EFFICIENCY: LC FILTER REQUIRED WITH THE TRADITIONAL CLASS-D MODULATIONSCHEMEThe main reason that the traditional class-D amplifier-based on AD modulation needs an output filter is that theswitching waveform results in maximum current flow. This causes more loss in the load, which causes lowerefficiency. The ripple current is large for the traditional modulation scheme, because the ripple current isproportional to voltage multiplied by the time at that voltage. The differential voltage swing is 2 × VCC, and thetime at each voltage is half the period for the traditional modulation scheme. An ideal LC filter is needed to storethe ripple current from each half cycle for the next half cycle, while any resistance causes power dissipation. Thespeaker is both resistive and reactive, whereas an LC filter is almost purely reactive.The TPA3131/32D2 modulation scheme has little loss in the load without a filter because the pulses are shortand the change in voltage is VCC instead of 2 × VCC. As the output power increases, the pulses widen, makingthe ripple current larger. Ripple current could be filtered with an LC filter for increased efficiency, but for mostapplications the filter is not needed.An LC filter with a cutoff frequency less than the class-D switching frequency allows the switching current to flowthrough the filter instead of the load. The filter has less resistance but higher impedance at the switchingfrequency than the speaker, which results in less power dissipation, therefore increasing efficiency.
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