D 类 LC 滤波器设计器

ApplicationReportSLAA701A–October2016–RevisedNovember2016

LCFilterDesign

ABSTRACT

Inhigher-powerclass-Damplifiers,generallyabove10Wofoutputpower,afilterontheoutputofthe

amplifierisrequired.Thefilterispassiveinnatureandusesbothaninductorandacapacitoroneach

outputterminal.Therefore,itisreferredtoasanLCfilter.PropercomponentselectionoftheLCfilteris

criticaltomeetthedesiredaudioperformance,efficiency,EMC/EMIrequirements,andcostfortheend

application.ThisapplicationreportservesasaguidetoaidinthesectionofLCfiltercomponentsfor

class-Damplifierstomeettarget-designgoalsoftheendsystem.

Contents

1Class-DOutputConfigurations3

1.1Bridged-TiedLoad(BTL)3

1.2ParallelBridge-TiedLoad(PBTL)3

1.3Single-Ended(SE)4

2Class-DModulationSchemes5

2.1AD(Traditional)Modulation5

2.2BDModulation6

3Class-DOutputLCFilter7

3.1OutputLCFilterFrequencyResponseProperties7

3.2Class-DBTLOutputLCFilterTopologies8

3.3Single-EndedFilterCalculations9

3.4Type-1FilterAnalysis10

3.5Type-2FilterAnalysis12

3.6HybridFilterforADModulation14

3.7ADModulationWithType-1orType-2Filters17

3.8LCFilterQuickSelectionGuide17

4InductorSelectionforHigh-PerformanceClass-DAudio18

4.1InductorLinearity18

4.2RippleCurrent20

4.3MinimumInductance21

4.4CoreLoss22

4.5DCResistance(DCR)23

4.6InductorStudyWiththeTPA3251Device24

5CapacitorConsiderations28

5.1Class-DOutputVoltageOverview28

5.2CapacitorRatingsandSpecifications30

5.3CapacitorTypes35

6RelatedCollateral40

ListofFigures

1Stereo(Two-Channel)BTLClass-DAmplifier3

2MonoPBTLClass-DAmplifier4

3FourSingle-EndedOutputs4

4AD(Traditional)Modulation5

5BDModulation6

Alltrademarksarethepropertyoftheirrespectiveowners.

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LCFilterDesign

1

6EffectofQonFrequencyResponse7

7Single-EndedLCFilter9

8Type-1FilterforADModulation10

9Type-1FilterEquivalentCircuit10

10Type-1Single-EndedEquivalentCircuit1111Type-1LCFilterResponseWithC

BTL=0.68µFandLBTL=10µH1212Type-2FilterforBDorADModulation12

13Type-2FilterEquivalentCircuit13

14Type-2FilterSingle-EndedEquivalentCircuit1315Type-2LCFilterResponseWithC

g=1.5µFandLBTL=10µH1416HybridFilterforADModulation15

17HybridFilterSingle-EndedEquivalentCircuit1518HybridLCFilterResponseWithC

BTL=0.63µF,Cg=0.12µF,andLBTL=10µH1619Type-1ADModulationFilterConvertedtoType-217

20TypicalInductorSaturationCurve18

21TPA3251EVMTHD+NvsOutputPower,4Ω19

22TPA3251EVMTHD+NvsSignalFrequency,20W,4Ω19

23PVDD/2Common-ModeVoltage20

24PWMVoltageWaveform21

25InductorVoltageandCurrent21

26InductorCore-LossModel22

27TPA3251PowerDissipationWithInductorDCRPVDD=30V,600kHz,2×BTL,4Ω23

28TPA3251THD+NvsOutputPowerforVariousInductors600kHz,36V,4Ω26

29TPA3251THD+NvsFrequencyforVariousInductors20W,600kHz,36V,4Ω26

30Class-DSEFilter-ADofBDMode28

31LCFilterFrequencyResponse28

32Class-DLCFilterOutput29

33Class-DLCFilterOutputWithRipple29

34EquivalentSeriesResistance32

35DissipationFactor32

36KemetPHE426HB7100JR06Capacitor37

37VishayMMKP383Capacitor37

38ACVoltageRatinglessthan85°C37

39ACVoltageRatingbetween85°Cand105°C37

40Film-CapacitorTemperatureCoefficient38

41CeramicCapacitor%CapacitanceChangevsDCVoltage39

ListofTables

1Class-DFilterTypesandTheirSEEquivalentCircuits8

2FilterComponents–R

BTL=8Ω173FilterComponents–R

BTL=6Ω174FilterComponents–R

BTL=4Ω175AverageChangeinInductancefor10InductorSamples186R

P,DissipationFactor,andIdlePowerMeasuredfor10Inductors227ResultsofVariousInductors25

8RecommendedInductorsWiththeTPA32xxClass-DFamily27

9CapacitorRatingsandSpecifications30

10CapacitorTypeComparison35

11Capacitor-TypeToleranceComparison35

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Class-DOutputConfigurations

12CapacitorReliabilityParameterComparison36

13Film-CapacitorKeyParameters36

14ParameterandDescriptionsforSelectingMetalizedFilmCapacitors38

15RemainingCodeOptions40

1Class-DOutputConfigurations

SomeTIclass-Daudioamplifierssupportmultipleoutputconfigurationsinasingledevice.Thisallowsfor

ahighlevelofflexibilityfortheendapplication.

1.1Bridged-TiedLoad(BTL)

Bridge-tiedload(BTL)isthemostcommonoutputconfigurationforaclass-Damplifier.ABTL

configurationconsistsofoneamplifierdrivingonesideofaloadandanotheramplifier,withaninverted

signalfromthefirstamplifier,drivingtheothersideoftheload.Thisresultsin2×morevoltageswing

acrosstheloadforagivensupplyvoltagewhencomparedtoasingle-endedconfigurationwhereoneside

oftheloadistiedtotheamplifieroutputandtheothersidetoground.Twicethevoltageswingacrossthe

loadequatestoa4×powerincreasebecauseP=V2/R.So,aBTLloadconfigurationoffers4×more

powertotheloadthanasingle-endedconfigurationfromthesamesupplyvoltage.

Becauseeachsideoftheloadisdriven,theloadisnotground-referenced.Therefore,thevoltageacross

theloadmustbemeasureddifferentiallyrelativetoground.

OutA

OutB

Class-D

Amplifier

OutC

OutD

Figure1.Stereo(Two-Channel)BTLClass-DAmplifier

1.2ParallelBridge-TiedLoad(PBTL)

Parallelbridged-tiedload(PBTL)isanoutputconfigurationthattakesastereoBTLamplifierandconnects

theoutputsinparallelforasinglemonochannel.Althoughthemaximumoutputvoltageswingisthesame

foraBTLoutputconfiguration,themaximumcurrenthasbeenincreasedbecauseeachoutputsharesthe

loadcurrent.Thisoftenallowsforlower-impedanceloadstobedrivenwithhigheroutputpowerwhen

comparedtoBTLwiththesamesupplyvoltage.TheamplifiercurrentlimithasdoubledcomparedtoBTL.

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LCFilterDesign

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Class-DOutputConfigurations

OutA

OutB

Class-D

Amplifier

OutC

OutD

Figure2.MonoPBTLClass-DAmplifier

1.3Single-Ended(SE)

Inasingle-ended(SE)configuration,onlyoneoutputisusedtodrivetheloadratherthanapairofoutputs

operatingoutofphase,asfoundinBTLandPBTLconfigurations.Forthisreason,onlyhalfoftheSignal

swingisavailablecomparedtoBTLoraquarterofthetotaloutputpower.Howeverthisconfigurationcan

allowforfourchannelswithasinglestereoBTLamplifierasshowninFigure3.Someamplifiersalsoallow

acombinationof1×BTLand2×SEchannelsforsupportof2.1audiosystemswithasingledevice.

DuetothePWMmodulationofaclass-Damplifier,aDCvoltageofPVDD/2orhalfofthesupplyvoltage

ispresentaftertheLCfilter.InSEmode,becausethespeakerisnowground-referenced,eitheraDC

blockingcapacitororsomeothermeansofreferencingthespeakertoPVDD/2isnecessarysothatno

DCvoltageappearsacrossthespeaker.

OutA

OutB

Class-D

Amplifier

OutC

OutD

Figure3.FourSingle-EndedOutputs

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Class-DModulationSchemes

2Class-DModulationSchemes

ThissectiondescribeshowanalogsignalsareconvertedtoPWMsignalstodrivetheMOSFETsinthe

outputbridge.Mostclass-Damplifierscanbeclassifiedasusingoneoftwomodulationtechniques,AD

(traditional)orBDmodulation.

2.1AD(Traditional)Modulation

Thetraditionalswitchingtechnique(ADmodulation)modulatesthedutycycleofarectangularwaveform,

suchthatitsaveragecontentcorrespondstotheinputanalogsignal.TheBTLoutputs(seeFigure4)are

theinverseofeachother.ADmodulationhasnosignificantcommon-modeswitchingcontentinitsoutput.

However,thereisacommon-modeDCvoltageduetotheaveragevalueofthePWMswitching.Because

bothsidesoftheloadseethisDCvoltagelevel,itdoesnotcontributetopowerdissipationacrossthe

load.ThisDCvoltageisequaltoPVDD/2,orhalfofthesupplyvoltage.TheTPA312xD2familyemploys

ADmodulation.AllTASmodulatorscanbeconfiguredforADmodulation.

Figure4.AD(Traditional)Modulation

Becausetheswitchingwaveformisnearlyentirelydifferential,aBTL-connectedloadacrosstheA-legand

B-legseesthefullswitchingwaveform.Atidle,theamplifierswitchesatthenominalPWMfrequencywith

a50%dutycycleacrosstheload.Thiscausessignificantcurrentflowandpowerdissipationintotheload.

AnLCfilterisnecessarytoreducethecurrenttoasmallresidualrippleforgoodefficiency.

Generally,thelowertheripplecurrentforanADmodulationclass-Damplifier,thebettertheefficiencydue

toreducedloaddissipationandreducedI2RlossacrossRDS(on)oftheoutputFETs.

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Class-DModulationSchemes

2.2BDModulation

TheBDmodulationswitchingtechniquemodulatesthedutycycleofthedifferenceoftheoutputsignals

suchthatitsaveragecontentcorrespondstotheinputanalogsignal.TheBTLoutputs(seeFigure5)are

nottheinverseofeachother.BDmodulationhassignificantcommon-modecontentinitsoutput.Some

TASmodulatorscanbealsobeconfiguredforBDmodulation.

Figure5.BDModulation

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Class-DOutputLCFilter

3Class-DOutputLCFilter

3.1OutputLCFilterFrequencyResponseProperties

Thefrequencyresponseofthesecond-orderclass-DLCoutputfilteriscriticalwhenselectingthe

componentvaluesfortheinductorandcapacitor.TheLCfilterresponsealsovarieswithspeakerload

impedance.TheloadimpedancedeterminesthedampingratiooftheoutputLCfilterandisclassifiedas

overdamped,criticallydamped,orunderdamped.Itisalsoimportanttounderstandthespeakerload

impedancevariationsfortheapplicationandselecttheLandCvaluesthatsuittheexpectedload

variations.Ideally,theLCfiltervalueisselectedforacriticallydamped,flatpassband,andphase

response.Twoconsiderationswhenselectingcomponentsforthesecond-orderlow-passfilteristhecut-

offfrequencyandQfactorordampingratio.

Figure6.EffectofQonFrequencyResponse

TIrecommendsusingasecond-orderButterworthlow-passfilterbecauseofitsflatpass-bandandphase

response.TIdoesnotrecommendtheuseofLCfiltersthatpeakexcessively,liketheunderdampedfilter

responseshowninFigure6.Athighfrequency,thepeaksaregenerallyharshtothehumanearandcan

alsotriggertheprotectioncircuitry,suchasovercurrent,ofsomeamplifiers.However,overdampedfilters

resultinattenuationofhigh-frequencyaudiocontentintheaudioband.

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Class-DOutputLCFilter

3.2Class-DBTLOutputLCFilterTopologies

Forclass-Damplifiers,thereareprimarilytwofiltertypesuseddependingonthemodulationscheme.The

Type-1filterisadifferentialfilterusedforADmodulationamplifiersonly.TheType-2filterisacommon-

modefilterprimarilyusedforBDmodulation.

Table1showseachfiltertypeandtheassociatedsingle-endedequivalentthatisusedlaterinthissection

forfrequencyresponseanddampinganalysis.Thesingle-endedequivalentisusedtomakethe

computationsforeachfiltertypeeasier.

Table1.Class-DFilterTypesandTheirSEEquivalentCircuits

Class-DBTLFilterTypes

Type-1Type-2Hybrid

LBTL

LBTL

LBTL

Vout+

Vout+

Vout+

Cg

Cg

CBTL

CBTL

RBTL

RBTL

RBTL

Vout-

Vout-

Vout-

Cg

Cg

LBTL

LBTL

LBTL

Type-1Single-EndedEquivalentType-2Single-EndedEquivalentHybridSingle-EndedEquivalent

LBTL

LBTL

LBTL

++

++

+

+

Vin

C=2xCBTLRL=

RBTL/2

Vout

VinC=Cg

RL=Vout

Vin

RBTL/2C=2xCBTL+Cg

RL=

RBTL/2

Vout

__

__

_

_

Class-D

Modulation:AD

Class-D

Modulation:

BDorAD

(seeSection3.7)

Class-D

Modulation:AD

FilterType:DifferentialFilterType:CommonModeFilterType:Hybrid

CBTL=Differentialbridgedtiedloadcapacitor

Cg=Single-endedcapacitortoground

RBTL=Differentialloadimpedance

LBTL=Seriesinductor

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Class-DOutputLCFilter

3.3Single-EndedFilterCalculations

Sincethegoalistousethesingle-endedequivalentofeachfiltertypeforeasyanalysis,thefrequency

responseofthesingle-endedfiltermustbeknown.

L

+

+

Vin

CRLVout

_

_

Figure7.Single-EndedLCFilter

Theequationsforthesingle-endedLCfiltershowninFigure7follow:

f

0

1

w

0

==-

CutofffrequencyofsingleendedLCfilter

22LC

pp´

(1)

w0=2pf0Conversionbetweenradiansandfrequencyinhertz

(2)

C

QRQualityFactorQ

=

L

L

(3)

11

z==

2Q

2´R

L

C

L

DampingRatio

(4)

Aspreviouslymentioned,itisusuallydesirabletodesignafilterthatiscriticallydampedwithaButterworth

response.Forthistypeoffilter,Q=0.707=1/√2.BysubstitutingQ=1/√2intoequationsforCut-off

FrequencyandQualityFactorpreviouslylisted,LandCvaluescanbederivedforacriticallydamped

systemassumingthedesiredcut-offfrequency,ωo,isknown.

R2

´

L

LInductorvalueforcriticallydampedButterworthfilter

=

w

0

(5)

1

CCapacitorvalueforcriticallydampedButterworthfilter

=

w´´

R2

0L

(6)

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Class-DOutputLCFilter

3.4Type-1FilterAnalysis

TheType-1filterisadifferentialfilterdesignedforADmodulationclass-Damplifiers.ADmodulationhas

nosignificantcommon-modecontentonitsoutputs,thusonlythedifferentialmodeisneededforanalysis.ThedifferentialBTLcomponents,CBTLandRBTLoftheADmodulationLCfiltershowninFigure8,canbe

simplifiedtoasingle-endedequivalent.

LBTL

Vout+

+

CBTL

Vout

_

RBTL

Vout-

LBTL

Figure8.Type-1FilterforADModulation

Frominspection,theType-1filtercanbesplitintoanequivalentcommon-modefilterasshowninFigure9.

CBTLmustbescaledbyafactorof2sinceinthecommon-modemodel,thecapacitorsappearinseries.

LBTL

Vout+

+

2xCBTL

Vout

_

RBTL/2

+

2xCBTL

Vout

_

Vout-

RBTL/2

LBTL

Figure9.Type-1FilterEquivalentCircuit

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Class-DOutputLCFilter

Nowthesingle-endedequivalentcanbedrawnasshowninFigure10.Thedifferencebetweentheoriginalfilterandsingle-endedequivalentisthatCBTLhasbeenmultipliedbyafactoroftwoandRBTLhasbeen

dividedbyafactoroftwo.

Whenusingthesingle-endedequivalentcircuittofindthefiltercomponentvalues,CBTL=C/2.

LBTL

+

+

Vin

C=2xCBTLRL=

RBTL/2

Vout

_

_

Figure10.Type-1Single-EndedEquivalentCircuit

3.4.1Type-1FrequencyResponseExample

UsingtheequationsoutlinedinSection3.3,afilterisdesignedwithatargeted40-kHzbandwidthand4-Ωspeakerload.

TheLandCvaluescanbecalculatedasfollows:

R

R´2

BTL

L

L=whereR=,R=4W,w=2pf,andf=

LBTL000

w2

0

R

BTL

´2

22

2

L===11.25mH

2pf2p´40000

0

40kHz

Theneareststandardinductorvalueis10µH.

L=L=10mH

BTL

R1

BTL

C=whereR=,R=4W,w=2pf,andf=40kHz

LBTL000

w´R´2

2

0L

111

C====1.4mF

R2400002

w´R´2p´´2

BTL

0f2

L

0

2p´´

2

C

C==0.70mF»0.68mF

BTL

2

Thestandardcapacitorvalueis0.68µF.

Usingthestandardinductorandcapacitorvaluesfromthepreviouscalculations,theQfactorfor4Ωis:

R2C-

´´

6

C1.3610

BTLBTL

QR20.737

====

L6

L2L1010

-

BTL

´

(7)

WiththetargetedQof1/√2≈0.707andthecut-offfrequencyis:

f

0

111

====

2LC2L2C2(1010)(1.3610)

p´p´´

--66

66

--

BTLBTLp´´´

43156Hz

Thepeakingatcut-offfrequencyindBis:

Peaking(w)=20logQ=-2.65dB

010

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LCFilterDesign

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Class-DOutputLCFilter

Usingthetransferfunction(Equation8),thefrequencyresponseoftheselectedLCfiltercanbeplottedfor

differentspeakerloadstoprovideacompletevisualevaluationoftheLCfilterresponsewithvarious

speakerloads.

H(s)

Diff

V(s)1

OUT

==

V(s)

L

INBTL2

1s2Cs

+´´´+

BTL

R

BTL

2

(8)

20

10

0

-10

-20

2:-303:

4:6:8:

-40

101001k10k100k1M

Frequency(Hz)

D001

Figure11.Type-1LCFilterResponseWithCBTL=0.68µFandLBTL=10µH

3.5Type-2FilterAnalysis

TheType-2class-Dfilterisacommon-modefilterdesignedforBDorADmodulationamplifiers.Sincethe

Type-2filteriscommonmode,itcanbeeasilyconvertedintoitsequivalentsingle-endedform.Thecommon-modeBTLcomponentsCganddifferentialloadRBTLoftheLCfiltershowninFigure12,canbe

simplifiedtoasingle-endedequivalent.

LBTL

Vout+

+

Cg

Vout

RBTL_

Vout-

Cg

LBTL

Figure12.Type-2FilterforBDorADModulation

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Class-DOutputLCFilter

Frominspection,theType-2filtercanbesplitintoanequivalentcommon-modefilterasshownin

Figure13.

LBTL

Vout+

+

Cg

Vout

_

RBTL/2

+

Cg

Vout

_Vout-RBTL/2

LBTL

Figure13.Type-2FilterEquivalentCircuit

Nowthesingle-endedequivalentcanbedrawn.Theonlydifferenceforsingle-endedanalysisisthatRBTL

hasbeendividedbyafactoroftwo.

LBTL

+

+

VinC=Cg

RL=

RBTL/2

Vout

_

_

Figure14.Type-2FilterSingle-EndedEquivalentCircuit

3.5.1Type-2FrequencyResponseExample

UsingtheequationsoutlinedinSection3.3,afilterisdesignedwithatargeted40-kHzbandwidthand4-Ωspeakerload.

R

R´2

BTL

L

L=whereR=,R=4W,w=2pf,andf=

LBTL000

w2

0

R

BTL

´2

22

2

L===11.25mH

2pf2p´40000

0

40kHz

Theneareststandardinductorvalueis10µH.

L=L=10mH

BTL

R1

BTL

C=whereR=,R=4W,w=2pf,andf=40kHz

LBTL000

w´R´2

2

0L

111

C====1.4mF

R2400002

w´R´2p´´2

02f´2

LBTL

p´

0

2

C=C=1.4mF»1.5mF

g

Thestandardcapacitorvalueis1.5µF.

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Class-DOutputLCFilter

Usingthestandardinductorandcapacitorvaluesfromthepreviouscalculations,theQfactorfor4Ωis:

RC

-

6

C1.510

BTLg´

QR20.775

====

L6

L2L1010-

BTL

´

WiththetargetedQof1/√2≈0.707andthecut-offfrequencyis:

f

0

111

====

2LC2LC2(10106)(1.5106)

p´p´

--

BTLg

p´´´

41093Hz

Thepeakingatthecut-offfrequencyindBis:

Peaking(w)=20logQ=-2.22dB

010

UsingthetransferfunctionbelowthefrequencyresponseoftheselectedLCfiltercanbeplottedfor

differentspeakerloadstoprovideacompletevisualevaluationoftheLCfilterresponsewithvarious

speakerloads.

H(s)

Diff

V(s)1

OUT

==

V(s)

L

INBTL2

1sLCs

+´+´+

BTLg

R

BTL

2

(9)

20

10

0

-10

-20

2:-303:

4:6:8:

-40

101001k10k100k1M

Frequency(Hz)

D002

Figure15.Type-2LCFilterResponseWithCg=1.5µFandLBTL=10µH

3.6HybridFilterforADModulation

Forsomeapplications,itmaybebeneficialtouseahybridfiltercombiningtheType-1andType-2filtersforanADmodulationamplifier.ByaddingCgtotheType-1filter,thehigh-frequencydecouplingtoground

isimproved,sincetheamplifierADPWMmodulationisneverperfectlydifferential.ForthisconfigurationTIrecommendsCg=0.1×CBTL.

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Class-DOutputLCFilter

LBTL

Vout+

+

Cg

CBTL

Vout

RBTL

_

Vout-

Cg

LBTL

Figure16.HybridFilterforADModulation

FromthepreviousstudyoftheType-1andType-2filters,theHybridFiltersingle-endedequivalentcanbe

easilydrawn.

+

+

Vin

C=2xCBTL+Cg

RL=

RBTL/2

Vout

_

_

Figure17.HybridFilterSingle-EndedEquivalentCircuit

3.6.1HybridFilterFrequencyResponseExample

UsingtheequationsoutlinedinSection3.3,afilterisdesignedwithatargeted40-kHzbandwidthand4-Ωspeakerload.

R

R´2

BTL

L

L=whereR=,R=4W,w=2pf,andf=

LBTL000

w2

0

R

BTL

´2

22

2

L===11.25mH

2pf2p´40000

0

40kHz

Theneareststandardinductorvalueis10µH.

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Class-DOutputLCFilter

L=L=10mH

BTL

R1

BTL

C=whereR=,R=4W,w=2pf,andf=40kHz

LBTL000

w´R´2

0L

111

C====1.4mF

R2400002w´p´´

R´222

BTL

0

0

L2

pf´´

2

C=2´C+CwhereC

BTLgg

»

2´C

BTL

10

2´C

BTL

C=2´C+=1.4mF

BTL

10

C2´C

BTL

C==0.63mFandC==0.12mF

BTLg

2.210

Thestandardcapacitorvaluesare0.63µFand0.12µF.

Usingthestandardinductorandcapacitorvaluesfromthepreviouscalculations,theQfactorfor4Ωis:

R2.2C-

6

C´1.38´10

BTLBTL

QR20.743

L2L1010-

====

L6

BTL

´

WiththetargetedQof1/√2≈0.707andthecut-offfrequencyis:

f

0

111

====

2LC2L2.2C2(1010)(1.3810)

p´p´´

66

--

BTLBTL

p´´´

42843Hz

Thepeakingatthecut-offfrequencyindBis:

Peaking(w)=20logQ=-2.58dB

010

UsingthetransferfunctionbelowthefrequencyresponseoftheselectedLCfiltercanbeplottedfor

differentspeakerloadstoprovideacompletevisualevaluationoftheLCfilterresponsewithvarious

speakerloads.

V(s)1

OUT

H(s)==

Diff

V(s)

L

IN

BTL2

1sL2CCs

+´+´´++

BTLBTLg

R

BTL

2

(10)

20

10

0

-10

-20

2:-303:

4:6:8:

-40

101001k10k100k1M

Frequency(Hz)

D003

Figure18.HybridLCFilterResponseWithCBTL=0.63µF,Cg=0.12µF,andLBTL=10µH

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Class-DOutputLCFilter

3.7ADModulationWithType-1orType-2Filters

SincetheCgcapacitorsontheType-2filtershareacommonnodethroughground,differentialsignalswill

seeacapacitancevalueofCg/2sincethecapacitorsappearinseriesfordifferentialsignalsonly.

Therefore,withproperselectionoftheCgcapacitancevalue,theType-2canalsobeusedforAD

modulationclass-Damplifierswhichrequireadifferentialfilter.ConversionfromType-1toType-2onlyrequiresscalingofCBTLbyafactorof2.Thatis,Cg=2xCBTLforidenticalfiltercut-offfrequencyand

dampingfactor.

LBTL

Vout+

+

Vout+

Cg=2xCBTL

+

CBTL

Vout

Vout

_

_

RBTL

Cg=2xCBTL

RBTL

Vout-

Vout-

LBTL

LBTL

Figure19.Type-1ADModulationFilterConvertedtoType-2

3.8LCFilterQuickSelectionGuide

ThefiltercomponentsinthefollowingtablesarebasedonaType-2filterconfigurationpreviouslyshown.

ThetableparametersareindicatorsoftheLCfilterresponsewhenselectingtheinductorandcapacitor

values.TheQgivesinsightintothedampingofthefilterandwhetherthereispeakingathighfrequency.Theƒ0istheresonantcut-offfrequencyofthefilterandprovidesinsightonthebandwidthofthefilter.The

peakingat20-kHzisanindicatoroftheflatnessofthefilterintheaudioband.BasedonthePWM

frequencyoftheclass-Damplifieritisagoodpracticetoaccesstheamountofthecarrierfundamental

frequencythefilterattenuates.Higherattenuationhelpsminimizetheriskofcarriersignalinterference.

Table2.FilterComponents–RBTL=8Ω

Qƒ0(kHz)Peaking@

20-kHz(dB)

LBTL(µH)Cg(µF)Attenuation@

400-kHz(dB)

Attenuation@

600-kHz(dB)

0.70860–0.051150.47–33–41

0.196730.927100.47–29–37

Table3.FilterComponents–RBTL=6Ω

Qƒ0(kHz)Peaking@

20-kHz(dB)

LBTL(µH)Cg(µF)Attenuation@

400-kHz(dB)

Attenuation@

600-kHz(dB)

0.63949–0.408150.68–36–44

0.639610.122100.68–33–40

0.777880.06770.47–26–34

Table4.FilterComponents–RBTL=4Ω

Qƒ0(kHz)Peaking@

20-kHz(dB)

LBTL(µH)Cg(µF)Attenuation@

400-kHz(dB)

Attenuation@

600-kHz(dB)

0.79150–0.429101.0–36–44

0.756600.06771.0–33–40

SLAA701A–October2016–RevisedNovember2016

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LCFilterDesign

17

InductorSelectionforHigh-PerformanceClass-DAudio

4InductorSelectionforHigh-PerformanceClass-DAudio

4.1InductorLinearity

TheinductanceversuscurrentprofilefortheinductorusedintheoutputLCfilterofaclass-Damplifiercan

significantlyimpactthetotalharmonicdistortion(THD)performance.

Anidealinductormaintainsthespecifiedinductancevaluenomatterwhatcurrentpassesthroughit.

However,real-worldinductorsalwayshavedecreasinginductancewithincreasingcurrent.Atsomepoint,

thecurrentlevelsaturatestheinductorandtheinductancefallsoffseverely.ThisisoftenspecifiedasIsat.

Becauseinductorlinearityisafunctionofcurrent,inductordistortionishigherwithlower-impedanceloads.

Lsat

Current(Amps)

Figure20.TypicalInductorSaturationCurve

KeepinmindthattheinductancechangeattheIsatcurrentratingvariesbetweenmanufacturersandeven

inductortypes.SomemanufacturersspecifyIsatata30%orhigherchangeininductance.Useofthis

inductorallthewaytotheIsatratingforanLCclass-Dfilterresultsinvery