Preliminary
The speech compression allows a higher gain in the
transmit channel, i.e. the microphone gets more
sensitive at low sound pressure levels on the
microphone, which enables the user to move further
away from the telephone. This means that a constant
signal is provided on the line practical independent of
the microphone signal level. Any reduction of gain by
the compressor in the transmit channel will
automatically be given to the receive channel.
Switching Converter
The ac ringing signal is utilised to extract the power
necessary to the tone ringer circuit. A switch mode
power supply is used to obtain a high efficiency dc
conversion.
This approach allows the use of the same
loudspeaker and amplifiers for both loudhearing and
tone ringing. It also allows an acoustic feedback of
the melodies during programming with the same
sound pressure level as during ringing.
When a ringing signal is applied, PD is pulled high
and the oscillator is enabled. The switching converter
is controlled by the output CM, which is turned high
and low with a duty cycle controlled by the voltage at
V
PP
.
When off-hook the switching converter has a high
impedance (CM low) to avoid any influence on the
transmission and on pulse dialling.
AS2520/21/20B/21B
The smoothing capacitor should be in the range of 10
to 68 nF. The choke coil must have an inductance of
>1mH and a dc resistance of < 15 ohm.
1µ5
La
510
Lb
1µ5
510
33 n
30V
BC
327
5k6
10 k
BC
547
2.2 mH
CM
V
PP
470 µ
5V1
V
PP
Vss
P
Figure 7: Switching Converter
Tone Input
The tone input is a digital input in ringing mode and
during melody feedback. The digital melody signal
(PDM = pulse density modulation) is directly applied
to the TI input (see also application notes for further
details).
During DTMF dialling the DTMF signal is applied
through a capacitor to the TI input and will be fed to
the line (pin LI) and to the receive output (RO) as
confidence tone.
Rev. 5.1
Page 8
May 1999
AS252x
AMSCO [ AUSTRIAMICROSYSTEMS AG ]
