TDA7491P Datasheet, PDF (34/42 Pages) STMicroelectronics – 2 x 10-watt dual BTL class-D audio amplifier

Applications information
TDA7491P
5.6.1
Reconstruction low-pass filter
Standard applications use a low-pass filter before the speaker. The cut-off frequency should
be higher than 22 kHz and much lower than the output switching frequency. It is necessary
to choose the L-C component values depending on the loud speaker impedance. Some
typical values, which give a cut-off frequency of 27 kHz, are shown in Figure 45 and
Figure 46 below.
Figure 45. Typical LC filter for an 8-Ω speaker
Figure 46. Typical LC filter for a 4-Ω speaker
5.6.2
Filterless modulation
TDA7491P can be used without a filter at the IC outputs, because the frequency of the
TDA7491P output is beyond the audio frequency, the audio signal can be recovered by the
inherent inductance of the speaker and natural filter of the human ear.
The reconstruction of the audio signal on the load is usually achieved using a complete LC
filter (such as a Butterworth) solution that guarantees good audio performance, high
efficiency and EMI suppression. The LC component values should be computed by
considering the target audio band and the PWM switching frequency. The cut-off frequency
must lie well below the switching frequency and above the upper audio frequency. In
particular, the following schematic gives a guideline for a cut-off frequency of about 30 kHz
for both 6- and 8-Ω speakers.
Thanks to its advanced modulation approach, aimed to improve both driving efficiency and
radiating emissions, the device is even able to drive a load with a very low component count.
With this cost-saving filtering scheme the TDA7491P complies with the EMI specifications
FCC class B. Figure 47 on page 35 shows the simplified schematic adopted for the test and
the relevant emission curve at full output power.
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