CS4525 datasheet(54/91 Pages) CIRRUS

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DS726A1

CS4525

6.4

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).

6.4.1

Half-Bridge Output Filter

Figure 25 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 15 shows the component values

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

imately 35 kHz.

C2 is the DC-blocking capacitor. Table 16 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

Ω

22 µH

1.0 µF

Ω

33 µH

0.68 µF

Ω

47 µH

0.47 µF

Table 15. Low-Pass Filter Components - Half-Bridge

Load

Corner Frequency

Ω

40 Hz

1000 µF

58 Hz

680 µF

120 Hz

330 µF

Ω

39 Hz

680 µF

68 Hz

390 µF

120 Hz

220 µF

Ω

42 Hz

470 µF

60 Hz

330 µF

110 Hz

180 µF

Table 16. DC-Blocking Capacitors Values - Half-Bridge

OUTx

220 pF

Ω

Figure 25. Output Filter - Half-Bridge

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