Design of an integrated analog controller for a class-D audio amplifier

An integrated analog controller for a self-oscillating class-D audio power amplifier is designed in a 0.35 ¿m CMOS technology for a 3.3 Volt power supply. It is intended to be used with an external output stage and passive filter, for medium power applications of upto a few 100 Watts. The controller was optimized with regard to its loop gain to suppress the distortion of the output stage. In typical commercially available output stages, the distortion is dominated by dead time effects and the THD can be as low as 20 dB. The controller uses self-oscillation to generate the carrier. To control the self-oscillation a second order phase shift network is embedded in the loop. To increase the loop gain a fifth-order loop filter is added. For a switching frequency of 400kHz the controller achieves a loop gain of 51 dB, nearly flat over the audio band. For reasons of flexibility, the order of the controller is made programmable, as well as the dead time and the delay in the loop. Full spice simulations of the controller combined with an external 120 Watt output stage indicate that a THD of upto 80 dB (better than 0.01%) can be obtained even under the worst case condition of a dead time of 50 ns.