2.1-channel applications using two devices

AN4569

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DocID026803 Rev 1

3.1.5

Main filter

Figure 16. Schematics for main filter section

The purpose of the main filter is to cut off the frequency above the audible range of 20 kHz.

The main filter is designed using the Butterworth method to define the cutoff frequency. In

order to have a clean amplifier, a cutoff frequency above 20 kHz is mandatory.

For class D amplifiers, the output load is a part of the filter. The following formulas are used

to verify the values of LC components.

Equation 5

2.1 channels, two devices - main filter

The following table shows the recommended values for the components.

Table 3. Component values for 2.1-channel applications, two devices

8 Ω

6 Ω

4 Ω

3 Ω

L1S, L2S, L3S, L4S, L1M, L2M

22 μH

15 μH

10 μH

10 μH

C18S, C19S, C13M

470 nF

680 nF

1 μF1 μF

C11

C12

C13

1

2









-----------------------------------------------------

====

L1S

L2S

L3S

L4S

L1M

L2M

2





2





----------------------------------------------

=====

=

=

1

2











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=