Design Support


This application report investigates reducing and eliminating the LC output filter traditionally used in class-D audio power amplifier applications. The filter can be completely eliminated if the designer is using a predominatly inductive speaker; however, the supply current and the EMI are higher than if using the full second-order Butterworth low-pass filter. The designer can use half of the components in the originally recommended second-order Butterworth low-pass filter. The half and no filter class-D applications outperform the full second-order Butterworth filter in total harmonic distortion plus noise (THD+N) and intermodulation distortion (IMD). This document shows speaker requirements with and without a filter, fidelity and electromagnetic interference (EMI) results, and indicates what type of filter fits various system requirements.