthetimethattransistor Q1isintheONstate.WhentransistorQ1isturnedoff,theenergyistransferredinserieswithV in tothe outputfiltercapacitor(C out )andload(R L ).Thisconfigurationallowstheoutputvoltagetobesettoany valuegreaterthanthatofinput.Thefollowingequationscanbeusedtocalculatetheoutputvoltage: V out =V in (t on /t off )+V in or V out =V in ((t on /t off )+1) Figure8.BoostSwitchingRegulator Asanexample,supposethattransistorQ1isoff,theinductorcurrentiszero,andoutputvoltageisatits nominalvalue.Atthistime,loadcurrentisbeingsuppliedonlybyC out ,anditwilleventuallyfallbelow nominalvalue.Whentheoutputvoltagefallsbelowthenominalvalue,itissensedbythecontrolcircuit, whichinitiatesanoncycle,drivingtransistorQ1intosaturation.Currentstartstoflowfrominputthrough theinductorandQ1,anditrisesatarateofΔI/Δt=V/L.Thevoltageacrosstheinductorisequalto V in –V sat ,andthepeakcurrentisroughlyalinearfunctionoft,asshownhere: I L =((V in –V sat )/L)t Whentheon-timeiscompleted,Q1turnsoff,andthemagneticfieldintheinductorstartstocollapse, generatingareversevoltagethatforwardbiasesD1,supplyingenergytoC out andR L .Theinductorcurrent decaysatrateofΔI/Δt=V/L,andthevoltageacrossitisequaltoV out +V F –V in .Thecurrentatany instantiscalculatedasshownhere: I L =I L(pk) –((V out +V F –V in )/L)t Assumingthatthesystemisoperatinginthediscontinuousmode,thecurrentthroughtheinductor reacheszeroafterthet off periodiscompleted.ThentheI L(pk) attainedduringt on mustdecaytozeroduring t off ,andaratiooft on tot off canbewrittenasshownhere: ((V in –V sat )/L)t on =((V out +V F –V in )/L)t off ∴t on /t off =(V out +V F –V in )/(V in –V sat ) Thevolt-timeproductoft on mustbeequaltothatoft off ,andtheinductancevaluedoesnotaffectthis relationship. TheinductorcurrentchargestheoutputfiltercapacitorthroughD1duringt off .Iftheoutputvoltageisto remainconstant,thenetchargepercycledeliveredtooutputfiltercapacitormustbezero(Q+=Q–). I chg t off =I dischg t on Figure9showstheboostswitchingregulatorwaveforms.Byobservingthecapacitorcurrentandmaking somesubstitutioninthepreviousequation,aformulaforpeakinductorcurrentcanbeobtained. (I L(pk) /2)t off =I out (t on +t off ) ∴I L(pk) =2I out (t on /t off +1) SLVA252B–September2006–RevisedNovember2007ApplicationoftheMC34063SwitchingRegulator9 SubmitDocumentationFeedback www.ti.com 4.4InductanceCalculation 4.5OutputVoltageRipple BoostSwitchingRegulator Thepeakinductorcurrentisalsoequaltothepeakswitchcurrent,sincethetwoareinseries.Byknowing thevoltageacrosstheinductorduringt on andtherequiredpeakcurrentfortheselectedswitchconduction time,aminimuminductancevaluecanbedetermined: L min =((V in –V sat )/I pk(switch) )t on(max) Calculatetheoutputripplevoltagefromtheknownvaluesoft on ,t off ,peakinductorcurrent,outputcurrent, andoutputcapacitorvalue.ThecapacitorcurrentwaveformsisdepictedinFigure9,t1beingthe discharginginterval.Solvingfort1inknowntermsyields: Duringt off ,thecurrentislinearwithnegativeslope,–ΔI L /t off ic(t)=–(I pk /t off )×t V(t)=–1/C out ∫(I pk /t off )×tdt =–I pk /(C out ×t off )xt 2 /2+constant Theaxisoftheparabolpasswaschosenbythemaximumsoconstant=0. =–I pk /(C out ×t off )×t 2 /2 V(-τ)=–I pk /(C out ×t off )×τ 2 /2,τistimefromic(t)=maxtoic(t)=0 (t off –τ)/ off =I out /I pk ,trianglegeometry τ=t off ×(I pk –I0)/I pk (1) V(-τ)=–I pk /2(C out ×t off )×(t off ) 2 ×(I pk –I0) 2 /ΔI L 2 V(-τ)=–t off ×(I pk –I0) 2 /(2C out ×I pk )(2) Energyconservationintheoutputcapacitor:Q+=Q– (I pk –I0)×τ/2=(t off –τ)×I0/2+I0×t on (3) Equation1andEquation2give: t off ×(I pk –I0) 2 /2∆IL=I0/2×t off ×(1–(ΔI L –I0)/ΔI L )+I0×t on =t off ×I0 2 /2ΔI L +I0xt on t off ×((I pk –I0) 2 –I0 2 )/2I pk =I0×t on (I pk –2I0)×t off /2=I0×t on Theinductorripplecurrent: I pk =2I out ×(1+t on /t off )(4) Fromoutputcapacitorrippleperiodicityandcontinuity: V(–τ)=V ripple(pp) BysubstitutingEquation4inEquation3: V ripple (C out )=I out (t off +2t on ) 2 /2C(t off +t on ) Ift on =6.5t off ,then: V ripple (ESR)=2I out ×(1+t on /t off )×ESR 10ApplicationoftheMC34063SwitchingRegulatorSLVA252B–September2006–RevisedNovember2007 SubmitDocumentationFeedback [...]... the following: Vout = Vin(ton/toff) Q1 Vin Vout D1 + Cin L + Cout RL GND Figure 10 Switching Inverter Regulator The inverter converter operates identically to that of the boost converter The voltage across the inductor during ton is Vin – Vsat but, during toff, the voltage is equal to the negative magnitude of Vout + VF The VLT time-product of ton must be equal to that of toff, a ratio of ton to toff... toff t1 Q– ton Vripple(p-p) Vout –Vout – Vpk Figure 11 Inverter Switching Regulator Waveforms 12 Application of the MC34063 Switching Regulator SLVA252B – September 2006 – Revised November 2007 Submit Documentation Feedback www.ti.com Selecting the Right Inductor 6 Selecting the Right Inductor Proper inductor selection is crucial to the performance of the switching regulator' s design The switching regulator. .. (|Vout| + VF)toff ∴ ton/toff = (|Vout| + VF)/(Vin – Vsat) The derivations and the formulas for Ipk(switch), L(min), and Cout are the same as that of the boost converter Figure 11 shows the voltage-inverter switching regulator waveforms SLVA252B – September 2006 – Revised November 2007 Submit Documentation Feedback Application of the MC34063 Switching Regulator 11 www.ti.com Inverting Switching Regulator. .. voltage-inverting switching regulator is shown in Figure 10 The energy is stored in the inductor during the conduction time of Q1 Upon the Q1 turn off, the energy is transferred to the output filter capacitor and load In this configuration, the output voltage is derived only from the inductor This allows the magnitude of the output to be set to any value It may be less than, equal to, or greater than that of the. .. safety-critical applications (such as life support) where a failure of the TI product would reasonably be expected to cause severe personal injury or death, unless officers of the parties have executed an agreement specifically governing such use Buyers represent that they have all necessary expertise in the safety and regulatory ramifications of their applications, and acknowledge and agree that they are... continuous-mode operation come at the expense of a larger inductor Once the minimum inductor and peak current value are determined, the inductor can be selected Most manufacturers provide the following data in their data book: • Inductance value • DCR (dc resistance) of the winding • DC saturation current • RMS current • Package type, size, and pattern The geometry and the shape of the inductor chosen can have... can be used However, the bobbin core inductors can generate more EMI, as the open core does not confine the flux within the core and can affect nearby sensitive circuits SLVA252B – September 2006 – Revised November 2007 Submit Documentation Feedback Application of the MC34063 Switching Regulator 13 IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections,... constitute a license from TI to use such products or services or a warranty or endorsement thereof Use of such information may require a license from a third party under the patents or other intellectual property of the third party, or a license from TI under the patents or other intellectual property of TI Reproduction of TI information in TI data books or data sheets is permissible only if reproduction... two mode of operation: • Continuous mode • Discontinuous mode Each mode has characteristically different operating characters and, therefore, can affect the regulator performance and requirements In many applications, the continuous mode is the preferred mode of operation, since it offers greater output power with lower peak currents, wider input range, and lower output ripple These advantages of continuous-mode... for all legal, regulatory and safety-related requirements concerning their products and any use of TI products in such safety-critical applications, notwithstanding any applications-related information or support that may be provided by TI Further, Buyers must fully indemnify TI and its representatives against any damages arising out of the use of TI products in such safety-critical applications TI . 4SLVA252B–September2006–RevisedNovember2007 SubmitDocumentationFeedback www.ti.com 2FunctionalDescription FunctionalDescription Theoscillatoriscomposedofacurrentsourceandsink,whichchargeanddischargetheexternaltiming capacitor(C T )betweenanupperandlowerpresetthreshold.Thetypicalchargeanddischargecurrents are35mAand200mA,respectively,yieldingapproximatelya6:1ratio.Thus,theramp-upperiodissix timeslongerthanthatoftheramp-downperiod(seeFigure3).Theupperthresholdisequaltointernal referencevoltageof1.25V,andthelowerthresholdisapproximatelyequalto0.75V.Theoscillatorruns continuouslyataratecontrolledbythevalueofC T . Duringtheramp-upportionofthecycle,alogic1ispresentattheAinputoftheANDgate.Iftheoutput voltageoftheswitchingregulatorisbelownominal,alogic1isalsopresentattheBinput.Thiscondition setsthelatchandcausestheQoutputtobealogic1,enablingthedriverandoutputswitchtoconduct. Whentheoscillatorreachesitsupperthreshold,C T startstodischarge,andalogic0ispresentattheA inputoftheANDgate.Thislogiclevelisalsoconnectedtoaninverterwhoseoutputpresentsalogic1to theresetinputofthelatch.ThisconditioncausesQtogolow,disablingthedriverandoutputswitch.A logictruthtableofthesefunctionalblocksisshowninTable1. Table1.LogicTruthTableofFunctionalBlocks ANDGateInputsLatchInputs ActiveConditionofOutput Comments TimingCapacitor,C T Switch ABSR Regulatoroutputisgreaterthanor Beginrampup000 equaltonominal(B=0). Nochange,becauseBwas0before Beginrampdown000 C T rampdown. Nochangeeventhoughregulator outputlessthannominal.Output Rampingdown0010 switchcannotbeinitiatedduringR T rampdown. Nochange,becauseoutputswitch Rampingdown0010 conditionwasterminatedwhenA=0. Regulatoroutputbecamelessthan nominalduringC T rampup(whenB Rampingup10 changedto1).Partialoncyclefor outputswitch. Regulatoroutputbecamegreaterthan orequaltonominal(Bchangedto0) Rampingup101 duringrampupofC T .Nochange, becauseBcannotresetthelatch. Completeoncycle,becauseB=1 Beginrampup1 beforeC T rampupstarted. Outputswitchconductionisalways Beginrampdown1 terminatedwhenC T isrampingdown. TheoutputofthecomparatorcansetthelatchonlyduringtherampupofC T andcaninitiateapartialor fulloncycleofoutputswitchconduction.Oncethecomparatorhassetthelatch,itcannotresetit .The latchremainssetuntilC T beginsrampingdown.Thus,thecomparatorcaninitiateoutputswitch conductionbutcannotterminateit,andthelatchisalwaysresetwhenC T beginsrampingdown .The comparator’soutputisatalogic0whentheoutputvoltageoftheswitchingregulatorisabovenominal. Undertheseconditions,thecomparator’soutputcaninhibitaportionoftheoutputswitchoncycle,a completecycle,acompletecycleplusaportionofonecycle,multiplecycle,ormultiplecyclesplusa portionofonecycle. SLVA252B–September2006–RevisedNovember2007ApplicationoftheMC34063SwitchingRegulator5 SubmitDocumentationFeedback www.ti.com 3BuckRegulator R L C out D1 Q1 C in V in GND V out L + + 3.1BuckConverterOperation 3.2Time-OnandTime-OffCalculation 3.3SwitchPeakCurrentCalculation BuckRegulator Figure6showsthebasicbuckswitchingregulator.Q1interruptstheinputvoltageandprovidesavariable duty-cyclesquarewavetoanLCfilter.Thefilteraveragesthesquarewaveandproducesadcoutput voltagethatcanbesettoanylevellessthantheinputbycontrollingthepercentconductiontimeofQ1to thatofthetotalswitchingcycletime. V out =V in (%t on ) or V out =V in (t on /(t on +t off )) Figure6.BuckRegulator Asanexample,supposethatthetransistorQ1isoff,theinductorcurrent(I L )iszero,andtheoutput voltageisatitsnominalvalue.TheoutputvoltageacrosscapacitorC out willultimatelydecaybelowthe nominaloutputlevel,becauseitistheonlysourceofsupplycurrenttoloadR L .Thisvoltagedeficiencyis sensedbytheswitchingcontrolcircuitandcausesQ1toturnon.Theinductorcurrentstartstoflowfrom V in throughQ1andC out inparallelwithR L ,anditrisesatarateofΔI/Δt=V/L.Thevoltageacrossthe inductorisequaltoV in –V sat –V out ,andtheinductorpeakcurrentatanyinstantiscalculatedasshown here: I L =((V in –V sat –V out )/L)t Attheendoftheonperiod,Q1isturnedoff.Asthemagneticfieldintheinductorstartstocollapse,it generatesareversevoltagethatforwardbiasesD1,andthepeakcurrentdecaysatarateofΔI/Δt=V/L asenergyissuppliedtoC out andR L .Thevoltageacrosstheinductorduringthisperiodisequalto V out +V F ofD1.Thecurrentasafunctionoftimeiscalculatedasshownhere: I L =I L(pk) –((V out +V F )/L)t WhereV F istheforwardvoltageofD1. Asanexample,supposethatduringquiescentoperation,theaverageoutputvoltageisconstant,andthe systemisoperatinginthediscontinuousmode.ThenI L(pk) attainedduringt on mustdecaytozeroduring t off ,andaratiooft on tot off canbedetermined. ((V in –V sat –V out )/L)t on =((V out +V F )/L)t off ∴t on /t off =(V out +V F )/(V in –V sat –V out ) Thevolt-timeproductoft on mustbeequaltothatoft off ,andtheinductancevalueisnotafactorwhen determiningtheirratio.Iftheoutputvoltageinsideaswitchingperiodistoremainconstant,theaverage currentintotheinductormustbeequaltotheoutputcurrentforacompletecycle.Thepeakinductor currentwithrespecttooutputcurrentis: (I L(pk) /2)t on +(I L(pk) /2)t off =I out t on +I out t off ∴I L(pk) =2I out ApplicationoftheMC34063SwitchingRegulator. 4 6BuckRegulator 6 7BuckSwitchingRegulatorWaveforms 8 8BoostSwitchingRegulator 9 9BoostSwitchingRegulatorWaveforms 11 10SwitchingInverterRegulator 11 11InverterSwitchingRegulatorWaveforms 12 ListofTables 1LogicTruthTableofFunctionalBlocks. SLLLinear ABSTRACT Thisapplicationreportprovidesthefeaturesthatarenecessarytoimplementdc-to-dc fixed-frequencyschemeswithaminimumnumberofexternalcomponentsusingthe MC34063. Thisdevicerepresentssignificantadvancementsineaseofusewithhighly efficientand,yet,simpleswitchingregulators.Theuseofswitchingregulatoris becomingmorepronouncedoverthatoflinearregulators,becauseofthesizeand power-efficiencyrequirementofnewequipmentdesigns.Theuseofswitching regulatorsincreasesapplicationflexibilityandreducesthecost. Contents 1MC34063Description