SA2531 A/B/C/E/G/U
FUNCTIONAL DESCRIPTION
Power On Reset
The on chip power on reset circuit monitors the supply voltage (VDD). When VDD rises above
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 + ILine.R1
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 VSS 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 Characteristic or Output impedance of the SA2531 is set within the IC and adjusted by
MaskOptions. Availableoptionsarefor600Ω and1000Ω. Whenthe1000Ω optionisselected
then a capacitor may be added to the circuit at pin CI to add a reactive element and make the
output impedance complex.
Oscillator
AlltheTimingFunctionsoftheSA2531arebasedonaClockFrequencyof3.58MHz. Acrystal
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 600Ωversions and 37dB for 1000Ω
versions (see test circuit figure 5). The microphone input is differential with an input
impedance of 25 kΩ.
ThesoftclipcircuitlimitstheoutputvoltageatLIto2.0VPEAK (seefigures8&9). Theattack
time is 30µs/6dB and the decay time is 20 ms/6 dB. When mute is active, during dialling
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sa me s
SAMES [ SAMES ]
