5-20
Functional
Description
Portable
PLUS
Computer
STEP1, STEP2, STEP4,
and
STEP8 are the four digital inputs to
the
LCD negative supply
and
deter-
mine its
output
voltage. These four
inputs
are provided by the PPU circuitry. Each
input
represents a
logic
0
when
pulled
down
to ground, or a logic 1
when
high impedance. The four
inputs
form a
binary value ranging from
0 to 15, with STEP8 the most significant bit. A value of 0 for the four
inputs causes
the
LCD negative supply to produce
-13V
(dark LCD). As the value of
the
four inputs
increments to
15, the
output
voltage changes in evenly-spaced steps
up
to
-8V
(light LCD). The
overall function is
that
of
aD/A
converter
that
ranges from
-13
to - 8 Vdc.
Two
miscellaneous connections to other circuitry are
VREF
and
VccS.
VREF
is a 1.235V ± 1 % refer-
ence voltage provided by the
VCC regulator circuitry. VccS is the switched supply voltage described
above in
the
VCC regulator section. VccS
is
used in on-off control of
the
LCD negative supply.
5.15.6
VLCD
Switch
Circuit
When
LCDON*
is driven low by the PPU, current flows through A2R108 pins 7
and
8 to
turn
on
A2Q76. A2Q76
then
passes current from
VccS
through A2R108, pins 5
and
6
to
turn
on
A2Q77. With
A2Q77
on, VLCD is pulled
down
to the negative voltage generated at VFETBIAS.
When
the
PPU
drives
LCDON*
high, A2Q76 is turned off. This interrupts
the
base current to A2Q77,
so
that
A2Q77
turns
off
and
VLCD is disconnected from
VFETBIAS.
5.15.7
VFETBIAS Supply Circuit
The circuit which produces VFETBIAS is a switching dc-to-dc converter, the basic parts
of
which are
an
oscillator, a transistor switch,
an
inductor,
and
a dc rectifier.
The
output
of
the oscillator is A2U52, pin
7.
When
this output goes low, it turns
on
the transistor
switch A2Q75. While the switch is
on, current from
VBAT
flows into inductor A2L50, storing energy.
As
long as A2Q75
is
turned on, the current
and
stored energy in A2L50 increase. The oscillator
output
eventually goes high, turning A2Q75 off, interrupting the current flow from
VBAT
through
A2L50.
Because current flow cannot stop instantaneously in an inductor, current begins to flow through
A2CR54 in the
forward
direction. This current discharges A2C62, so the voltage
at
VFETBIAS is nega-
tive. While A2Q75 is turned off, the current in
A2L50 decreases,
and
energy stored in A2L50 is
transferred to A2C62. Assume
that
the VLCD switch
is
on
, so the
LCD
loads the circuit: during
the
off-time of A2Q75, A2L50 supplies the load current as well as the current
to
charge A2C62 (nega-
tively). During the on-time of A2Q75, all the load current
is
supplied by A2C62. No current flows
from A2Q75 to
the
load because during the on-time A2CR54 is reverse-biased. A2CR54
and
A2C62
form the dc rectifier of the dc-to-dc converter. The frequency of oscillation
and
capacitance
of
A2C62
are selected so
that
there is little ac ripple voltage across the load during each cycle.
5.15.8
Variable Control
of
the
VFETBIAS Supply
VFETBIAS is approximately equal to -
VBAT
x (ton/toff)
under
proper loading conditions (where
ton
is .
the
'on'
time of A2Q75
and
toft is the
'oW
time). The function of
the
A2U52 oscillator is to provide
control of
ton
and
toft according to this formula, given
the
STEP8 through STEP1
inputs
.
A2U52 is a low-power comparator.
It
has
an
open-collector
output
(pin
7)
that pulls to
ground
when
the
voltage
at
the
'+'
input (pin
2)
is less
than
the
voltage
at
the
'-'
input
(pin 3).
When
the
'+'
input
is at a higher voltage
than
the'
-'
input,
the
output
becomes a high impedance
and
is
pulled
up
to
VBAT
by A2R110
and
A2R109.
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