User Guide

After Sales
Technical Documentation
System Module
THX–4
Amendment 10/97 Page 4–13
Charging Control
The circuit is basically a power switch, V403, which connects an external
constant current source to the battery. V403 is driven by a modified long
tail pair (emitter–coupled pair) which provides a safety mechanism to stop
excessive voltages being placed across the phone’s internal circuitry. This
long tail pair consists of V420 and V421 in a negative feedback
configuration, with the control input applied to the base of V421 and the
feedback to the base of V420. The potential divider R465 // R514 and
R464 is used to scale the voltage on the anode of diode V508 and place it
on the base of V420. By long tail pair action, the divided voltage will be
the same as the base voltage of V421, and the voltage applied across the
battery is therefore clamped. Note, however, that this clamping only takes
place under fault conditions.
When V403 is switched on, the parallel combination of R467 and R468
provide sufficient base current to ensure that it is in a state of saturation.
The switching circuit is normally controlled by the microprocessor’s CSW
output (P94–SCK2_PW13) which drives V421 via smoothing circuit R466,
C461 and R470. However, at power up (i.e. the insertion of the battery or
of a charger), the PSL overrides it for a period of about 1s by holding the
XRES line low.
The voltage at which the battery is clamped can be set to one of three
levels by using the PAV_EN line to switch V406B and the XRES line to
switch V406A. When charging normally, the processor’s CSW line is at
4.8V nominal and the resultant maximum voltage on the battery will be
11.2V. During a call, however, PAV_EN is asserted and the effect of R456
on the division ratio is consequently removed, causing the limiting voltage
on the battery to become 8.8V. During reset, the Zener diode V506
places 2.6V on the base of V421 and the battery voltage will then not be
able to exceed 6.5V.
Once fully charged, the CSW line is switched using PWM for battery
maintenance.
Display Lighting
Control of the keyboard display lighting is performed by V700, R716 and
R713; control of the LCD lighting is performed by V701, R705 and R716.
All lighting is enabled by a high level on pin 44 (P71) of D404.
Audio Circuit Description
Transmit (TX) Signal Path
The TX audio signal is processed in the NANTIC and fed, via the MOD
line, to the synthesizer. NANTIC contains the following stages for TX
signal processing: