Class D topology utilizes pulse-width modulation (PWM) with a triangle-wave generator. This slide shows a simplified block diagram of a PWM-based, half-bridge, Class D amplifier. It consists of a pulse-width modulator made of a comparator and triangle-wave generator, two output switches (e.g. power MOSFETs), and an external low pass filter to recover the amplified audio signal. The switching frequency for NXP’s Class D amplifiers is typically between 250kHz to 450kHz. The output square wave is pulse-width modulated by the input audio signal, and is accomplished by comparing the input audio signal to an internally generated triangle-wave oscillator. It drives the power output driver switching stage, creating the amplified digital signal before the low pass filter (e.g. Butterworth filter). As shown, this PWM output has a varying duty cycle proportional to the audio signal. After passing through the LPF, the PWM modulated carrier is filtered out and an amplified sinusoidal audio signal drives the speaker.