Epson PX-8 Technical Manual page 45

REV.-A
2.2.5 DC Voltage Regulators
The MAPLE board is powered by the main or auxiliary +5V Ni-Cd battery. For circuits such as an
I/O control section which requires different operating voltages and any special section which re-
quires a larger current, however, voltage regulators are used to convert the battery voltage to the
required voltages and prevent the circuit operation from being affected from a voltage drop due to
use of large amount of current. The internally used voltage regulators are summarized below:
(1) +5V regulator
Purpose: ROM capsule power source.
Control: Enabled when the ROM capsule is reread.
Output voltage: +5V
(2) LCD drive source regulator
Purpose:
Control:
LCD drive power source.
Always enabled.
Output voltage: -1 5V (The LCD is actually driven by a voltage of 20V obtained using
the potential difference from the +5V supply.)
(3) RS-232C level source regulator
Purpose: Sources for the RS-232C levels of ± 8V.
Control:
Output voltage:
Enabled only when the RS-232C or serial interface is operated.
±8V
The individual regulators are detailed in the following:
2.2.5.1 +5V Regulator
This regulator supplies power to the ROM capsule. When accessed, the ROM generates such a
large transient current that, if it were directly powered by the battery, a momentary low voltage
condition would occur due to a voltage drop along the power line, precluding normal operation.
To prevent this, the regulator is provided as a power buffer. The circuit operation is detailed as
follows (Refer to Fig. 2-14):
SWPR +5V
• The SWPR signal is low when power is off
and is inverted high at pin 10 of the IC
120. This signal maintains transistor 028
in conduction, holding the collector low.
Thus, the switching signal fed from pin 2
of IC 140 does not appear at the lower ter-
minal of the capacitor C28 (collector of
028) and the transistor 019 is cut off,
generating no output voltage .
• When power is off, transistor 028 is cut
off by the high level of the SWPR signal,
and a clock signal of approximately 35 kHz
is fed to pin 14 of IC 240. This causes a
pulse signal at the collector of the transis-
tor 019, which repeats, switching transis-
tor 05 on and off.
2-21
+s
7
120
10K
6~
10 3OK R72
Oscillator
circuit
35 kHz clock
R90
68K 12
Q047 C39
R73
30K
Constant-voltage
circuit
converter
= =======t==~~=GND
VB
Fig. 2-14 +5V Regulator Circuit