5·14
Functional
Description
Portable
PLUS
Computer
There are three open-drain logic outputs from the VCC regulator: they pull
down
to
OV
to represent
~
logic 0
and
are open circuits to represent logic
1.
Their function
is
to feed status information to the
PPU circuitry. CARDSIN* pulls
down
to
OV
to indicate that both
PLUGINl*
and
PLUGIN2* are
grounded, meaning that cards are plugged into both slots. A high impedance at CARDSIN* indicates
to the
PPU that at least one card_has been removed,
and
that the PPU should not allow the user to
~
-
-
~-------'-
-
-
-
~
-
--
---
--
_._--
-----
_
turn
the
~
uter
on.
1-
0W
BATTERY
pulls
down
to
OV
to indicate that the battery voltage
is
above 1
T
5:"
8v.-wnen
the battery voltage goes below
S.8V,
LOW
BATTERY
becomes a high impedance, indicat-
ing to the
PPU
that a low-battery message should be presented to the user. SHUTDOWN is low to
indicate that the battery voltage
is
above~,
and
is
high impedance to indicate that the battery I
voltage
is
below S.
6V.
When the
PPU
sees SHUTDOWN become a high impedance, it forces the
computer into sleep mode to avoid battery damage from
over-dischargi
..D
g.
~
______
__
_
-,
Two
other signals connect the vee regulator with other circuits.
VREF
is
a 1.23SV ± 1 % reference
voltage provided
by
the vce regulator for the
LCD
negative supply circuit.
VFETBIAS
is
a negative
voltage provided
by
the LCD negative supply for the VCC regulator while the computer is awake.
When
in sleep mode,
VFETBIAS
is
set to
OV.
One
regulator circuit provides both supply output voltages. The output of this circuit
is
VccDS. The
regulator
is
a linear design: a single pass transistor (A2Q63, connected between
VBAT
and
VccDS)
is
controlled
by
a differential amplifier which turns on the proper amount to maintain VccDS
at
SV
in
awake
mode
(or 3.
2SV
in sleep mode). As the current required by the load increases, the differential
amplifier provides more base current to A2Q63. This circuit
is
current-efficient: at high current loads,
the only battery current
not
delivered to the load
is
the A2Q63 base current.
The differential amplifier consists of A2QS9,
A2Q60, A2Q61, A2Q62,
and
associated resistors.
VREF,
a 1.23SV reference voltage,
is
provided to the base of A2Q61. Note that the current in A2R71 is
essentially
zero, so there
is
no voltage across the resistor. When the circuit
is
stable, the voltage
at
the
base
of
A2Q62
is
also
VREF.
Resistors A2R79, A2R80,
and
A2R81 form a voltage divider that determines the output voltage
of
the
VccDS regulator. When the computer
is
in sleep mode, SLEEP
is
at logic 1, A2R80
is
effectively in
parallel with A2R81,
and
VccDS
is
regulated to
3.2SV.
When the computer is awake, SLEEP is at logic
0, A2R80
is
effectivly in parallel with A2R79, and VccDS
is
regulated to Sv. (Note that the base
current of A2Q62
is
not zero,
and
this affects the operation of the voltage divider.)
The other supply output,
VccS,
is
merely the output of a transistor switch connected to VccDS. When
SLEEP
is
high (3.2SV), A2Q70
is
turned
off. The
Vgs
of
-1.62V
(determined
by
the voltage divider
between
SLEEP
and
VFETBIAS)
is not negative enough to turn
on
the transistor. When SLEEP is low,
VFETBIAS
is in the range
-7V
to
-13V
as provided by the LCD negative supply circuit. The voltage
at
A2Q70 gate is
-3.SV
or lower. This with a source voltage of
+SV
at A2Q70 gives a Vgs of
-8.SV
or
lower, sufficient to
turn
on A2Q70.
If
PLUGINl*
or PLUGIN2*
is
not grounded, A2QSS or A2QS4, respectively,
turn
on.
If
either tran-
sistor turns
on
while the computer
is
in awake mode, A2QS6 turns on,
putting
the vce regulator into
the
shutdown
state. The voltage
at
the emitters of A2Q61
and
A2Q62
is
pulled above
VREF
through
A2R121, which
damps
off both transistors. This turns off the differential amplifier
and
A2Q63, the
pass transistor. Within the computer circuitry there are current paths between
VBAT
and
VccDS
other
than
the VCC regulator. These current paths would hold VccDS up at about
IV
in the
shutdown
state
if it weren't for A2Q88. Bias current
is
provided through A2R121
and
A2R78 to
turn
on
A2Q88
when
the vce regulator
is
in the
shutdown
state, which holds VccDS
down
to less
than
O.
IV
.
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