tea1085t NXP Semiconductors, tea1085t Datasheet - Page 10

• Manufacturer Part Number: tea1085t
• Description: Listening-in Circuit For Line-powered Telephone Sets
• Manufacturer: NXP Semiconductors

Philips Semiconductors
Power down input (PD) pin 19
During pulse dialling or register recall (timed loop break)
the telephone line is interrupted, thereby breaking the
supply to the transmission and listening-in circuits. The
capacitor connected to V
listening-in circuit during the supply breaks.
By making the PD input HIGH during the loop break the
requirement on the capacitor is eased and, consequently,
the internal (standby) current consumption I
V
transmission circuit is not affected transistors TR1 and
TR2 are inhibited and the bias current is reduced from
equation:
I
(where 4.2 V
2V
Ra = an internal resistor of typical 60 k
Larsen limiter current stabilizer (SIC) pin 17
A current reference is set by resistor R36 between SIC and
V
current is given by the following equation:
March 1992
SUP(PD)
handbook, full pagewidth
BB
SS
0.4 mA to
Listening-in circuit for line-powered
telephone sets
d
. The preferred value is 120 k . The internal reference
is reduced from 3.8 mA to 400 A typical. So that the
= the voltage drop across 2 internal diodes ( 1.3 V)
= I
BIAS(PD)
55 A with V
V
SUP
= (V
V M
C23
C22
V
SUP
BB
BB
SUP
provides the supply for the
Fig.8 Larsen limiter preamplifier and voltage/current converter.
R26
R25
2V
3 V)
= 4.5 V in the following
d
) / Ra
LAI
LAI
R28
R27
R29
BBO
V BB
(Fig.4) at
QLA
V QLA
V BB
V SS
10
C24
I
Changing the value of R36 will affect the timing of the
Larsen level limiter system.
Larsen limiter preamplifier (LAI1/LAI2 and QLA) pins
9/10 and 11
This circuit amplifies the microphone signal to a level
suitable for the Larsen limiter detector. The gain is set by
external components (see Fig.8).
Normally the gain is set to the same level as the
microphone amplifier of the transmission circuit, this
ensures that the output signal level at output QLA is equal
to the line signal level.
The gain between QLA and the microphone input is given
by the following equation (the high-pass filter is not taken
into account):
A
R25 = R26 = 10 k
The gain can be adjusted between 30 dB (R29 = 316 k )
and 52 dB (R29 = 4 M ). The impedance result of R28 and
R27 in parallel must be equal to R29
(e.g. R27 = R28 = 2
SIC
pre
= 1.25 / R36; when R36 = 120 k , I
R30
R31
= V
QLA
DTI
/ V
M
= R29 / R26; in the basic application
LLC
R29).
TEA1085; TEA1085A
THL1
R35
DETECTOR
I DCA
LARSEN
C25
Preliminary specification
THL2
DCA
R32
R33
R34
MGR037
SIC
= 10.5 A