TPA2001D2

1-W FILTERLESS STEREO CLASS-D AUDIO POWER AMPLIFIER

SLOS292A – MARCH 2000 – REVISED APRIL 2000

6

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

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

Figure 2. The TPA2001D2 Output Voltage and Current Waveforms Into an Inductive Load

efficiency: why you must use a filter 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

× VDD and the time at each voltage 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 TPA2001D2 modulation scheme has very little loss in the load without a filter because the pulses are very

short and the change in voltage is VDD instead of 2 × VDD. As the output power increases, the pulses widen

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 cut-off 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.