User Guide

After Sales
Technical Documentation
Desktop Charger CHH–6
Page 13Original, 11/94
Watchdog Circuit
The processor generates positive pulses to output PC7 (pin 18). These
pulses are received by transistor V160 and discharging capacitor C161.
The capacitor is charged through resistor R163. This voltage is fed to
regulator N100 shutdown input (pin 3) through transistor V162. If the
processor stops, the voltage across the capacitor increases and the
regulator is switched off.
5V Internal Supply
An accurate 5V internal voltage supply is provided by N100 a precision
regulator. The output of this may be switched off using the shutdown input
(pin 3), which is controlled by the watchdog circuit. The input to the regulator
is filtered by R107, R108 and C105, which removes noise due to the phone
operation.
Flat Battery Operation
When a very flat battery is fitted (ie. voltage <4.3V) diode V129 is turned ON,
charging the battery with a very low current: the current is limited by R109 and
R110.
The charge transistor V130 must be turned off during this low voltage
period, else the input to the voltage regulator, N100, is pulled low, causing
the output voltage to drop.
The low charge current via diode V129 is sufficient to rapidly change the
voltage of a flat battery, since the battery has a high input impedance in
this low charge state.