Preliminary

AS2520/21/20B/21B

Rev. 5.1

Page 8

May 1999

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

When off-hook the switching converter has a high

impedance (CM low) to avoid any influence on the

transmission and on pulse dialling.

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.

CM

2.2 mH

BC

327

470 µ

La

Lb

1µ5

1µ5

510

510

5k6

33 n

10 k

5V1

BC

547

30V

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.