DS749A1

15

CS4412

4.4

Output Filters

The filter placed after the PWM outputs can greatly affect the output performance. The filter not only reduces

radiated EMI (snubber filter), but also filters high frequency content from the switching output before going

to the speaker (low-pass LC filter).

4.4.1

Half-Bridge Output Filter

Figure 5 shows the output filter for a half-bridge configuration. The transient-voltage suppression circuit

(snubber circuit) is comprised of a resistor (20

Ω, ¼ W) and capacitors (220 pF) and should be placed as

close as possible to the corresponding PWM output pin to greatly reduce radiated EMI.

The inductor, L1, and capacitor, C1, comprise the low-pass filter. Along with the nominal load impedance

of the speaker, these values set the cutoff frequency of the filter. Table 3 shows the component values for

L1 and C1 based on nominal speaker (load) impedance for a corner frequency (-3 dB point) of approxi-

mately 35 kHz.

C2 is the DC-blocking capacitor. Table 4 shows the component values for C2 based on corner frequency

(-3 dB point) and a nominal speaker (load) impedances of 4

Ω, 6 Ω, and 8 Ω. This capacitor should also

be chosen to have a ripple current rating above the amount of current that will passed through it.

Load

L1

C1

4

Ω

22 µH

1.0 µF

6

Ω

33 µH

0.68 µF

8

Ω

47 µH

0.47 µF

Table 3. Low-Pass Filter Components - Half-Bridge

PWM

Output

220 pF

C1

20

Ω

L1

C2

+-

*Diode is Zetex

ZHCS400 or

equivalent

VP

Figure 5. Output Filter - Half-Bridge