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TDA7350A

If the voltage present across the load becomes rapidly zero (due to the fast switch off) a small pop occurs,

depending also on Cout,Rload.

The parameters that set the switch off time constant of the st-by pin are:

■

the st-by capacitor (Cst-by)

■

the SVR capacitor (Csvr)

■

resistors connected from st-by pin to ground (Rext)

The time constant is given by :

T

≈ Csvr · 2000Ω // Rext + Cst-by · 2500Ω // Rext

The suggested time constants are :

If Rext is too low the Csvr can become too high and a different approach may be useful (see next section).

Figg 43, 44 show some types of electronic switches (

µP compatible) suitable for supplying the st-by pin (it

≤ 150mV).

Also for turn off pop the bridge configuration is superior, in particular the st-by pin can go low faster.

5.4 Global Approach to Solving Pop Problem by Using the Muting/Turn On Delay Function

In the real case turn-on and turn-off pop problems are generated not only by the power amplifier, but also

(very often) by preamplifiers,tone controls, radios etc. and transmitted by the power amplifier to the loud-

speaker.

A simple approach to solving these problems is to use the mute characteristics of the TDA7350A. If the

SVR pin is at a voltage below 1.5 V, the mute attenuation (typ)is 30dB .The amplifier is in play mode when

Vsvr overcomes 3.5 V.

With the circuit of fig 45 we can mute the amplifier for a time Ton after switch-on and for a time Toff after

switch-off. During this period the circuitry that precedes the power amplifier can produce spurious spikes

that are not transmitted to the loudspeaker.

This can give back a very simple design of this circuitry from the pop point of view. A timing diagram of

this circuit is illustrated in fig 46.

Other advantages of this circuit are:

– A reduced time constant allowance of stand-by pin turn off. Consequently it is possible to drive all the

car-radio with the signal that drives this pin.

– A better turn-off noise with signal on the output. To drive two stereo amplifiers with this circuit it is

possible to use the circuit of fig 47.

Figure 43.