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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

Supply; SUP, SREF, VBB, VSS and VA

The line current is divided into ITR for the TEA1060 and ISUP for the TEA1085/TEA1085A.

The supply arrangement is illustrated in Fig.4.

handbook, full pagewidth Iline

ISUP

SUP

ICC R1 ITR R20

Vint

LINE

VCC

LN

TEA1060

VEE

SLPE

SREF

TEA1085

TEA1085A

R9

TR1

VBB

TR2

IBBO

VOLTAGE

STABILIZER

C20

IBIAS

VSS

VA

R38

MGR034

ITR is constant: ITR = Vint / R20; ISUP = Iline − ICC − ITR

Fig.4 Supply arrangement.

Where:

Vint

R20

ICC

is an internal temperature compensated

reference voltage with a typical value of

315 mV between SUP and SREF

is a resistor between SUP and SREF

is the internal current consumption of the

TEA106X (≈ 1 mA)

A practical value for R20 is 150 Ω. This value of resistance

produces a value for ITR = 2 mA and ISUP = Iline − 3 mA.

The TEA1085/TEA1085A stabilizes its own supply voltage

at VBB. Transistor TR1 provides the supplies for the

internal circuits. TR2 is used to minimize the signal

distortion on the line by momentarily diverting the input

current to VSS whenever the instantaneous value of the

voltage VSUP drops below the supply voltage VBB. VBB is

fixed to a typical value of 3.6 V but can be increased by

means of an external resistor (R38) connected between

VA and VSS or decreased by connecting this resistor

between VA and VBB. The minimum level on VBB is

restricted to 3.0 V; the level of the VBB limiter is also

affected (see application report for further information).

The supply at VBB is decoupled by a 470 µF capacitor.

The DC voltage (VSUP − VSS) is determined by the

transmission IC (VLN−SLPE); thus:

VSUP − VSS = VLN−SLPE + Vint.

The minimum DC voltage that can be applied to this input

is VBB(max) + 0.4 V.

Where: VBB(max) is the worst case supply voltage (this

depends on the setting of R38, which is connected

between VA and VSS).

The internal current consumption of the

TEA1085/TEA1085A (ISUP0) is typically 4.2 mA (where

VSUP − VSS = 4.5 V, MUTE off). Thus the current available

for powering the loudspeaker is ISUP − ISUP0.

The current ISUP0 consists of a bias current of ≈ 0.4 mA for

the circuitry connected to SUP and current IBB0 of ≈ 3.8 mA

which is used for the circuitry connected to VBB (see Fig.4).

March 1992

7

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