User Manual

The TPA005D02 Audio Power Amplifier Evaluation Module

3-8

Details

3.2.3.2 Output Filter

Class D amplifiers require special filtering at the output to remove the

high-frequency switching component and accurately reconstruct the audio

signal. The output filter is a low-pass filter (LPF) which sets the high frequency

–3-dB point of the bandwidth. The major consideration here is how to choose

the components and set the desired –3-dB point.

Filter Design Goals

A second-order low-pass filter is used for the output filter. The Butterworth filter

is characterized by a flat response over the pass band and less attenuation

after the cutoff frequency. The order of the filter determines how many poles

exist that are at the same frequency, with each pole providing –20 dB per

decade of signal attenuation for a total of –40 dB per decade in this circuit. The

cutoff frequency (f

can be determined using the equation

(2 LC

)

where L is the inductance and C is the equivalent capacitance. The values

used in the output filter are 15 µH for the inductor, and 0.22 µF and 1 µF

capacitors in parallel for an equivalent capacitance of 2.22 µF, setting the

cutoff frequency to 27.5 kHz. The main purpose of this filter is to reduce the

switching frequency to an acceptable level and not attenuate the audio band.

The 250 kHz amplifier output signal is then reduced by –40 dB to one percent

of its pre-filter value.

The considerations for inductor selection are inductance, continuous and peak

current ratings, dc series resistance, and the packaging. The inductance was

chosen based on common inductance and capacitance values, to be 15 µH.

Class D Output Filter Design Methodology

The output filter attenuates the high switching frequency. A second-order

Butterworth low-pass filter was chosen for its flat pass band, good phase

response, and low parts count (it requires only an inductor and a capacitor).

The normalized transfer for the Butterworth filter is

H(s)

s 1

The next step is to realize the circuit and develop a transfer function. The filter

for a single-ended application is shown in Figure 3–6.

Figure 3–6. Single-Ended Class D Output FIlter

–