Fig.4

Effect of Inductance and Capacitance on Damping

Underdamped

Overdamped

Frequency

Output

Figure 18. Effect of inductance and Capacitance on Damping

If a sharper cutoff is preferred, to attenuate better the carrier frequency Butterworth filters of the 3rd or

of the 4rth order could be used.

For example with a 3rd order filter (fig.19) the obtained frequency response is shown in fig.20.

The equations used to domension the passive components are the following:

L1

=

0.5

2

π

L2

=

1.5

2

π

C1

=

1.33

2

π

For Rload = 8ohm, Fc = 30Khz resulted L1 = 20

µH, L2 = 63µH, C = 0.88µF

RL

C1

L1

L2

Figure 19

Figure 20. 3rd order Butterworth filter

Components choice:

Not only the right choice of the LC network values is important, but it is also the type of the passive com-

ponents in the Class-D amplifier performances evenmore in terms of Losses and Harmonic distortion.

CAPACITOR: The losses in the filter capacitor due essentially to the ESR will be neglegible if Multilayer

film capacitors are used. Multilayer Mylar, Polypropilen or Policarbonate film capacitors are the re-

comended choice, we advise to avoid using ceramic capacitors for such kind of application.

INDUCTOR: The losses in the inductor at low frequencies are mainly due to the coil winding series re-

sistance. At higher frequencies, where the Skin Effect becomes of importance, a multiwire winding could

be effective to obtain the maximum in terms of efficiency. However for most of the applications the use

of a single wire winding with adequate cross section is enough.

The inductor used for the filter must sustain a DC current greater than the specified current limitation

value (see the datasheet of the device of interest for the value to consider) without saturation and the

coil material must show very low hysteresis losses.

AN1013 APPLICATION NOTE

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