U4090B-P Datasheet, PDF (9/35 Pages) ATMEL Corporation – Monolithic Integrated Feature Phone Circuit EMI Improved

U4090B-P
There are four major supply states:
1. Speech condition: In speech condition the system is supplied by the line current. If the
LIDET-block detects a line voltage above the fixed threshold (1.9 V), the internal signal
VLON is activated, thus switching off RFD and RPC and switching on all other blocks of
the chip. At line voltages below 1.9V, the switches remain in their quiescent state as
shown in Figure 3-4 on page 10.
OFFSACOMP disables the group listening feature (SAI, SA, SACL, AFS) below line
currents of approximately 10 mA.
2. Power down (pulse dialing): When the chip is in power-down mode (PD = high), e.g.,
during pulse dialing, the internal switch QS shorts the line and all amplifiers are
switched off. In this condition, LIDET, voltage regulators and IMPED CONTR are the
only active blocks.
3. Ringing: During ringing, the supply for the system is fed into V_B via the ringing power
converter (RPC). The only functional amplifiers are in the speaker amplifier section
(SAI, SA, SACL).
4. External supply: In an answering machine, the chip is powered by an external supply
via pin V_B. This application allows the possibility to activate all amplifiers (except the
transmit line interface TXA). Selecting IMPSEL = high impedance activates all switches
at the ES line.
3.2 Acoustic Feedback Suppression
Acoustical feedback from the loudspeaker to the handset microphone may cause instability in
the system. The U4090B-P offers a very efficient feedback suppression circuit, which uses a
modified voice switch topology. Figure 3-4 on page 10 shows the basic system configuration.
Two attenuators (TX ATT and RX ATT) reduce the critical loop gain by introducing an externally
adjustable amount of loss either in the transmit or in the receive path. The sliding control in block
ATT CONTR determines, whether the TX or the RX signal has to be attenuated. The overall loop
gain remains constant under all operating conditions.
Selection of the active channel is made by comparison of the logarithmically compressed TX-
and RX- envelope curve.
The system configuration for group listening, which is realized in the U4090B-P, is illustrated in
Figure 3-6 on page 11. TXA and SAI represent the two attenuators, the logarithmic envelope
detectors are shown in a simplified way (operational amplifiers with two diodes).
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