High-Frequency Digital Current Mode Control Architectures for Class-D Audio Amplifiers

Low power audio signals are amplified using a power amplifier before feeding to the loudspeaker for efficient reproduction of audio sounds. High frequency class-D amplifier is a preferred choice because of its high power density and high efficiency. The feedback control of Class-D amplifiers is designed to deliver a constant gain and phase shift over an audio frequency range of 20 Hz to 20 kHz in presence of L-C filter mismatch, loudspeaker dynamics, and input supply variations. However, fixed-frequency current-mode control suffers from sub-harmonic instability for such wide duty ratio operation and requires adaptive external ramp compensation for stabilization. This digest explores a constant OFF-time digital current-mode control which achieves fast tracking performance and constant gain and phase shift over an audio frequency range with non-idealities in circuit parameters. Further, the event-based sampling mechanism in constant OFF-time modulator achieves inherent current-loop stability over wide duty-ratio, which eliminates the requirement of external ramp compensation. The small-signal control-to-output transfer function of the current-mode controlled Class-D audio amplifier with the approximated loudspeaker impedance is derived and verified using the SIMPLIS simulations. The Class-D audio amplifier hardware prototype is made using a 80V GaN-devices and the proposed method is implemented using FPGA platform and the test results demonstrate the proposed control scheme.