TDA7491LP Datasheet, PDF (29/37 Pages) STMicroelectronics – 2 x 5-watt dual BTL class-D audio amplifier

TDA7491LP
Applications information
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 32 and
Figure 33 below.
Figure 32. Typical LC filter for an 8-Ω speaker
Figure 33. Typical LC filter for a 4-Ω speaker
5.6.2
Filterless modulation
TDA7491LP can be used without a filter at the IC outputs, because the frequency of the
TDA7491LP 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 34 on page 30 shows the simplified schematic adopted for the test and
the relevant emission curve at full output power.
Doc ID 13541 Rev 5
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