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
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