User Guide

After Sales
Technical Documentation
Handsfree Desktop Charger CHH–7
Original, 11/94 Page 21
Watchdog Function
Under normal operating conditions the processor is pulsating the
WATCHDOG–line in order to reset the power–off function of regulator
N102. If the processor is not functioning correctly and hence the regulator
does not send the pulse to the XPWROFF–line within the 100ms timeout
period, the regulator reverts to its power–off state resetting the processor
and switching the VAREF and +VAN voltages off. The regulator and the
processor can be restarted by pressing the discharge button (S300) once
or switching the power supply (connected to X100) off and on. The supply
voltage (+VA) for the regulator must be greater than 8.1V in order for the
regulator to switch on.
Switch Mode Power Supply (SMPS)
C100 and C101 are the EMI capacitors. L100 and L101 are EMI
suppressor chokes. V100 is the voltage suppressor (zener) diode.
The microprocessor D300 pin 2 starts the SMPS by starting reference
regulator N103. V115–1 leads the initial current to totally empty the
battery. V106 is a constant current source which sets the voltage value of
R118. The same voltage appears across the current shunt resistor R103
when the SMPS is working. N101–a is a pulse width modulator. N101–b,
together with R122, C103 and V114–1, generates a minimum pulse width.
R127 and R521 are input and output voltage correction resistors. V103
limits the comparator power supply +VS. V115–2 and V372 limits the
comparator output voltage. V341–1 and C341 form a bootstrap circuit,
which acts like a voltage doubler and generates the positive gate voltage
to switch the fet V102 gate. L111 (L109), R130 and C340 are
EMI–suppression components. L108 is the main SMPS inductor. V110
limits the maximum output voltage.
Battery Charging
Charging current for the phone battery is supplied when the processor
switches the SMPS on after the phone is connected to the system
connector X701 on the DC4H module. The charger voltage should be
between 13.5V and 21.0V. The phone battery’s charging state is indicated
by the red (V344 / V345) and green (V333) LEDs. Because the phone
and the spare battery cannot be charged simultaneously, the charging
current of the phone must be detected: this is achieved by a differential
amplifier (N250–A) and corresponding resistors.
Charging current for the spare battery is fed in via switching transistor
V221 and Schottky diode V220. The transistor is controlled by the
processor. When the phone battery is not being charged and hence the
spare battery is allowed to charge, the transistor is on and a constant
current is supplied to the battery (the red LED V331 lights). In the pulsed
charging mode, charging current is adjusted by pulsing this charge current
(the green LED V330 lights).