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
AMSCO [ AUSTRIAMICROSYSTEMS AG ]
