2.1-channel application using one device

AN4569

10/29

DocID026803 Rev 1

2.1.5

Main filter

Figure 6. Schematic 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 the LC components.

Equation 2

Main filter equation for SE and BTL

Following table show typical component value for standard output load.

Table 1. Component values for 2.1-channel, single-device applications

8 Ω

6 Ω

4 Ω

3 Ω

L1, L2

47 μH

33 μH

22 μH

15 μH

C11, C12

390 nF

470 nF

680 nF

1 μH

L3, L4

22 μH

15 μH

10 μH

10 μH

C13

470 nF

680 nF

1 μF1 μF

C11

C12

C13

1

2









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



L3

L4

2









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

====

=

=

=

L1

L2







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

===

1

2











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

=