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EffectsofApplyingaSquareWaveIntoaSpeaker
Efficiency (theoretical, %) +
R
L
ǒ
R
L
) r
ds(on)
Ǔ
100% +
8
(8 ) 1.3)
100% + 86%
(1)
P
(total)
+
P
O
Efficiency
+
8.5 W
0.86
+ 9.88 W
(2)
Other losses + P
(total)
(measured) * P
(total)
(theoretical) + 10.49 * 9.88 + 0.61 W
(3)
P
(dis)
+ 0.61 W * (12 V 22 mA) + 0.35 W
(4)
WhentoUseanOutputFilterforEMISuppression
TPA3008D2
SLOS435A–MAY2004–REVISEDJULY2004
APPLICATIONINFORMATION(continued)
Audiospecialistshaveadvisedforyearsnottoapplyasquarewavetospeakers.Iftheamplitudeofthe
waveformishighenoughandthefrequencyofthesquarewaveiswithinthebandwidthofthespeaker,the
squarewavecouldcausethevoicecoiltojumpoutoftheairgapand/orscarthevoicecoil.A250-kHzswitching
frequency,however,doesnotsignificantlymovethevoicecoil,astheconemovementisproportionalto1/f
2
for
frequenciesbeyondtheaudioband.
Damagemayoccurifthevoicecoilcannothandletheadditionalheatgeneratedfromthehigh-frequency
switchingcurrent.Theamountofpowerdissipatedinthespeakermaybeestimatedbyfirstconsideringthe
overallefficiencyofthesystem.Iftheon-resistance(rds(on))oftheoutputtransistorsisconsideredtocausethe
dominantlossinthesystem,thenthemaximumtheoreticalefficiencyfortheTPA3008D2withan8-Ωloadisas
follows:
Themaximummeasuredoutputpowerisapproximately8.5Wwithan12-Vpowersupply.Thetotaltheoretical
powersupplied(P(total))forthisworst-caseconditionwouldthereforebeasfollows:
Theefficiencymeasuredinthelabusingan8-Ωspeakerwas81%.Thepowernotaccountedforasdissipated
acrossther
DS(on)
maybecalculatedbysimplysubtractingthetheoreticalpowerfromthemeasuredpower:
Thequiescentsupplycurrentat12Vismeasuredtobe22mA.Itcanbeassumedthatthequiescentcurrent
encapsulatesallremaininglossesinthedevice,i.e.,biasingandswitchinglosses.Itmaybeassumedthatany
remainingpowerisdissipatedinthespeakerandiscalculatedasfollows:
Notethatthesecalculationsarefortheworst-caseconditionof8.5Wdeliveredtothespeaker.Becausethe0.35
Wisonly4%ofthepowerdeliveredtothespeaker,itmaybeconcludedthattheamountofpoweractually
dissipatedinthespeakerisrelativelyinsignificant.Furthermore,thispowerdissipatediswellwithinthe
specificationsofmostloudspeakerdriversinasystem,asthepowerratingistypicallyselectedtohandlethe
powergeneratedfromaclippingwaveform.
DesigntheTPA3008D2withoutthefilterifthetracesfromamplifiertospeakerareshort(<50cm).Powered
speakers,wherethespeakerisinthesameenclosureastheamplifier,isatypicalapplicationforclass-Dwithout
afilter.
Mostapplicationsrequireaferritebeadfilter.TheferritefilterreducesEMIaround1MHzandhigher(FCCand
CEonlytestradiatedemissionsgreaterthan30MHz).Whenselectingaferritebead,chooseonewithhigh
impedanceathighfrequencies,butlowimpedanceatlowfrequencies.
UseaLCoutputfilteriftherearelowfrequency(<1MHz)EMI-sensitivecircuitsand/ortherearelongwiresfrom
theamplifiertothespeaker.
WhenbothanLCfilterandaferritebeadfilterareused,theLCfiltershouldbeplacedascloseaspossibleto
theICfollowedbytheferritebeadfilter.
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