16

DS786A1

CS4412A

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.

Each output pin must be connected to two Schottky diodes—one to ground and one to the VP supply.

These diodes should be placed within 12 mm of the corresponding OUTx pin. The requirements of this

diode are:

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 4 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.

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 4. Low-Pass Filter Components - Half-Bridge

OUTx

220 pF

C1

20

Ω

L1

C2

+-

*Diode is Rohm

RB160M-30 or

equivalent

VP

Figure 5. Output Filter - Half-Bridge