as2521t austriamicrosystems, as2521t Datasheet - Page 4

• Manufacturer Part Number: as2521t
• Description: Telephone Speech Circuit With Loudhearing And Handsfree
• Manufacturer: austriamicrosystems

Preliminary
Functional Description
The AS252x contains all the voice circuits needed in
a high feature telephone instrument, i .e.:
The
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.
L +
Rev. 5.1
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
line
300
30
Z
B
adaptation
LLC
STB
V
Vss
CS
LS
DD
RI
LI
includes
CONVERTER
SWITCHING
EXTRACTION
LINE DRIVER
SYNTESIZER
CI
V
IMPEDANCE
CM
CONTROL
DD
POWER
ST-AMP
AS2520/21/20B/21B
DC
PD
line
SS
V
DD
driver,
PD
Figure 2: Block Diagramme
DETECTOR
LEVEL
V
LE
PP
INTERFACE
ac
MT or PD
LOGIC
Page 4
HS
RX-AGC
CONTROL
POWER
MT
VOICE
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.
TX-AGC
&
VOL
RING
MT
A
TI
GND
HS
INPUT
PDM
DETECTOR
LEVEL
A
MI-AMP
MI-AMP
RO-AMP
LO-AMP
V
AS2520/21/20B/21B
PP
Vss
P
LSI
M3
RTH
M1
M2
M4
A
RO
LO
GND
May 1999