Self-Tuned Class-D Audio Amplifier With Post-Filter Digital Feedback Implemented on Digital Signal Controller

Abstract

This article describes the designing process of a low-cost class-D audio amplifier with digitized global feedback after the output filter, using a signal microcontroller. Various approaches to the feedback-based distortions elimination of class-D amplifiers are discussed. Hardware architecture of the author’s designed class-D amplifier with digital audio input interface, output voltage feedback path relays on SAR-ADC, and output power stage in multiphase bridge tide load configuration is shown. The selection method of the output low-pass filter parameters and power stage phase count was explained. A model of class-D amplifier with PID digital compensator and control loop interfaces is presented. Efficient high-speed digital PID compensator implementation on fixed-point 16-bit DSP core is exhibited. Procedure for control loop gain and phase plots measurements and self-tuning of PID feeding under damped second-order plants is presented along with simulations of loop properties influences on tuning effects. The results of the prototype device measurements such as Bode plots or output voltage frequency spectrum are presented. A big effort has been put on a useful, complete and transparent description for the development process of the presented class-D amplifier arrangement. The described class-D amplifier is the first solution with single feedback control from the output of LC filter based on a microcontroller with embedded ADC and PWM hardware with ensuring full audio bandwidth reproduction width THD distortion level below 0.25% at 1 kHz. The amplifier has been designed with the solicitude of device cost by using low-volume inductors in the output filter and by utilization of the built-in microcontroller ADC and PWM interfaces.