LC Filter Design (Rev. A) - Texas Instruments

1 1 SLAA701A October2016 RevisedNovember2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedLC FilterDesignAll trademarksare the propertyof October2016 RevisedNovember2016LC FilterDesignABSTRACTIn higher-powerclass-Damplifiers,generallya bove10 W of outputpower,a filteron the outputof theamplifieris filteris passivein natureand usesbothan inductorand a capacitoron ,it is referredto as an LC the LC filteriscriticalto meetthe desiredaudioperformance,efficiency,EMC/E MIrequirements,and costfor the a guideto aid in the sectionof LC filtercomponentsforclass-Damplifiersto meettarget-designgoalsof the end (BTL).. (PBTL).. (SE).. (Traditional) OutputLC AD ModulationWithType-1or Resistance(DCR).

2 Of Figures1 Stereo(Two-Channel)BTL (Traditional) October2016 RevisedNovember2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedLC FilterDesign6 Effectof Q on AD FilterResponseWithCBTL= F and LBTL= 10 BD or AD FilterResponseWithCg= F and LBTL= 10 AD FilterResponseWithCBTL= F, Cg= F, and LBTL= 10 ModulationFilterConvertedto +Nvs OutputPower,4 ..1922 TPA3251 EVMTHD+Nvs SignalFrequency,20 W, 4 ..1923 PVDD/ 2 30 V, 600 kHz,2 BTL,4 ..2328 TPA3251 THD+Nvs OutputPowerfor VariousInductors600 kHz,36 V, 4 ..2629 TPA3251 THD+Nvs Frequencyfor VariousInductors20 W, 600 kHz,36 V, 4 ..2630 Class-DSE Filter - AD of BD VoltageRatingless than85 VoltageRatingbetween85 C and 105 CapacitanceChangevs DC of Tables1 Class-DFilterTypesand TheirSE RBTL= 8.

3 173 FilterComponents RBTL= 6 ..174 FilterComponents RBTL= 4 ..175 AverageChangein Inductancefor 10 , DissipationFactor,and Idle PowerMeasuredfor 10 AOut BOut COut October2016 RevisedNovember2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedLC Descriptionsfor SelectingMetalizedFilm class-Daudioamplifierssupportmultipleout putconfigurationsin a allowsfora high levelof flexibilityfor the end (BTL)Bridge-tiedload (BTL)is the mostcommonoutputconfigurationfor a BTLconfigurationconsistsof one amplifierdrivingone side of a load and anotheramplifier,with an invertedsignalfromthe first amplifier,drivingthe otherside of the resultsin 2 morevoltageswingacrossthe load for a givensupplyvoltagewhencomparedto a single-endedconfigurationwhereone sideof the load is tied to the amplifieroutputand the otherside to voltageswingacrosstheload equatesto a 4 powerincreasebecauseP = V2/ R.

4 So, a BTL load configurationoffers4 morepowerto the load thana single-endedconfigurationfromthe of the load is driven,the load is not ,the voltageacrossthe load mustbe measureddifferentiallyrelativeto Stereo(Two-Channel) (PBTL)Parallelbridged-tiedload (PBTL)is an outputconfigurationthat takesa stereoBTL amplifierand connectsthe outputsin parallelfor a maximumoutputvoltageswingis the samefor a BTL outputconfiguration,the maximumcurrenthas beenincreasedbecauseeachoutputsharesthel oad oftenallowsfor lower-impedanceloadsto be drivenwith higheroutputpowerwhencomparedto BTL with the amplifiercurrentlimit has doubledcomparedto AOut BOut COut DClass-DAmplifierOut AOut BOut COut October2016 RevisedNovember2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedLC FilterDesignFigure2.

5 (SE)In a single-ended(SE)configuration,only one outputis usedto drivethe load ratherthana pair of outputsoperatingout of phase,as foundin BTL and this reason,only half of the Signalswingis availablecomparedto BTL or a quarterof the total configurationcanallowfor four channelswith a singlestereoBTL amplifieras shownin Figure3. Someamplifiersalso allowa combinationof 1 BTL and 2 SE channelsfor supportof audiosystemswith a to the PWMmodulationof a class-Damplifier,a DC voltageof PVDD/ 2 or half of the supplyvoltageis presentafterthe LC SE mode,becausethe speakeris now ground-referenced,eithera DCblockingcapacitoror someothermeansof referencingthe speakerto PVDD/ 2 is necessaryso that noDC voltageappearsacrossthe October2016 RevisedNovember2016 SubmitDocumentationFeedbackCopyright 2016.

6 TexasInstrumentsIncorporatedLC FilterDesign2 Class-DModulationSchemesThis sectiondescribeshow analogsignalsare convertedto PWMsignalsto drivethe MOSFETsin be classifiedas usingone of two modulationtechniques,AD(traditional)or BD (Traditional)ModulationThe traditionalswitchingtechnique(AD modulation)modulatesthe duty cycleof a rectangularwaveform,suchthat its averagecontentcorrespondsto the BTL outputs(seeFigure4) arethe inverseof modulationhas no significantcommon-modeswitchingcontentin its ,thereis a common-modeDC voltagedue to the averagevalueof the the load see this DC voltagelevel,it doesnot contributeto DC voltageis equalto PVDD/ 2, or half of the TPA312xD2familyemploysAD TASmodulatorscan be configuredfor AD AD (Traditional)

7 ModulationBecausethe switchingwaveformis nearlyentirelydifferential,a BTL-connectedload acrossthe A-legandB-legseesthe full idle, the amplifierswitchesat the nominalPWMfrequencywitha 50%duty cycleacrossthe causessignificantcurrentflow and powerdissipationinto the LC Filter is necessaryto reducethe currentto a smallresidualripplefor ,the lowerthe ripplecurrentfor an AD modulationclass-Damplifier,the betterthe efficiencydueto reducedload dissipationand reducedI2R loss acrossRDS(on)of the October2016 RevisedNovember2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedLC ModulationThe BD modulationswitchingtechniquemodulatesthe duty cycleof the differenceof the outputsignalssuchthat its averagecontentcorrespondsto the BTL outputs(seeFigure5)

8 Arenot the inverseof modulationhas significantcommon-modecontentin its be also be configuredfor BD BD Filter7 SLAA701A October2016 RevisedNovember2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedLC FilterDesign3 Class-DOutputLC FilterFrequencyResponsePropertiesThe frequencyresponseof the second-orderclass-DLC outputfilter is criticalwhenselectingthecomponentvaluesf or the inductorand LC Filter responsealso varieswith load impedancedeterminesthe dampingratioof the outputLC Filter and is classifiedasoverdamped,criticallydamped, or is also importantto understandthe speakerloadimpedancevariationsfor the applicationand selectthe L and C valuesthat suit the ,the LC Filter valueis selectedfor a criticallydamped,flat passband,and considerationswhenselectingcomponentsfor the second-orderlow-passfilter is the cut-off frequencyand Q factoror Effectof Q on FrequencyResponseTI recommendsusinga second-orderButterworthlow-passfilter becauseof its flat pass-bandand doesnot recommendthe use of LC filtersthat peakexcessively,like the underdampedfilterresponseshownin Figure6.

9 At high frequency,the peaksare generallyharshto the humanear and canalso triggerthe protectioncircuitry,suchas overcurrent,of ,overdampedfiltersresultin attenuationof high-frequencyaudiocontentin the = CgVin+_Vout+_RL = RBTL/2C = 2x CBTL + CgVin+_Vout+_RL = RBTL/2 LBTLLBTLC = 2x CBTLRL = RBTL/2 Vin+_Vout+_RBTLVout+LBTLVout -CgCgLBTLCBTLRBTLVout+LBTLVout -CgCgLBTLLBTLCBTLRBTLVout+LBTLVout -Class-DOutputLC October2016 RevisedNovember2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedLC FilterTopologiesFor class-Damplifiers,thereare primarilytwo Filter typesuseddependingon the is a differentialfilter usedfor AD Type-2filter is a common-modefilter primarilyusedfor BD showseachfilter type and the associatedsingle-endedequivalentthat is usedlaterin this sectionfor frequencyresponseand single-endedequivalentis usedto makethecomputationsfor eachfilter type Class-DFilterTypesand TheirSE EquivalentCircuitsClass-DBTLF ilterTypesType-1 Type-2 HybridType-1 Single-EndedEquivalentType-2 Single-EndedEquivalentHybridSingle-Ended EquivalentClass-DModulation:ADClass-DMod ulation:BD or AD( )Class-DModulation:ADFilterType:Differen tialFilterType:CommonModeFilterType.

10 HybridCBTL= Differentialbridgedtied load capacitorCg= Single-endedcapacitorto groundRBTL= Differentialload impedanceLBTL= Seriesinductor0 LCCapacitor valueforcriticallydampedButterworthfilte 12rR=w 0 LLInductor value for critically dampedButterworth fi terR2l =wLCRL11 Damping Ratio2Q2z == LCQRQ uality Factor QL=002 Conversion between radians and frequency in hertzw = pf001 Cutoff frequency of single ended LC filter22 L Cw==-pp fLCRLVoutVin+_+ Filter9 SLAA701A October2016 RevisedNovember2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedLC goal is to use the single-endedequivalentof eachfilter type for easyanalysis,the frequencyresponseof the single-endedfilter mustbe Single-EndedLC FilterThe equationsfor the single-endedLC Filter shownin Figure7 follow:(1)(2)(3)(4)As previouslymentioned,it is usuallydesirableto designa Filter that is criticallydampedwith a this type of Filter ,Q = 1 / 2.