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SA2531 A/B/C/E/G/U

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FUNCTIONAL DESCRIPTION

Power On Reset

The on chip power on reset circuit monitors the supply voltage (V

approx. 1.2V, a power on reset occurs to assure correct start-up and the RAM is cleared.

DC Conditions

The normal operating range is from 13mA to 100 mA.

Operating range with reduced

performance is from 5mA to 13mA. In the operating range all functions are operational.

At line currents below 13mA the SA2531 provided an additional scope below 4.5V to allow

parallel operation. (See Figure 12).

The dc characteristic (excluding diode bridge and Pulsing transistors) is determined by the

voltage at LI and the resistor R1 as follows:

VLS = VLI + I

The voltage at LI is 4.5V.

During pulse dialling the speech circuit and other parts of the device not required are in a

power down mode to save current. The CS pin is pulled to V

shunt transistor on to keep a low voltage drop at the LS pin during make periods.

AC Impedance

The Characteristic or Output impedance of the SA2531 is set within the IC and adjusted by

Mask Options. Available options are for 600

Ω and 1000Ω. When the 1000Ω option is selected

then a capacitor may be added to the circuit at pin CI to add a reactive element and make the

output impedance complex.

Oscillator

All the Timing Functions of the SA2531 are based on a Clock Frequency of 3.58MHz. A crystal

or ceramic resonator of this frequency should be connected to the OSC pin. In practise minor

deviations from the nominal frequency may occur due to the characteristics of the frequency

reference device used and so it is recommended that care is taken in the selection of

components.

Typically a small value capacitor (

≤47pF) should be connected in parallel with the Frequency

Reference to ensure start-up and/or operation at the nominal frequency.

Speech Circuit

The speech circuit consists of a transmit and a receive path born with soft clip, mute, line loss

compensation and side tone cancellation.

Transmit

The gain of the transmit from M1/M2 to LS is 35dB for 600

Ω versions and 37dB for 1000Ω

versions (see test circuit figure 5). The microphone input is differential with an input

impedance of 25 k

Ω.

The soft clip circuit limits the output voltage at LI to 2.0V

time is 30µs/6dB and the decay time is 20 ms/6 dB. When mute is active, during dialling