4 Output Filter Computations

As described in the device data sheets (see the Appendix), depending on the selected mode, the CLASSD amplifier can have either a single-ended or a differential output. A specific type of pulse width modulation (PWM) is associated with each case.

For a single-ended output (CLASSD_MR.PWMTYP=0), the PWM acts only on the falling edge of the PWM waveform (trailing edge PWM).

Figure 4-1. Output Waveform Modulation Principle for PWMTYP=0 (Modes 2 and 4)

For a differential output (CLASSD_MR.PWMTYP=1), both the rising and falling edges of the PWM waveform are modulated (symmetric PWM). The modulation principles for both types of PWM are illustrated in the following figures. In particular, when describing a null input, if PWMTYP=0 (trailing edge PWM), the output waveform is a square wave with a 50% duty cycle. With the same input and PWMTYP=1, the differential output waveform is close to zero, although some minor width spikes remain).

Figure 4-2. Output Waveform Modulation Principle for PWMTYP=1 (Modes 1 and 3)

Consequently, for a low-cost implementation in mode 1, the classical L-C low-pass filter can be removed.