Datasheet L3281AD1, L3281AB Datasheet (SGS Thomson Microelectronics)

LOW VOLTAGE TELEPHONE SPEECH CIRCUITS

OPERATIONDOWNTO 1.6V / 6.5mA
DTMF & BEEP TONE INPUTS
EXTERNAL MUTING FOR EARPHONE AND

MICROPHONE
SUITABLE FOR DYNAMIC EARPHONE AND

DYNAMIC OR ELECTRETMICROPHONE
AGC CONTROL ON BOTH SENDING AND

RECEIVING

DIP14

L3281

PRELIMINARY DATA

SO14

DESCRIPTION

The L3281 is an electronic speech circuit devel­oped to replace hybrid circuits in telephone sets
that can be operated in parallel with other
phones.

BLOCK DIAGRAM

ORDERING NUMBERS:

L3281AB L3281AD1

June 1993

This is advancedinformationon a new product now in development or undergoing evaluation. Details are subject to change without notice.

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L3281

PIN CONNECTION (top view)

ABSOLUTE MAXIMUM RATINGS

Symbol Parameter

V

L

I

L

P

tot

T

op

T

j

Line Voltage (3 ms pulse) 15 V
Line Current 150 mA
Total Power Dissipation, T
Operating Temperature – 20 to 55 °C
Junction Temperature – 65 to 150 °C

THERMAL DATA

Symbol Parameter

R

th j-amb

Thermal Resistance Junction Ambient Max 90 130 °C/W

Value

DIP-14 SO-14

=55°C 1.0 0.6 W

amb

Value

DIP-14 SO-14

Unit

Unit

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TEST CIRCUITS
Figure1.

L3281

Figure2. Figure3.

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L3281

ELECTRICAL CHARACTERISTICS IL= 20 to 100mA; R4 =(51Ω // diode) + 33Ω;

T=25°C; f =1kHz; Unless OtherwiseSpecified

Symbol Parameter Test Condition Min. Typ. Max. Unit

V

l

Line Voltage IL= 6.5mA

I

= 20mA

L

I

= 50mA

L

I

= 80mA

L

CMRR Common Mode Rej. Ratio 50 dB

DG

THD

DG

THD

MU
MU

G
R

THD

G
R

THD

I

G

N
Z
G

N
Z

Z
V

I

beep
beep

V

leak

tx

tx

ml

rx

rx

ro

m

so

ro

mf

mf

beep

z

Sending Gain Vmi= 10mV; IL= 20mA 30 31.5 33 dB
Delta Sending Gain Vmi= 10mV; IL= 70mA – 7.2 –5.7 –4.2 dB

tx

Sending Distortion Vso= 700mV; IL= 20mA 5 %

tx

Sending Noise Vmi= 0V; IL= 50mA – 70 dB
Mic. Input Impedance Vmi= 10mV 40 kΩ
Receiving Gain IL= 20mA; Vri= 0.2V – 10.7 – 9.2 – 7.7 dB
Delta Receiving Gain IL= 70mA; Vri= 0.2V – 7.2 – 5.7 – 4.2 dB

rx

Receiving Distortion Vro= 350mV; Load = 350Ω

rx

V

= 300mV; IL=10mA

ro

Receiving Noise Vri= 0V 100 µV
Rec. Output Impedance Load = 200Ω;Vro= 50V 10 Ω
Sidetone V

= 10mV 10 20 dB

mi

Line Match. Impedance Vri= 0.2V 500 600 700 Ω
Sending Output Voltage IL= 6.5mA; THD = 5% 100 mV
Receiving Output Current IL= 6.5mA; THD = 5% 0.5 mAp
Mute InputLow Dialing Mode 50 100 µA

lo

Mute InputOpen Speaking Mode 1 µA

hl

DTMF Gain V

= 10mV 14.5 16 17.5 dB

mf IN

DTMF Input Impedance 5 10 kΩ
DTMF Distortion V

mf

Beeptone Gain V

= 140mV 5 %

mf LN

= 25mV 8.5 dB

beep IN

Beeptone Input Impedance 5.5 8 kΩ
Beeptone Distortion V

= 100mV; IL= 20mA 0.5 5 %

beep IN

Zener Voltage (Pin 5) Iz= 1 mA 4.2 5.1 6.2 V
Leakage Current,V

=3V 20 µA

pin5

1.65

3.4

6.0
8

3.7

6.5

9.5

5
5

V
V
V
V

%
%

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LOGIC OF MUTE SWITCHING

MUTE DTMF BEEP MIC IMP RX IMP

LOW ACTIVE TO LINE ACTIVETO EARPHONE MUTED MUTED
(DIAL) OUTPUT OUTPUT
OPEN ACTIVE TO LINE ACTIVETO EARPHONE ACTIVE ACTIVE

(SPEECH) OUTPUT OUTPUT

L3281

CIRCUITDESCRIPTION

TWOTO FOUR WIRE CONVERSION
The L3281AB is based on a Wheastone bridge

configuration.To balance the bridgethe following
relation mustbe satisfied:

ZI / /

Zm

R2

=

Zb

R3

Figure4: 2/4 Wire Conversion

TheAC signal from the microphone is sent to one
diagonalof the bridge (pins 1 and 3). A small per­centage of the signal power is lost on Zb (being
Zb > (Zm//Zi)); the main part is sentto the line via
R2.

In receivingmode, the AC signal coming from the
LINEis sensed across the second diagonal of the
bridge(pins12 and 2).

Theimpedance Zm and Zb can be complex.

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L3281

DCCHARACTERISTIC

The fig.5 shows the equivalentsimplified circuit of
the DC regulator that provides to give the oppor­tuneDC impedanceZdc.

+ 1

= 2















⋅Z4

+ VD+ VR1

+ VD + VR1




Idc

⋅Z4



VL =



RB





VL =

Idc ⋅Z4)⋅

(




sinceRA =RB

VL =

When IL 18 mA and considering neglectible the
VD + VR1 variation versus line current :

ZDC =

At IL = 6.5 mA no current flows through Z4 but
only in the rest of the circuit for internal biasing
(Io;Ia). The bias current Io is fixed by the resistor
R2. The line voltage in thiscase is :

(RA + RB)

⋅



RA



RB



(Idc⋅Z4 ⋅2) + VD + VR1

∆ VL
∆ Idc

equivalent.
Thezener voltage will be:



Vz =

It is possible to supply 1mA to the electrete volt­age if VL> (1mA + Iz) ⋅ Rz + Vz

Figure6: LowVoltageSpeech Circuit.

Figure7: ZenerEquivalent.

70K



13.6K



+ 1



⋅Vbe




VL = Ia RA + VR1 = 1.6 V

The Fig.6 shows the DC characteristic (voltage
between pin 12 and pin 3 versus line current).
The device own an equivalent zener voltage at
pin 5 that can be used as supply voltage for elec­tret microphone(see Block Diagram).

The value of the resistor R2 and the capacitor C2
should be chosen in order to not affect the AC
line inpedance. The Fig.7 shows the zener

Figure5: EquivalentSimplified Circuit

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L3281

ACCHARACTERISTIC

The AC Impedance measured at line terminals is
equalto:

Zm =(R1

The value of the capacitor C1 must be In the
rangeof 22 µF to 100 µF.

The external resistor R1 canbe replacedby a re­sistor/capacitornetworkin order to realizea com­plex Impedance Zm.

TRANSMITTING CIRCUIT

The first block of the TXstage is basically a differ­ential amplifier which converts voltage to current.
The inputs are internally polarized at 300 mVdc.
The differential Input impedanceis 60 KΩ to allow

Figure8: EquivalentTransmitting Circuit.

+

1

) //(R2+ R3+ Zb )

jwC1

a good matching to microphone. The AGC in TX
is function of voltage at pin 14 in order to de­crease to max gain of 5.5dB to 6.0dB when the
linecurrent increases.

RECEIVING CIRCUIT

Fig.9 shows the equivalent receiving circuit. The
differential input of RX signal across R2+R3 is
transferred to the AGC block when the mute sig­nal (pin 10) is not active.

The AGC in RX is a function of the voltage at pin
14 and decreases the gain when the line current
increases(5.5dBto 6.0dB).

The final stage is a single ended amplifier with
low output impedance optimized to drive mag­netic/dynamictransducers.

Figure9: EquivalentReceivingCircuit.

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L3281

DIP14 PACKAGE MECHANICAL DATA

DIM.

MIN. TYP. MAX. MIN. TYP. MAX.

a1 0.51 0.020

B 1.39 1.65 0.055 0.065

b 0.5 0.020

b1 0.25 0.010

D 20 0.787

E 8.5 0.335

e 2.54 0.100

e3 15.24 0.600

F 7.1 0.280

I 5.1 0.201

L 3.3 0.130

Z 1.27 2.54 0.050 0.100

mm inch

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SO14PACKAGE MECHANICAL DATA

L3281

DIM.

A 1.75 0.069
a1 0.1 0.2 0.004 0.008
a2 1.6 0.063

b 0.35 0.46 0.014 0.018

b1 0.19 0.25 0.007 0.010

C 0.5 0.020
c1 45° (typ.)

D 8.55 8.75 0.336 0.344

E 5.8 6.2 0.228 0.244

e 1.27 0.050

e3 7.62 0.300

F 3.8 4.0 0.15 0.157

L 0.5 1.27 0.020 0.050
M 0.68 0.027
S8°(max.)

MIN. TYP. MAX. MIN. TYP. MAX.

mm inch

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L3281

Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no responsibility for the
consequences of use of such information nor for any infringement of patents or other rights of third partieswhichmay result from its use. No
license is granted byimplication orotherwise under any patent or patent rights ofSGS-THOMSON Microelectronics. Specifications mentioned
in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied.
SGS-THOMSON Microelectronics products are not authorized for useascritical componentsinlife support devices or systems withoutexpress
written approval of SGS-THOMSON Microelectronics.

 1994 SGS-THOMSON Microelectronics - All RightsReserved

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