16

DS690PP1

CS44130

4.4

Output Filter

The RC 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 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 (5.6

close as possible to the corresponding PWM output pin. This will 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 2 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 3 shows the component values for C2 based corner frequency (-

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

a ripple current rating above the amount of current that will pass 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 2. Low-Pass Filter Components - Half-Bridge

Corner Frequency

C2

36 Hz

1000 µF

54 Hz

680 µF

110 Hz

330 µF

Table 3. DC-Blocking Capacitors Values - Half-Bridge

Figure 5. Output Filter - Half-Bridge

PW M

Output

560 pF

C1

5.6

Ω

L1

C2

+-

*Diode is Zetex

ZHCS400 or

equivalent

VP