Feedforward Capacitor to Improve Stability and …

1 CO1 1211 11C =+25 pF2R R R - fffp1 SLVA466A December2011 RevisedJune2017 SubmitDocumentationFeedbackCopyright 2011 2017,TexasInstrumentsIncorporatedFeedfor wardCapacitorto ImproveStabilityand BandwidthWiththe TPS621-Familyand TPS821-FamilyApplicationReportSLVA466A December2011 RevisedJune2017 FeedforwardCapacitorto ImproveStabilityandBandwidthWiththe TPS621-Familyand TPS821-FamilyMatt commonmethodto improvethe stabilityand bandwidthof a powersupplyis to use a feedforwardcapacitor,whichis a capacitorplacedacrossthe zeroand a pole (fZ< fP)

2 To the controlloop,whichcan be strategicallyplacedto improvethe be measuredin boththe transientresponseand bodeplot of the designstrategiesfor placingthe poleand zeroat the optimalfrequencyto improvetransientresponseand circuitstabilityfor the TPS62130/40/50/60/70 DCS-Control foundin theTI applicationreportOptimizingtheTPS62130/4 0/50/60 a trademarkof othertrademarksare the propertyof SimplifiedSchematicWithInternaland ExternalFeedforwardCapacitorsShownThe traditionalmethodof designingwith a feedforwardcapacitoris to add an externalcapacitor(Cff)

3 Inparallelwith the high-sidefeedbackresistor,R1 in Figure1. The capacitorvalueis chosenbasedon thevaluesof the feedbackresistorsto placethe geometricmeanof the pole and zeroat the applicationreportOptimizingTransientResp onseof InternallyCompensateddc-dc ConvertersWithFeedforwardCapacitor(SLVA2 89) presentsan equationto calculatethe TPS6213x/4x/5x/6x/7xfamilyof integratedcircuitshas an internal25-pF feedforwardcapacitor,the equationmustbe modifiedas shownin Equation1.(1)60-40 Gain180-180 Phase1k1 MFrequencyP12 ff1=2 (R // R )CfpZ1 ff1=2 R December2011 RevisedJune2017 SubmitDocumentationFeedbackCopyright 2011 2017,TexasInstrumentsIncorporatedFeedfor wardCapacitorto ImproveStabilityand BandwidthWiththe TPS621-Familyand TPS821-FamilyWherefCOis the unitygain crossoverfrequencyof the controlloop withoutthe externalfeedforwardcapacitorinstalledand R1 and R2 are the high-sideand low-sidefeedbackresistors, showsthe locationsof the feedbackresistorsand capacitorvalueplacesthe geometricmeanof the zeroand pole at fCO.

4 Whichprovidesthe best the zeroand pole are calculatedusingEquation2 and Equation3. Notethat the poleis alwaysat a higherfrequencythanthe zerobecauseR1is alwayslargerthanR1//R2.(2)(3)As an example,take a 12-Vin, a H inductor,200- F outputcapacitor,R1as 10 k , and R2as . First,the loop gain withoutthe feedforwardcapacitorneedsto bemeasuredto find the unitygain applicationreportHowto MeasureControlLoopof TPS62130/40/50/60/70 DCS-Control Devices(SLVA465) explainshow to take a loop measurementon the DCS-Control topologyand Figure2 showsthe crossoverfrequencyis ,soEquation1 is usedto calculatethat 941 pF of extracapacitanceis showsthe new loopresponsewith an added1000-pFCff comparedwith the responsewithoutthe Measuringthe GainCrossoverFrequencyWithouta FeedforwardCapacitorWith CffWithout CffWithout CffWith December2011 RevisedJune2017 SubmitDocumentationFeedbackCopyright 2011 2017.

5 TexasInstrumentsIncorporatedFeedforwardC apacitorto ImproveStabilityand BandwidthWiththe TPS621-Familyand TPS821-FamilyFigure3. LoopGainWithand WithoutFeedforwardCapacitorFigure4 showsthe load transientresponsebeforeand afteraddingthe additionalCff. The increaseinloop bandwidthdue to the Cff is seenin the bodeplot, whereasthe transientresponseshowsthe tangiblebenefitof the gain in transientresponseis fasterand the voltageundershootis less withthe externalCff. In addition,no ringingoccursin the outputvoltagebecausethe phaseis not allowedtodip as low due to the with the externalCff, the load regulationis reducedsomewhatas shownby the differentfinal valuesof the outputvoltagein Figure4.

6 This effectis morepronouncedforsmallerexternalCff this reason,adjustingthe resistorsand usingonly the internal25-pFCff isthe preferredmethodof loop responseoptimizationas discussedin the next LoadTransientResponseWithand WithoutFeedforwardCapacitor1 REF2 OUTREFR VR =VV -OUT1 COREFV1R =2 k! VR = k! V -1 REF2 OUTREFR VR =VV -11R = k!2 (1 MHz)(25 pF) pChoosingFeedbackResistorsBasedon December2011 RevisedJune2017 SubmitDocumentationFeedbackCopyright 2011 2017,TexasInstrumentsIncorporatedFeedfor wardCapacitorto ImproveStabilityand BandwidthWiththe TPS621-Familyand TPS821-Family2 ChoosingFeedbackResistorsBasedon InternalCffThe secondand preferredmethodof designinga TPS62130/40/50/60/70powersupplywith a feedforwardcapacitoris to choosethe feedbackresistorsbasedon the uniqueto this familyof devicesbecauseof the benefitof usingthismethodis the abilityto eliminateone componentfromthe circuitto reduceproductioncostswhilekeepingsuperio rload regulationin first step is to measurethe controlloop with

7 Resistorsthat placethe zerodue to the internalfeedforwardcapacitorat a frequencyhigherthanthe gain feedforwardcapacitordoesnot havean effectat frequencieshigherthan100 kHz (seeSection3), chooseR1suchthatthe frequencyof the zerois at leastgreaterthan100 completelyeliminatethe effectof the zero sphaseresponse,the zeromustbe placedone decadeabovethe gain ,placingthe zeroat 1 MHzis sufficientfor all gain , an appropriatehigh-sideresistorcan be calculatedas shownby the followingequations.(4)Therefore,usinga resistoris sufficientfor any LC be chosenfor the desiredoutputvoltageusingEquation5.

8 (5)For example,for a desiredoutputof V, R2is calculatedas:(6)FromEquation2 and Equation3, the zeroand pole are placedat 1 MHzand , controlloop gain can thenbe measuredto find the unitygain crossoverfrequencywithoutany effectfromthe internalCff. The followingequationsthencan be usedto calculateR1and R2to repositionthegeometricmeanof the pole and zeroto the crossoverpoint,with Cff equalto 25 pF.(7)(8)Continuingwith the previousexample,for a a crossoverfrequencyof ,R1andR2are calculatedto be 432 k and 137 k , showsthe increasedloop bandwidthand phasewith the new causesthe improvedtransientresponsein Figure6, whichalso has no ringingand excellentload CffWith Cff180-180 Phase1k1 MFrequency60-40 GainOriginal ResistorsNew InternalCff5 SLVA466A December2011 RevisedJune2017 SubmitDocumentationFeedbackCopyright 2011 2017.

9 TexasInstrumentsIncorporatedFeedforwardC apacitorto ImproveStabilityand BandwidthWiththe TPS621-Familyand TPS821-FamilyFigure5. LoopGainBeforeand AfterOptimizingResistorsFigure6. LoadTransientResponseBeforeand AfterOptimizingResistorsWith CffWithout CffOriginal ResistorsNew Resistors180-180 Phase1k1 MFrequency60-40 GainFeedforwardCapacitorfor GainCrossoverFrequenciesAbove100 December2011 RevisedJune2017 SubmitDocumentationFeedbackCopyright 2011 2017,TexasInstrumentsIncorporatedFeedfor wardCapacitorto ImproveStabilityand BandwidthWiththe TPS621-Familyand TPS821-Family3 FeedforwardCapacitorfor GainCrossoverFrequenciesAbove100 kHzAt loop bandwidthsabove100 kHz.

10 The feedforwardcapacitorhas limitedor no effecton the is a characteristicof the DCS-Control topologyas implementedon the TPS62130/40/50/60 and Figure8 showthe resultsof optimizingthe feedbackresistorsfor the Hinductorand 22- F outputcapacitor,whichhas a unitygain crossoverfrequencyof lack ofany significantchangein the bodeplot and load transientshowsthat the Cff has no LoopGainfor H Inductorand 22- F CapacitorFigure8. LoadTransientResponsefor H Inductorand 22- F December2011 RevisedJune2017 SubmitDocumentationFeedbackCopyright 2011 2017,TexasInstrumentsIncorporatedRevisio nHistory4 ConclusionThis applicationreportpresentsmethodsof improvingthe load transientresponseof theTPS62130/40/50/60/70familyof devicesby usinga externalcapacitoror adjustingthe feedbackresistorsare two waysto improvethe loop responsefor loopbandwidthsbelow100 mostdesigns.