Multitone RPR 551IS, Техническое руководство

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RPR 550IS Series
Page 3 - 16 TM1188 Issue 1
LCD 5V Power Supply
52. When the display module is to be enabled, microprocessor IC3 generates a low signal
from pin 11 enabling TR19b via R73. Transistors TR4 and TR22 subsequently
conduct. A self-oscillating converter is formed by TR22 and L4. On negative cycles
C39 charges via D10b. On positive cycles the voltage rises on both plates of C39 by
the same amount to maintain the potential difference between them. Diode D10a
anode potential becomes more positive than the cathode and it conducts charging
C54.
53. Voltage control of the output is by negative feedback from TR21b and TR19a. TR21a
provides a reference voltage. Any changes affecting the voltage on the base of TR21b
cause TR21b collector to alter. This changes the V
BE
of TR19a and, via TR19b and
TR4, the amount of base drive of oscillator TR22. Diodes D4 and D5 infallibly limit the
output voltage of the convertor under fault conditions.
Low Battery Indication
54. When the battery voltage falls below a certain level (nominally 1.2V) the battery low
chip, IC7, generates a signal to microprocessor IC3 pin 44 which activates visual and
audible warnings to the user.
ABSENCE RACK REGISTRATION AND CHARGING
55. When fitted with a rechargeable cell and inserted into an absence/charging rack D12
and R11 allow charging current to flow through the external contacts into the battery.
The reverse biasing of the diode prevents battery voltage from being present on the
external contacts. If the pager is switched off, placing it in the rack automatically turns
it on.
56. The EEPROM is programmed by microprocessor IC3 with data being entered from the
absence rack connections via R67 and R65.