Preliminary

AS2520/21/20B/21B

Rev. 5.1

Page 5

May 1999

DC Conditions

The normal operating range (off-hook) is from 13 mA

to

100

mA.

Operating

range

with

reduced

performance is from 5 mA to 13 mA (parallel

operation).

In

the

normal

operating

range

all

functions are operational.

In the line hold range from 0 to 5 mA the device is in

a power down mode and the voltage at LI is reduced

to maximum 3.5V.

The dc characteristic (excluding diode bridge) is

determined by the voltage at LI and the resistor R1 at

line currents above 13 mA as follows:

V

The voltage at LI is 4.5V.

Below 13 mA the AS252x provides an additional

slope in order to allow parallel operation (see figure

3).

100

90

80

70

60

50

40

30

20

10

0

0

1

2

3

4

5

6

7

8

(V)

VLS

VLI

(mA)

Line Current

Typically

No ac signals

Tamb: 25°C

Figure 3: DC Mask

When the PD pin is high (during pulse dialling) the

speech circuit and other part of the device not

operating are in a power down mode to save current.

The CS pin is pulled to V

SS

in order to turn the

external shunt transistor on to keep a low voltage

drop at the LS pin during make periods.

AC Impedance

The synthesised ac impedance of the circuit is set on

chip and by an external resistor and an external

capacitor (for complex impedance).

When R1 is set to 30 ohm, the ac impedance is 1000

ohm real, and the complex part can be set by a

capacitor connected to pin 2 (CI).

For 600 ohm telephones it is recommended to

connect a resistor and a capacitor from pin LS to V

SS

(see application notes). The dc resistance of R1

should be kept at 30 ohm to ensure correct dc

condition.

Return loss and sidetone cancellation can be

determined independent of each other (see figure 4).

Speech Circuit

The speech circuit consists of a transmit and a

receive path with soft clipping, mute, line loss

compensation and sidetone cancellation.

Transmit

The gain of the transmit path is 36.5 dB in handset

mode (from M1/M2 to LS) and 46.5 dB in handsfree

mode (from M3/M4 to LS). The microphone inputs

have an input impedance of 15 kohm.

The unique dual soft clipping control circuit limits the

output voltage at LI to 2V

clipping incorporates both a very fast control circuit to

eliminate harsh sidetone distortion and a slower

regulation circuit to limit the output voltage at 2V

PEAK

independent of the line impedance. The attack time

is 30 µs/6 dB. The overdrive range is 30 dB. When

mute is active, pin MT high, the gain is reduced by >

60 dB.

Receive

The gain of the receive path is 3 dB (test circuit

figure 8) from RI to

RO. The receive input is the

differential signal of RI and STB. Also the receive

channel provides soft clipping to avoid acoustic

shock and harsh distortion.

When mute is active during dialling the gain is

reduced by > 60 dB. During DTMF dialling a MF

comfort tone is applied to the receiver. The comfort

tone is the DTMF signal with a level that is -30 dB

relative to the line signal.

Volume Control

On the AS2520 the receive gain can be changed by

pressing the volume keys. The + key increases the

gain by 10 dB in 5 steps and the – key decreases the

gain by 10 dB in 5 steps. The gain is reset by next

off-hook. The volume can also be controlled via a

microcontroller.

The AS2521 uses a potentiometer to control the

receive gain. The volume is an indirect dc control to

avoid that noise is introduced from the potentiometer.

The volume control is common for both the earpiece

and

the

loudspeaker.

Any

increase

will

be

compensated to ensure acoustic stability.