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TEA1085

Listening-in circuit for line-powered telephone sets

Philips Electronics 

Philips Semiconductors

Listening-in circuit for line-powered

telephone sets

Preliminary specification

TEA1085; TEA1085A

Dynamic limiter (DLC) pin 23

To prevent distortion of the signal at the loudspeaker

outputs the gain of the amplifier is reduced rapidly when:

• the peaks of the signal at the loudspeaker outputs

exceed an internally determined threshold (voltage

limiter)

• the DC current into SUP is insufficient (current limiter)

• the voltage at VBB decreases below an internally

determined threshold, typically 2.9 V (VBB limiter)

The time in which the gain reduction is effected is the

'attack time'; this is very short in the first and third instance

and relatively long in the second instance. The circuit will

remain in the gain-reduced condition until the peaks of the

output signal remain below the threshold level. The gain

will then return to a nominal level after a time determined

by the capacitor connected to DLC (release time).

MUTE input (MUTE) pin 20; TEA1085A

This MUTE is provided with a logic input to operate with a

microcontroller for instance.

The loudspeaker amplifier is disabled when the MUTE

input is LOW (connected to VSS or open input). A HIGH

level at the MUTE input enables the amplifier in the

listening-in mode.

MUTE input (MUTE) pin 20; TEA1085

The MUTE function is provided with a toggle input and is

designed to switch between the standby condition and the

listening-in condition on the rising edge of the input MUTE

signal (see Fig.6).

In the basic application the MUTE input must be LOW

(connected to VSS). A simple push-button can be used to

operate the MUTE toggle (see Fig.7). Debouncing can be

realized by means of a small capacitor connected between

MUTE and VSS.

An internal start circuit ensures that the circuit always

starts up in the standby condition.

handbook, full pagewidth LSI1

MUTE

QLS1

standby

listening-in

standby

Fig.6 Mute toggle function of the TEA1085.

MGR036

handbook, full pagewidth

March 1992

VBB

MUTE

(a) Break contact.

10 kΩ

MUTE

MLA055

(b) Make contact.

Fig.7 Mute switch alternatives with the TEA1085.

9

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