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0 V
−12 V
+12 V
Current
OUTP
OUTN
Differential
Voltage
Across
Load
0 V
−12 V
+12 V
Current
OUTP
OUTN
Differential
Voltage
Across
Load
Output = 0 V
Output > 0 V
Efficiency:LCFilterRequiredWiththeTraditionalClass-DModulationScheme
TPA3008D2
SLOS435A–MAY2004–REVISEDJULY2004
APPLICATIONINFORMATION(continued)
Figure18.TheTPA3008D2OutputVoltageandCurrentWaveformsIntoanInductiveLoad
Themainreasonthatthetraditionalclass-Damplifierneedsanoutputfilteristhattheswitchingwaveformresults
inmaximumcurrentflow.Thiscausesmorelossintheload,whichcauseslowerefficiency.Theripplecurrentis
largeforthetraditionalmodulationscheme,becausetheripplecurrentisproportionaltovoltagemultipliedbythe
timeatthatvoltage.Thedifferentialvoltageswingis2xV
CC
,andthetimeateachvoltageishalftheperiodfor
thetraditionalmodulationscheme.AnidealLCfilterisneededtostoretheripplecurrentfromeachhalfcyclefor
thenexthalfcycle,whileanyresistancecausespowerdissipation.Thespeakerisbothresistiveandreactive,
whereasanLCfilterisalmostpurelyreactive.
TheTPA3008D2modulationschemehaslittlelossintheloadwithoutafilterbecausethepulsesareshortand
thechangeinvoltageisV
CC
insteadof2xV
CC
.Astheoutputpowerincreases,thepulseswiden,makingthe
ripplecurrentlarger.RipplecurrentcouldbefilteredwithanLCfilterforincreasedefficiency,butformost
applicationsthefilterisnotneeded.
AnLCfilterwithacutofffrequencylessthantheclass-Dswitchingfrequencyallowstheswitchingcurrenttoflow
throughthefilterinsteadoftheload.Thefilterhaslessresistancethanthespeaker,whichresultsinlesspower
dissipation,thereforeincreasingefficiency.
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