TPA3002D2

SLOS402C − DECEMBER 2002 − REVISED JANUARY 2004

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20

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

Figure 32. The TPA3002D2 Output Voltage and Current Waveforms Into an Inductive Load

Efficiency: LC Filter Required With the Traditional Class-D Modulation Scheme

The main reason that the traditional class-D amplifier needs an output filter is that the switching waveform

results in maximum current flow. This causes more loss in the load, which causes lower efficiency. The ripple

current is large for the traditional modulation scheme, because the ripple current is proportional to voltage

multiplied by the time at that voltage. The differential voltage swing is 2

is half the period for the traditional modulation scheme. An ideal LC filter is needed to store the ripple current

from each half cycle for the next half cycle, while any resistance causes power dissipation. The speaker is both

resistive and reactive, whereas an LC filter is almost purely reactive.

The TPA3002D2 modulation scheme has very little loss in the load without a filter because the pulses are very

making the ripple current larger. Ripple current could be filtered with an LC filter for increased efficiency, but

for most applications the filter is not needed.

An LC filter with a cutoff frequency less than the class-D switching frequency allows the switching current to

flow through the filter instead of the load. The filter has less resistance than the speaker, which results in less

power dissipation, therefore increasing efficiency.

Effects of Applying a Square Wave Into a Speaker

Audio specialists have advised for years not to apply a square wave to speakers. If the amplitude of the

waveform is high enough and the frequency of the square wave is within the bandwidth of the speaker, the

square wave could cause the voice coil to jump out of the air gap and/or scar the voice coil. A 250-kHz switching

for frequencies beyond the audio band.