AN1013 Просмотр технического описания (PDF) - STMicroelectronics
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AN1013 Datasheet PDF : 13 Pages
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AN1013 APPLICATION NOTE
Figure 18. Effect of inductance and Capacitance on Damping
Output
Underdamped
Overdamped
Fig.4
Frequency
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 ⋅
2π
Rload
⋅ Fc
L2
=
1.5 ⋅ Rload
2π ⋅ Fc
C1
=
2π
⋅
1.33
Rload
⋅
Fc
For Rload = 8ohm, Fc = 30Khz resulted L1 = 20µH, L2 = 63µH, C = 0.88µF
Figure 19
Figure 20. 3rd order Butterworth filter
L2
L1
C1
RL
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.
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