TDA7350 datasheet(17/22 Pages) STMICROELECTRONICS

17 / 22 page

TURN-OFF

A turn-off pop can occur if the st-by pin goes low

with a short time constant (this can occur if other

car radio sections, preamplifiers,radio.. are sup-

plied through the same st-by switch).

This pop is due to the fast switch-off of the inter-

nal current generator of the amplifier.

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 con-

stant 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 :

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

The suggested time constants are :

T > 120ms with Cout=1000

µF,RL = 4ohm,stereo

T > 170ms with Cout=2200

µF,RL = 4ohm,stereo

If Rext is too low the Csvr can become too high

and a different approach may be useful (see next

section).

Figg 40, 41 show some types of electronic

switches (

µP compatible) suitable for supplying

the st-by pin (it is important that Qsw is able to

saturate with VCE

≤ 150mV).

Also for turn off pop the bridge configuration is su-

perior, in particular the st-by pin can go low faster.

GLOBAL

APPROACH

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,ra-

dios etc. and transmitted by the power amplifier to

the loudspeaker.

A simple approach to solving these problems is to

use the mute characteristics of the TDA7350.

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 42 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 spuri-

ous spikes that are not transmitted to the loud-

speaker. 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

43. 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 44.

Figure 40

Figure 41

TDA7350A

17/22

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