The loop gain of this configuration is the following (fig. 13)

The feedback works properly only if fo (unity-gain frequncy) is lower than the switching frequency fsw.

The optimum behaviour is obtained with fo ~ 0.7fsw. For example, with fsw =120Khz, the capacitance

C4 (pin 8 DIP20, pin 5 MW15) is about 4.7nF.

If the switching frequency is changed, the C4 capacitance can be changed too to fit the following for-

mula:

1

2

⋅ π

R1

R1

⋅ C4

= 0.7 fsw

[12]

From [12] it is possible to calculate the value of C4 vs switching frequency fsw:

C4

=

1

2

⋅ π

R1

R1

+ R2

[13]

where:

R1 = 30K

Ω; R2 = 45KΩ;

=

R1

⋅ R2

R1

+ R2

= 18K

Ω

Av = 100;

fsw : switching frequency

With these values in [13], it is possible to obtain the final formula of C4 vs fsw to be used:

C4

= 5.05E −4

F

fsw

[14]

For example, with fsw =120Khz, with [14] C4 = 4.2nF.

A.1) Output LC filter dimensioning

The low pass filter placed after the switching stage is dimensioned to eliminate the high frequency PWM

waves and to feed the Audio signal to the loudspeaker.

The TDA7480/81 operate at a switching frequency of 120Khz, the LC low pass filter suggested in the ap-

plication circuit is a typical 2 pole Butterworth designed to have a cutoff frequency outside the Audio

band (fc = 30KHz).

This filter topology was chosen because it offers the feature to be maximally flat in the passband.

The filter frequency response will be flat only if it is properly loaded on the specified

loudspeaker impedance for example 8 ohm (fig.14),

Figure 13

AN1013 APPLICATION NOTE

9/13