OutputCurrent

+VCC

0V

+VCC

0V

+VCC

0V

–VCC

DifferentialVoltage

AcrossSpeaker

+VCC

0V

OutputCurrent

TPA3121D2

SLOS537B – MAY 2008 – REVISED JANUARY 2014

www.ti.com

APPLICATION INFORMATION

CLASS-D OPERATION

This section focuses on the class-D operation of the TPA3121D2.

Traditional Class-D Modulation Scheme

The TPA3121D2 operates in AD mode. There are two main configurations that may be used. For stereo

operation, the TPA3121D2 should be configured in a single-ended (SE) half-bridge amplifier. For mono

applications, TPA3121D2 may be used as a bridge-tied-load (BTL) amplifier. The traditional class-D modulation

scheme, which is used in the TPA3121D2 BTL configuration, has a differential output where each output is 180

degrees out of phase and changes from ground to the supply voltage, VCC. Therefore, the differential prefiltered

output

varies

between

positive

and

negative

VCC,

where

filtered

50%

duty

cycle

yields

0 V across the load. The class-D modulation scheme with voltage and current waveforms is shown in Figure 25

and Figure 26.

Figure 25. Class-D Modulation for TPA3121D2 SE Configuration

Figure 26. Class-D Modulation for TPA3121D2 BTL Configuration

Supply Pumping

One issue encountered in single-ended (SE) class-D amplifier designs is supply pumping. Power-supply pumping

is a rise in the local supply voltage due to energy being driven back to the supply by operation of the class-D

amplifier. This phenomenon is most evident at low audio frequencies and when both channels are operating at

the same frequency and phase. At low levels, power-supply pumping results in distortion in the audio output due

to fluctuations in supply voltage. At higher levels, pumping can cause the overvoltage protection to operate,

which temporarily shuts down the audio output.

12

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