TDA7491LP datasheet(29/37 Pages) STMICROELECTRONICS

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TDA7491LP

Applications information

Doc ID 13541 Rev 5

29/37

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.

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