ILA1068 datasheet(3/11 Pages) ETC

Part #	ILA1068

Description	Versatile Telephone Transmission Circuit with Dialler Interface

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ILA1068

3

FUNCTIONAL DESCRIPTION

Supplies VCC, LN, SLPE, REG and STAB

Power for the IC and its peripheral circuits is usually

obtained from the telephone line. The ILA1068

develops its own supply at VCC and regulates its

voltage drop. The supply voltage VCC may also be

used to supply external circuits, e.g. dialling and

control circuits.

Decoupling of the supply voltage is performed by a

capacitor between VCC and VEE; the internal voltage

regulator is decoupled by a capacitor between REG

and VEE.

The DC current flowing into the set is determined by

the exchange voltage (Vexch), the feeding bridge

resistance, (Rexch) and the DC resistance of the

telephone line (Rline).

An internal current stabilizer is set by a resistor of

3.6 K

Ω between the current stabilizer pin STAB and

VEE (see Fig.1).

Figure 1. Supply arrangement

If the line current Iline exceeds the current ICC + 0.5 mA

required by the circuit itself (approximately 1mA)

plus the current Ip required by the peripheral circuits

connected to VCC, then the voltage regulator diverts

the excess current via LN.

The regulated voltage on the line terminal (VLN) can

be calculated as:

VLN = Vref + ISLPE x R9

VLN = Vref + ((Iline - ICC - 0.5 x 10

3)-I

p) x R9,

where Vref is an internally generated temperature

compensated reference voltage of 4.2 V and R9 is an

external resistor connected between SLPE and VEE.

The preferred value for R9 is 20

Ω. Changing the

value of R9 will also affect microphone gain, DTMF

gain, gain control characteristics, side-tone level, the

maximum output

swing

on

LN

and

the

DC

characteristics (especially at lower voltages).

Under normal conditions, when ISLPE >> ICC + 0.5 mA

+ Ip , the static behaviour of the circuit is that of a

4.2 V regulator diode with an internal resistance

equal to that of R9. In the audio frequency range, the

dynamic impedance is largely determined by R1 (see

Fig.2).

The internal reference voltage can be adjusted by

means of an external resistor (RVA). This resistor,

connected between LN and REG, will decrease the

internal reference voltage; when connected between

REG and SLPE, it will increase the internal reference

voltage. Current (Ip) available from VCC for supplying

peripheral circuits depends on external components

and on the line current.

Leq = C3 x R9 x RP

RP = 17.5K

Ω

Figure 2. Equivalent impedance circuit

Microphone inputs MIC+ and MIC- and gain pins

GAS1 and GAS2

The circuit has symmetrical microphone inputs. Its

input impedance is 64 K

Ω (2 x32 KΩ) and its voltage

gain is typically 52 dB (when R7 = 68 K

Ω, see

Figure 5).

Dynamic,

magnetic,

piezo-electric

or

electret (with built-in FET source followers) can be

used.

The gain of the microphone amplifier can be adjusted

between 44 dB and 60 dB. The gain is proportional to

the value of R7 connected between GAS1 and GAS2.

An external capacitor C6 of 100 pF between GAS1

and SLPE is required to ensure stability. A larger

value may be chosen to obtain a firs t-order low-pass

filter. The cut-off frequency corresponds to the time

constant R7 x C6.

VCC

V

EE

ICC

I

SLPE

I

SLPE

I

C

I

line

R

line

R

exch

V

exch

LN

0.5 mA

+0.5 mA

R1

R9

STAB

SLPE

REG

C3

R5

C1

+

-

peripheral

circuit

DC

AC

IlA1068

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