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0 V

–5 V

+5 V

Current

OUTP

OUTN

Differential

Voltage

Across

Load

0 V

–5 V

+5 V

Current

OUTP

OUTN

Differential

Voltage

Across

Load

Output = 0 V

Output > 0 V

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

TPA2000D1

SLOS328F – JUNE 2000 – REVISED MARCH 2004

APPLICATION INFORMATION (continued)

Figure 10. The TPA2000D1 Output Voltage and Current Waveforms Into an Inductive Load

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

× V

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 TPA2000D1 modulation scheme has little loss in the load without a filter because the pulses are short and

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 that results in less power

dissipated, which increases efficiency.

10