Preliminary

AS2520/21/20B/21B

Rev. 5.1

Page 4

May 1999

Functional Description

The AS252x contains all the voice circuits needed in

a high feature telephone instrument, i .e.:

• line adaptation (ac impedance, dc characteristics,

2/4-wire conversion, power extraction)

• handset speech circuit

• loudhearing with enhanced anti-Larsen

• handsfree with dynamic loop gain control

• switching converter

The

line

adaptation

includes

line

driver,

ac

impedance (return loss), 2 to 4 wire converter, dc

mask and power extraction circuit for extracting the

maximum dc power from the line to supply the whole

device and peripheral circuits.

The handset speech circuit consists of a transmit and

a receive path with mute, dual soft clipping and line

regulation (pin option). A volume control is provided

with squelch and loop gain compensation to improve

signal-to-noise ratio and to assure acoustic stability.

Loudhearing

and

handsfree

functions

are

also

provided. The loudhearing function includes an anti-

Larsen circuit to prevent acoustic howling.

The handsfree circuit has a novel voice control

system

which

is

virtually

independent

of

any

background noise and works in a dynamic half

duplex mode as close to full duplex as the acoustic

loop gain allows.

The switching converter is used to extract the

available power from the ring signal and provides a

4V supply voltage. This allows the same loudspeaker

to be used for loudhearing/handsfree and tone

ringing.

LINE DRIVER

IMPEDANCE

SYNTESIZER

POWER

EXTRACTION

DC

CONTROL

VOICE

&

POWER

CONTROL

TX-AGC

RX-AGC

LE

HS

TI

LO

RO

RTH

M2

M1

CI

LI

CS

LS

Vss

RI

STB

MT

MT or PD

30

Ω

L +

AS2520/21/20B/21B

MT

PD

PD

LEVEL

DETECTOR

300

Ω

SS

SWITCHING

CONVERTER

CM

LSI

VOL

LOGIC

INTERFACE

LLC

LEVEL

DETECTOR

ST-AMP

RO-AMP

LO-AMP

MI-AMP

M3

MI-AMP

HS

M4

A

PDM

INPUT

RING

Figure 2: Block Diagramme