Preliminary
AS2520/21/20B/21B
Telephone Speech Circuit
with Loudhearing and Handsfree
Austria Mikro Systeme International AG
Key Features
u
Line/speech circuit, loudhearing, handsfree and
dc/dc converter on one 28 pin CMOS chip
u
Operating range from 13 to 100 mA (down to 5
mA with reduced performance)
u
Soft clipping control eliminating harsh distortion
u
Volume control of receive signal with squelch and
automatic loop gain compensation
u
Line loss compensation pin selectable
u
Low noise (max. - 72 dBmp)
u
Real or complex impedance adjustable
u
NET 4 compatible
u
Dynamically controlled voice switching
u
Same monitor amplifier for loudhearing, handsfree
and tone ringer
u
Very few external components
u
Power derived from ring signal by switching
converter during ringing
General Description
The AS2520/21/20B/21B are CMOS integrated
circuits that contain all the audio functions needed to
form a high comfort, line-powered telephone.
The devices incorporate line adaptation, speech
circuit, loudhearing and handsfree - all supervised by
the novel voice and power control circuit. A switching
converter is also provided for converting the ring
signal. The interface to a dialler/controller is made
very simple to allow easy adaptation to a telecom
microcontroller.
The AS2520 series incorporate volume control for the
earpiece and the loudspeaker (AS2520 digital with +/-
keys and AS2521 analogue with potentiometer). The
volume control circuit automatically compensates the
loop gain to ensure acoustic stability.
Typical Application
La
Package
Available in 28 pin SOP and DIP.
Lb
3V
1
4
7
2
5
8
0
3
6
9
#
DIALLER
µCONTROLLER
LCD DRIVER
TELEPHONE
SPEECH CIRCUIT
WITH
LOUDHEARING,
HANDSFREE,
DC/DC CONVERTER
HSM
*
HFM
AS2520
Figure 1: Typical Handsfree Telephone Application
Rev. 5.1
Page 1
May 1999
Preliminary
Pin Description
Pin #
1
2
3
Name
LS
CI
RO
Type
AI
AI
AO
Description
Line Current Sense Input
This input is used for sensing the line current.
Complex Impedance Input
AS2520/21/20B/21B
Input pin for the capacitor in the complex impedance.
Receive Output
This is the output for driving a dynamic earpiece with an impedance of 140
to 300 ohm.
Positive Voltage Supply
This is the supply pin for the circuit.
Analogue Ground
This pin is the analogue ground for the amplifiers.
Side Tone Balance Input
This is the input for the side tone cancellation network.
Line Loss Compensation Selection Pin
-6 dB from 45 mA to 75 mA;
LLC = V
DD
: High range
-6 dB from 20 mA to 50 mA;
LLC = A
GND
: Low range
gain independent of line current;
LLC = V
SS
: No regulation
Loudspeaker Amplifier Input
This is the input for applying the receive signal to the loudspeaker
amplifier.
Tone Input
This switchable input is intended for transmitting DTMF or other signals
like messages on TAMs (Telephone Answering Machines) onto the line in
off-hook conditions and when in ringing mode to apply a PDM signal to the
loudspeaker (see also table 1).
Receive Threshold Input
The sensibility of the receive peak detector can be adjusted by applying
the signal from RO to the RTH input through a voltage divider.
Converter Make Output
This is an output for controlling the external switching converter. It
converts the ring signal into a 4V supply voltage and is activated when PD
= high and HS, LE, MT = low.
Loudspeaker Power Supply
High power supply for the output driver stage.
Output for Loudspeaker
Output pin for an ac coupled 32
Ω
(25 to 50
Ω
)loudspeaker.
Negative High Power Supply
This pin is the negative high power supply for the loudspeaker amplifier.
Mute Input
Dialling mute input (see also table 1).
MT = V
DD
: Tx and Rx channels muted;
MT = V
SS
: Tx and Rx channels not muted.
Power Down Input
Input for powering down the speech circuit and loudhearing/handsfree
(see table 1).
Loudhearing Enable Input
Input for enabling loudhearing/handsfree, active high (see table 1).
Handset Switch Input
This is an input that is pulled high by the hook switch (handset) or µC
when off-hook (see table 1).
Page 2
May 1999
4
5
6
7
V
DD
A
GND
STB
LLC
Supply
Supply
AI
DI
8
LSI
AO
9
TI
AI
DI
10
RTH
AI
11
CM
AO
12
13
14
15
V
PP
LO
V
SSP
MT
Supply
AO
Supply
DI
16
PD
DI
17
18
LE
HS
DI
DI
Rev. 5.1
Preliminary
19
22
20
21
23
M1
M2
M4
M3
VOL
AI
AI
D/AI
AS2520/21/20B/21B
Microphone Inputs
Differential inputs for handset microphone (electret).
Handsfree Microphone Inputs
These are the input pins for the handsfree microphone (electret).
Volume Control Input
Volume control for the receive signal.
AS2520: Digital control with +/– keys or from µC;
AS2521: Analogue dc control with potentiometer.
Supply Source Control Output
This N-channel open drain output controls the external high power source
transistor for supplying (V
PP
) the loudspeaker amplifier in off-hook
loudhearing/handsfree mode.
Current Shunt Control Output
This N-channel open drain output controls the external high power shunt
transistor for the modulation of the line voltage and for shorting the line
during make period of pulse dialling.
Negative Power Supply
Line Input
This input is used for power extraction and line current sensing.
Receive Input
This is the input for the receive signal.
AI:
AO:
AI/O:
Analogue Input
Analogue Output
Analogue Input/output
24
SS
AO
25
CS
AO
26
27
28
DI:
DO:
DI/O:
V
SS
LI
RI
Supply
AI/O
AI
Digital Input
Digital Output
Digital Input/Output
Operating Modes
I/O Pins
MODE
Idle (on-hook)
Ringing
POT
POT/pulse dialling
POT/DTMF dialling
Handsfree
Handsfree/pulse dial
Handsfree/DTMF dial
Loudhearing
Loudhearing/pulse dial
Loudhearing/DTMF dial
TAM without LSP
TAM with LSP
Melody feedback
Test mode 1
Test mode 2
HS
0
0
1
1
1
0
0
0
1
1
1
1
1
0
0
0
Digital Inputs
LE
0
0
0
0
0
1
1
1
1
1
1
0
1
1
0
0
PD
0
1
0
1
0
0
1
0
0
1
0
1
1
1
0
1
MT
0
0
0
1
1
0
1
1
0
1
1
0
0
0
1
1
Table 1: Operating Modes
Rev. 5.1
Page 3
May 1999
Tone Input
TI
Not connected
PDM signal to LO (DI)
Not connected
Not connected
DTMF to LI and RO (AI)
Not connected
Not connected
DTMF to LI and RO (AI)
Not connected
Not connected
DTMF to LI and RO (AI)
Signal to LI (AI)
Signal to LI (AI)
PDM signal to LO (DI)
CM
Low
SW
Low
Low
Low
Low
Low
Low
Low
Low
Low
Low
Low
Low
LI
-
-
‘M1/M2’
V
BE
‘TI’
‘M3/M4’
V
BE
‘TI’
‘M1/M2’
V
BE
‘TI’
‘RI/STB’
‘RI/STB’
‘RI/STB’
‘LSI’
‘LSI’
‘RI/STB’
Outputs
RO
PD
-
‘RI/STB’
-
‘TI’
‘RI/STB’
LO
PD
‘TI’
-
-
-
‘LSI’
-
‘LSI’
‘LSI’
Reserved for testing
Reserved for testing
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
Preliminary
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
LS
= V
LI
+ I
LINE
ž R1
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).
8
(V)
7
AS2520/21/20B/21B
(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
PEAK
. Dual means that the soft
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.
Page 5
May 1999
VLS
6
5
4
3
2
1
0
0
10
20
30
40
50
Line Current
60
70
80
90
(mA)
100
VLI
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
Rev. 5.1
京公网安备 11010802033920号
EEWORLD
