How to Design a Simple Constant Current/Constant Voltage ...

1 1 SNVA829 June2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedHowto Designa SimpleConstantCurrent/ConstantVoltageBuc kConverterApplicationReportSNVA829 June2018 HowtoDesigna SimpleConstantCurrent/ConstantVoltageBuc kConverterABSTRACTT echnicalInformationaboutdesigninga constantcurrent,constantvoltage(CC/CV) designimplementationcan be challengingfroma complexity,efficiency,and cost LM5117devicewith its currentmonitor(CM)pin greatlysimplifiesthe designprocess,allowscurrentregulationwit houtusinga lossycurrentsenseresistor,and savescostin the designapproachusingthe LM5117CM pin and the LMV431for currentregulationandvoltageregulation, the SimpleCC/CVMethodUsingthe the of Figures1CC/CVImplementationWitha 30 V-to-54V Input,27 V-at-6A OutputCC/CVConverterUsingthe 30 30 VINWithIncreasingRLOAD(VOUT/ IOUT).. trademarksare the propertyof +To FBRSENSEVZRFBBuck Power Stage+ CC Mode = IOUT =(RINuVFB)/(RFBuRSENSE)CV Mode = VOUT ~VFB+VZRIN June2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedHowto Designa SimpleConstantCurrent/ConstantVoltageBuc kConverter1 IntroductionA DC-to-DCconverteris typicallyimplementedas a constantvoltage(CV) controlloopadjuststhe duty cyclein orderto maintaina constantoutputvoltageregardlessof changesto the inputvoltageand load constantcurrent(CC)converterregulatescur rentthe sameway:the controlloop adjuststhe duty cycleto maintaina constantoutputcurrentregardlessof changesto the inputvoltageand outputresistancecausesthe outputvoltageto adjustas the load resistancevaries.

2 The higherthe outputresistance,the greaterthe CC/CVconverterregulatesbothcurrentand voltagedependingon the the maximumoutputresistanceand resultingoutputvoltageso that componentsconnectedto the outputwon t be damaged,whichis CC/CVconverterusesare applicationsdrivinga light -emittingdiode(LED)or chargingbatteriesor currentis regulatedfor a rangeof outputresistances;shouldtheresistanceinc reasebeyonda certainlevel,the voltageis regulated,or clamped. Output-voltageaccuracymay be crucial,particularlyin batteryapplicationsand can set the voltageregulationpointas closeas possibleto the maximumsafe operatingvoltageratingof the ImplementingCC/CVFigure1 outlinesa typicaldiscreteimplementationof a converterrequiresa senseresistor(RSENSE), an amplifierand a voltageregulationcircuit(Vz).The currentflowingthroughRSENSE setsthe voltageacrossRFB, whichis the feedbackvoltageof a this way,the currentis ROUT increases,the voltageon the outputrisesto a pointwherethe Zenerdiodeconducts,and thedevicetransitionsfroma CC converterto a CV CC/CVImplementationWitha BuckTopologyAs previouslymentioned,the currentthroughRSENSE sets the feedbackvoltage,whichregulatesthe expressesthe relationshipbetweenthe outputcurrentand VFB:loss2 SENSEOUTSENSER I R uCLAMPREFZVVV OUTOUTOUTV I R uINFBOUTFBSENSER VIR Ru the TraditionalMethod3 SNVA829 June2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedHowto Designa SimpleConstantCurrent/ConstantVoltageBuc kConverter(1)Assuminga resistiveload,Equation2 governsthe voltageat the output:(2)Equation3 sets the voltageregulationlevel.

3 (3)As seenin Figure1, a Zenerdioderegulatesthe voltagein CV Zeneras a voltageclampyieldsrelativelypoorvoltage- accuracyperformancebecauseof the variationin Zenervoltagesfromdeviceto Zenerdiodesare sometimesusedin seriesto preventleakagecurrentflowingfromcathodet o anode,whichif presentcauseserrorsin the the TraditionalMethodThe traditionalmethodrequiresthe use of a senseresistorin serieswith the outputin orderto a result,resistivelosseswill impactefficiency;Equation4 showsthe lossesin the senseresistor(4)Higherlossesincreasethe operatingtemperatureand reducesystemefficiencybecausethe resistorhasall of the outputcurrentflowingthroughit. Costalso increasesbecauselow milliohmcurrent-senseresistorsare relativelyexpensivecomparedto a common-modevoltagerangeofthe amplifierneedsto be ratedto the high outputvoltagemightincreasethecost of the help savecosts,you coulduse a floatingbias supplyto reducethe common-modevoltagerangerequirement,but that will increasethe solutionpresentedin Figure1has manydisadvantages,includingaddeddesignco mplexity,boardreal estaterequired,cost and impacton SimpleCC/CVMethodUsingthe LM5117 DeviceThe LM5117is an emulatedpeakcurrent-modesynchronousbuckc ontrollersuitablefor high-current, majorbenefitof usingthe LM5117in a CC/CVapplicationis that it hasa currentmonitor(CM)

4 CM pin providesan accuratevoltagethat is proportionalto the outputcurrentof the designercan use the CM pin as the currentloop feedback,savingthe additionalcurrent-sensecircuitrythat the voltagepresenton the CMpin is accurateto 2%, providedthat the converteris set to forcedpulsedwidthmodulation(FPWM)or isin showsa V + V AveFBtFBbFBVCMR - 1 RV AveVCM 2 10 A 10 uu: uAveVCM 2 V AveOUTSSVCM 2 I R A uuu AvePEAKVALLEYSSVCMI+ IR A u uFBRSVOUT = (IOUTuROUT)VINLOHOCSCSGCMVOUTVOUTRFBbRFB tRBotVCLMV431 RTopVCR1 BAT54LM5117 VCMAveRefCC June2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedHowto Designa SimpleConstantCurrent/ConstantVoltageBuc kConverterFigure2. CLM5117CC/CVBasicImplementation6CC ProgrammingEquation5 describesthe relationshipbetweenthe CM voltageand IOUT:(5)Equation6 simplifiesEquation5:(6)As can be seen,the CM pin enablesthe designerto omit the seriespowerdissipativecurrent-senseresis torat the the current-senseresistorof the powerstageusedto generatea rampfor thecurrent-modepulse-widthmodulation(PWM ) the current-senseamplifiergain of the LM5117,whichhas a typicalvalueof AS= example,assumethat IOUT= 10 A and RS= 10 m.

5 UsingEquation7:(7)Settingthe resistordividernetworkfromthe CM pin to groundand connectingthe dividernodeto thefeedbackpin sets the V at the CM pin, selectingthe properresistor-dividerratiosets the set the resistor-dividervaluesfor 10-Acurrentregulation,selectRFBb= 10k and calculateRFBtusingEquation8:(8)whichyiel dsan RFBtvalueof 15 k .Rememberto accountfor the reductionin AScausedby placingseriesfilter resistorsfromRSand groundto CSand CSG, the LM5117datasheetfor moredetailson how resistorsin serieswith the current-sensepins affectthe gain of the ProgrammingCV programingis achievedby usingan LMV431as a voltageclamplevelof 12V. The forwardvoltagedropVfwdacrossBAT54is V, and the FB voltageof the LM5117is V. Thevoltageclampengageswhenthe voltageacrossR1 is equalto the voltagecalculatedusingEquation9:(9)CLAMP TopVCBotVCREFVR - 1 RV TopVCR 37 k :LMV431 REFR1 REFV V + V the LM51175 SNVA829 June2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedHowto Designa SimpleConstantCurrent/ConstantVoltageBuc kConverterTherefore,VR1= voltageat the referencepin of the LMV431mustbe a referencevoltageaboveVR1.

6 The LMV431hasa referencevoltageof , so the voltageat the referencepin of the LMV431is equalto the voltagecalculatedusingEquation10:(10)The refore,a VREF= is requiredfor the LMV431to conductcurrentfromits cathodeto 10 k and calculateRTopVCusingEquation11:(11)8 PowerDesignof the LM5117 The designapproachfor the powersectionof a CC/CVconverterusingthe LM5117is the sameas it is fora suggestscarryingout the designat the highestoutputpowerlevel(whichis thehighestoutputresistance)usingeitherWE BENCH Designeror the LM5117 QuickStartCalculator. Referas well to the LM5117datasheetfor guidanceon the designof the showsa 30 V-to-54V input,27 V-at-6A outputCC/CVimplementationusingthe June2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedHowto Designa SimpleConstantCurrent/ConstantVoltageBuc kConverterFigure3. A 30 V-to-54V Input,27 V-at-6A OutputCC/CVConverterUsingthe LM5117 VOUTIOUT03691215182124273001234567D002 Load June2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedHowto Designa SimpleConstantCurrent/ConstantVoltageBuc kConverter10 ResultsFigure4 showsthe efficiencyresultswith Efficiencyat 30 VINF igure5 showsload regulationand the voltagesetpointwith LoadRegulationat 30 VINWithIncreasingRLOAD(VOUT/ IOUT)Figure6 showsthe switchnode(CH3),VOUT ripple(CH1)and outputcurrent(CH4)at 30 VIN, 25 VOUTat6 June2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedHowto Designa SimpleConstantCurrent/ConstantVoltageBuc kConverterFigure7 showsthe load-transientperformanceVOUT(CH1)and outputcurrent(CH4)whensteppingaconstantr esistiveload from60 to 120.

7 Figure7. Load-TransientPerformance11 SummaryThe LM5117configuredas a CC/CVconverterprovidesaccuratecurrentreg ulation,whileofferingmanyadvantagesovert he designapproachis relativelysimple,and enablessignificantreductionsin size,cost,and ( TI ) technical,applicationor otherdesignadvice,servicesor information,including,but not limitedto,referencedesignsand materialsrelatingto evaluationmodules,(collectively, TI Resources ) are intendedto assistdesignerswho aredevelopingapplicationsthat incorporateTI products;by downloading,accessingor usingany particularTI Resourcein any way,you(individuallyor, if you are actingon behalfof a company,yourcompany)agreeto use it solelyfor this purposeand subjectto the termsofthis s provisionof TI Resourcesdoesnot expandor otherwisealterTI s applicablepublishedwarrantiesor warrantydisclaimersfor TIproducts,and no additionalobligationsor liabilitiesarisefromTI providingsuchTI reservesthe rightto makecorrections,enhancements,improvement sand otherchangesto its TI understandand agreethat you remainresponsiblefor usingyourindependentanalysis,evaluationa nd judgmentin designingyourapplicationsand that you havefull and exclusiveresponsibilityto assurethe safetyof yourapplicationsand complianceof yourapplications(andof all TI productsusedin or for yourapplications)with all applicableregulations,lawsand ,with respectto yourapplications,you haveall the necessaryexpertiseto createand implementsafeguardsthat (1)

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