t
Theory of Operation-2445A/2455A Service
before it
is
applied to voltage-reference
IC
U1290.
The
+2
.5 V output from
pin
2 of U1290
is
applied to the nonin-
verting input of operational amplifier U 1300C.
The
output
of U
1300C
is
the source of the + 1 0 V reference
level
used
by the various regulators. The output
level
is
set by
the voltage divider formed by
R1291,
R1293,
and
poten-
tiometer
R1292.
The Volt
Ref
Adjust pot
in
the
divider
allows the reference
level
to
be
precisely set. Zener diode
VR1292 prevents the reference from exceeding +
11
volts
should a failure
in
the reference circuitry occur.
+
87
V Regulator
The
+87
V Regulator
is
composed of 01220, 01221,
01222, 01223, U1281A,
and
their associated components.
The
circuit regulates
and
limits both the voltage
and
current of the supply output.
Initially,
as
power
is
applied, the voltage applied to
pin
2
of
U1281
A from the voltage divider formed by
R1227
and
R 1228
is
lower than the + 1 0 V reference
level
applied to
pin
3.
The
output of U1281A
is
forced
high,
reverse bias-
ing
the base-emitter junction of 01222
and
tu
ming it com-
pletely off. With 01222 off,
all
the current through
R1212
is
supplied
as
base
current to Darlington transistor pair
01221
and
01220,
and
maximum current flows
in
series-
pass transistor 01220.
This
charges
up
the various loads
on
the supply
line,
and
the output
level
charges positive.
As
the regulator output charges toward +
87
V,
the
voltage divider applies a positive-going voltage to the
inverting input of
U1281
A.
When
the output
level
reaches
+87
volts, the inverting input
reaches
the
+10
V refer-
ence
at the noninverting input. The output voltage at
pin
1
of U1281A will
go
negative
and
the base-emitter junction
of 01222 will
be
biased into the active region.
As
01222
turns on, base drive for the Darlington pair (01221
and
pass transistor 01220)
is
reduced.
The
output
will
be
held
at the
level
required ( +
87
V)
for voltage at the two inputs
of amplifier U1281A to
be
in
balance.
Current limiting is a foldback design
and
is
performed
by 01223
and
its associated components. Under normal
current demand conditions, 01223 is off. If the regulator
output current exceeds approximately 1 00mA
(as
it might
if a component
fails),
the voltage drop across
R1221
and
CR1220 reaches a point that forward biases 01223
via
the
bias
divider formed
by
R1222
and
R1223
.
As
01223
turns
on
, a portion of the base-drive current
to
01221 is
shunted away
by
01223. This reduces the base-drive
current
(and
thus the output current) of series-pass
transistor 01220.
3-44
II
+ 42 V Regulator
The
circuit configuration
and
operation of the +
42
V
Regulator
is
identical to that of the +
82
V Regulator.
Current limiting of the
+42
V supply occurs at approxi-
mately 400 mA. Base drive to Darlington pair 01241
and
01240
is
via
R1244
and
is
dependent
on
proper operation
of the +
87
Volt Regulator. This dependency ensures that
the relative polarities of the two supplies
are
never
reversed (preventing semiconductor -junction damage
in
the
associated load circuitry).
+ 15 V Regulator
The
+
15
V Regulator
uses
three-terminal regulator
U 1260
and
operational amplifiers U
1371
A
and
U
1371
B,
arranged
as
voltage sensors, to
achieve
regulation of the
+ 15 V supply. The three-terminal regulator holds its out-
put voltage at
pin
2 at 1
.25
volts more positive than the
reference input
level
at
pin
1.
The voltage at the reference
pin
is
established by current flow
in
either diode CR1262
or CR1263.
Resistors
R1261
and
R1262 at the regulator output
divide the +
15
V
level
down for comparison with the + 10
V reference applied to
pin
5 of operational amplifier
U1371B.
When
the
input voltage at
pin
6 (supplied by the
voltage divider)
is
lower than the + 10 V reference, the
output of amplifier U1371B is
high
and
the output voltage
of U1260
is
allowed to
rise.
As
the regulator output
reaches +
15
V,
the voltage
on
pin
6 of U
1371
B
approaches the
level
on
pin
5,
and
the amplifier begins
sinking current away from the reference
pin
of the three-
terminal regulator
via
diode CR1263. This lowers the volt-
age
on
the reference
pin
and
holds the output at +
15
V.
The
other voltage-sensing amplifier (U1371A) ensures
that
the
relative polarity between the + 15 V supply
and
the
+42
V supply
is
maintained, preventing component
damage
in
the
load
circuitry. Should the
+42
V supply
be
pulled
below +
15
V (excessive loading or supply failure),
the voltage at
pin
3 of U1371A falls below the voltage at
pin
2
and
the amplifier output voltage goes low. This for-
ward biases CR1262
and
lowers the reference voltage for
U1260, reducing the output voltage.
Current limiting for the + 15 V supply is provided
by
the
internal circuitry of the three-terminal regulator.
+ 5 V Regulator
Regulation of the + 5 V supply
is
provided
by
a circuit
similar to those of the
+87
V
and
the
+42
V Regulators.
As
long
as
the relative polarity between the +
15
V
and
the
+5
V supplies is maintained, base drive to 01281 is
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