Preliminary
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
AS2520/21/20B/21B
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.
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.
CI
30
Ω
L+
SS
V
PP
M1
MI-AMP
TX-AGC
M3
LEVEL
DETECTOR
MI-AMP
M4
LEVEL
DETECTOR
RTH
A
GND
M2
LI
LINE DRIVER
IMPEDANCE
SYNTESIZER
CS
POWER
EXTRACTION
MT or PD
HS
LLC
V
DD
LS
DC
CONTROL
VOICE
&
POWER
CONTROL
A
GND
RO-AMP
V
DD
300
Ω
Vss
RI
STB
PD
V
DD
RO
AS2520/21/20B/21B
ST-AMP
RX-AGC
MT
V
PP
A
PDM
INPUT
LO-AMP
LSI
LO
Z
B
SWITCHING
CONVERTER
LOGIC
INTERFACE
RING
CM
PD
LE
HS
MT
VOL
TI
Vss
P
Figure 2: Block Diagramme
Rev. 5.1
Page 4
May 1999
AMSCO [ AUSTRIAMICROSYSTEMS AG ]
