Reducing the output filter of a Class-D amplifier
Mike Score · 1999
A properly designed Class-D output filter limits supply current, minimizes EMI and protects the loudspeaker from switching waveforms. However, it also significantly increases the total design cost. The current recommended second-order output filter for the TI TPA005D02 is 30% of the audio power amplifier (APA) solution cost. This application note details the second-order Butterworth filter and two reduced filtering techniques, each providing a different price/performance node. The first alternative to the Butterworth filter reduces the output filter by half and the second option completely eliminates the filter. The filters were tested using Texas Instruments TPA005D02 Class-D APA. Design decisions based on the measured results and conclusions are drawn to provide practical solutions for applications with different price/performance nodes. Second-order Butterworth low-pass filter The second-order Butterworth low-pass filter is the most common filter used in Class-D amplifier applications. This filter uses two inductors and three capacitors for a bridged-tied-load (BTL) output, which can be seen in Figure 1. The primary purpose of this filter is to act as an inductor at the switching frequency. The inductor is needed to keep the output current constant while the voltage is switching. If the outputs do not see an inductive load at the switching frequency, the supply current will increase until the device is unstable. Higher inductance at the output yields lower quiescent current (supply current with no input) because it limits the amount of output ripple current. The filter protects the speaker by attenuating the switching frequency. The inductors, L1 and L2, and the capacitor C1 form a differential filter that has an attenuation slope of 40 dB per decade. The majority of the switching current flows through C1, leaving very little current to be dissipated in the speaker. The filter also reduces EMI. The previously described differential filter reduces differential radiation. Inductors L1 and L2 and capacitors C2 and C3 form common-mode filters that further reduce EMI. See the “TPA005D02 Class-D Stereo Audio Power Amplifier Evaluation Module User’s Guide” 1 for details on how the output filter was designed.