TDA1905 datasheet(10/14 Pages) STMICROELECTRONICS

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MUTING FUNCTION

The output signal can be inhibited applying a DC voltage VT to pin 4, as shown in fig. 29

Figure 29

The input resistance at pin 5 depends on the threshold voltage VT at pin 4 and is typically :

R5 = 200 K

Ω @ 1.9V ≤ VT ≤ 4.7V

muting-off

R5 =

Ω

≤ VT ≤ 1.3V

muting-on

≤ VT ≤ Vs

Referring to the following input stage, the possible attenuationof the input signal and therefore of the output

signal can be found using the following expression:

Considering Rg =10 K

Ω the attenuation in the

muting-on condition is typically AT = 60 dB. In the

muting-off condition, the attenuation is very low,

typically 1.2 dB.

A very low current is necessary to drive the thresh-

old voltage VT because the input resistance at pin

4 is greater than 150 K

Ω. The muting function can

be usedin many cases, when a temporaryinhibition

of the output signal is requested, for example:

– in switch-on condition, to avoid preamplifier

power-on transients (see fig. 22)

R8 • R5

+ 5

)

(

R8 • R5

+ R5

)

where R8

≅ 100 KΩ

– during switching at the input stages.

– during the receiver tuning.

The variable impedance capability at pin 5 can be

useful in many applications and two examples are

shown in fig. 25 and 27, where it has been used to

change the feedback network, obtaining 2 different

frequency responses.

TDA1905

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