Preliminary Data Sheet

AS2504

4/12

August 1998

Functional Description

Modes of operation

There are 3 possible modes:

Idle Mode: AS2504 is disconnected from the

a/b-line (= no line current) and VDD is not

supplied by auxiliary power. All internal circuits

are powered down. Input of digital control

signals is not possible.

Line Power Mode: AS2504 is connected to the

a/b-line and VDD is generated by the internal

power

extraction.

The

speech

circuit

is

powered up. The line interface circuit is

functional (DC-mask and AC-impedance are

synthesised). Input of digital control signals is

possible.

Aux. Power Mode: AS2504 is disconnected

from the a/b-line (= no line current) and VDD is

supplied by auxiliary power. The speech circuit

is powered up. The line interface circuit is not

functional

(DC-mask

and

AC-impedance).

Input of digital control signals is possible.

Start-up in Line Power Mode

As soon as AS2504 is supplied with line

charged up via the LI- and VDD-pin. After the

2V, the line interface circuit and the selected

signal path of the speech circuit is switched on.

This off-hook status is signalled to the internal

logic via the LS-pin (voltage level sensitive).

Start-up in Aux. Power Mode

As soon as AS2504 is supplied with auxiliary

power at the VDD-pin, the selected signal path

of the speech circuit is switched on. The line

interface circuit (DC-mask and AC-impedance)

is not functional. This on-hook status is

signalled to the internal logic via the LS-pin

(voltage level sensitive).

DC conditions

The normal operating mode is from 15mA to

100mA. An operating mode with reduced

performance is from 5mA to 15mA. In the line

hold range from 0mA to 5mA the device is in a

power down mode and the voltage at LI is

reduced to a maximum of 3.5V

The DC characteristic is determined by the

voltage at LI-pin and a 30

Ω resistor between

LI- and LS-pin. It can be calculated by the

Ω. The

t.m. voltage at the LI-pin is 4.5V. The

calculation leads to the following DC

resistances: 330

Ω to 75Ω at line currents from

15mA to 100mA.

With a HIGH at the MASK-pin, the speech

circuit and other parts of the device, which are

not needed for operation, are in power down

mode in order to save current. The CS-pin is

transistor fully on. This guarantees a low

voltage drop (<1V) at the LI-pin during make

periods of pulse dialling or flash.

2/4 wire conversion

AS2504 has a built-in dual Wheatstone bridge

with one common ground. This provides a

maximum of independence of AC-impedance

and side tone from each other. One can adapt

side tone without changing the AC-impedance.

AC-impedance

The AC-impedance of AS2504 is set to t.m.

1000

Ω. With the external capacitor at CI-pin it

can be programmed complex. With an external

resistor of approx. 1.5k

Ω connected to the LS-

pin it can be programmed to 600

33 * 30

Ω.

Side Tone

A good sidetone cancellation can be achieved

by using the following equation:

Transmit path

The gain of the M1/M2 —> LS is set to 37dB.

The input is differential with an impedance of

20k

Ω. The soft clip circuit limits the output

30µs/6dB and the decay time is 20ms/6dB.

The gain of the M3/M4 —> LS is set to 43dB.

The input is differential with an impedance of

10k

Ω. The soft clip circuit limits the output

30µs/6dB and the decay time is 20ms/6dB.