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0 V

–5 V

+5 V

Current

OUT+

OUT–

Differential

Voltage

Across

Load

0 V

–5 V

+5 V

Current

OUT+

OUT–

Differential

Voltage

Across

Load

Output = 0 V

Output > 0 V

EFFICIENCY: WHY YOU MUST USE A FILTER WITH THE TRADITIONAL CLASS-D

EFFECTS OF APPLYING A SQUARE WAVE INTO A SPEAKER

TPA2000D2

SLOS291F – MARCH 2000 – REVISED MARCH 2007

APPLICATION INFORMATION (continued)

Figure 13. The TPA2000D2 Output Voltage and Current Waveforms Into an Inductive Load

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

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 TPA2000D2 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

dissipated and increased efficiency.

Audio specialists have said 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 frequency,

the audio band. Therefore, the amount of cone movement at the switching frequency is very small. However,

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