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CHAPTER 3
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POWER SUPPLY BOARD
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3.1 GENERAL
The Power Supply Board controls power regulation and distribution to
the system, and also carries signals from the System board to the three
external device connectors and buzzer element.
Figure 3.1 is a schematic of the board circuit diagram. Connections
through to the slots are dealt with in section 4, and this section
concentrates on the power supply functions.
The board has been designed to operate under a wide range of
conditions. Supply voltage from the battery or external source may vary
between 5.5 and 11 volts. Regulated supplies are required at current
levels between 30 microamp and 170 milliamp in various operating states.
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3.2 POWER SUPPLY REQUIREMENTS
The power supply provides five supply rails to various parts of the
system :-
- The VCC1 rail supplies the main System board circuit, and is
required at all times. When the Organiser is off this rail
supports the real-time clock and system board RAM, and a rail
voltage of 3-5 volts is required with a load of typically 30
microamp. When the Organiser is on, this rail must be regulated
to 5 volt +/- 5 percent under loads up to 25 mA.
- The VCC2 rail supplies the system board LCD drivers. It is not
required when the Organiser is off, and when on should be
regulated to 5 volts +/- 5 percent with a load of typically 1 mA.
- The V_LCD rail is the negative voltage rail for the System board
LCD. It is not required when the Organiser is off, and when on
should be adjustable by the thumbwheel to provide a V_LCD voltage
of between +1.2 and -3 volts to set display contrast, with a load
of typically 100-200 microamp.
- The SVCC rail supplies the external slot devices. It is only
required when the slots need to be active, and so is switched
under software control. When on it should be regulated to 5 volts
+/- 5 percent with variable loads up to 170 mA.
- The SVPP rail is a secondary rail to slots 1 and 2 of the
Organiser. It is only required when the slots need to be active,
and is switched with SVCC. When on it is normally at a voltage of
4.5 volts, but when programming datapacks can be raised to 21
volts +/- 2 percent in a pulsed mode.
The power supply provides and switches these rails under control of
the system board, from a battery or external supply with a voltage of
between 5.5 and 11 volts.
The main operating states are defined by the ON_B and PACON_B signals
from the system board. ON_B is high when the Organiser is off, and is
pulled low by the control IC to switch the system on. PACON_B is
controlled directly by software from the processor port 6, and is pulled
low to switch the slot rails on. The main operating states are summarised
in the following table :-
ON_B PACON_B VCC1 VCC2 V_LCD SVCC SVPP
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1 high high 3-5 off off off off
2 low high 5 5 -3-+1 off off
3 low low 5 5 -3-+1 5 4.5
4 low low 5 5 -3-+1 5 21
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In state 1, only the VCC1 rail is active, and total system load on the
supply rail is typically 40 microamp, made up of 30 microamp load from the
system board and 10 microamp quiescent current in the power supply (i.e.
power required by the power supply itself in order to perform its
functions.).
In state 2, VCC1,VCC2 and V_LCD are active with VCC1 and VCC2 fully
regulated. In this state, total system power consumption is dependent on
the processor operating mode. If the processor is fully active and running
code, overall power consumption is typically 25 mA. When the processor is
in sleep mode, total power consumption is reduced to typically 4 mA.
In state 3, all rails are active with SVCC regulated to 5 volts, and
SVPP at 4.5 volts. Power consumption is dependent on the external devices
plugged in, but without any devices present it is typically 25 mA
internally since the processor is normally fully active in this state. The
power levels of each device type are listed in the relevant sections of the
manual.
In state 4, the SVPP rail is raised to 21 volts. This is a special
case and only occurs when programming datapacks, in a pulsed mode
controlled by the PULSE,READY,ALARM and OE_B signals.
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3.3 SUPPLY RAILS
VB is the main system supply rail from which all regulated supplies
can be derived. This is available at the top slot VB pin, and hence power
input to this pin can supply the whole system. This rail should be at a
voltage between 5.5 and 11 volts for proper system operation, when
supplying up to 200 mA peak system loads.
The battery supplies the VB rail via the forward diode D1. Where loss
of voltage across this diode would affect system performance, a direct
connection to the relevant regulators is also made (TR6 and TR10).
The PP3 battery should be of a type with a low internal resistance so
that it can supply peak system loads of 200 milliamp. Two types are
commonly used :-
- The long-life alkaline type is not rechargeable and has a typical
capacity of 400-500 mAh (milli-amp hours).
- The nickel-cadmium type is rechargeable and has a typical capacity
of 100 mAh between charges.
For both types, a new battery will give an off-load voltage of up to
10.5 volts, decreasing through its life to the minimum system supply
voltage of 5.5 volts.
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3.4 STANDBY REGULATOR
The Standby regulator supplies power to the VCC1 rail when the main
system regulators are shut down (i.e. with ON_B high). In this mode the
System board drains typically 30 microamp to keep the real-time clock
running and to retain data. The VCC1 voltage in this state should be
between 3 and 5 volts, and current consumption reduces with reducing
voltage.
The standby regulator is formed from TR1,TR2,TR3,R2,R3, and R4. TR3
is a pass transistor supplying current to VCC1 from the VB rail. TR1 and
TR3 are configured as a darlington pair, with base current supplied from
R2. Without any feedback this circuit would hold the VCC1 voltage at 1.2
volts (two VB-E drops) below the VB rail. Feedback is implemented through
TR2, which switches on with approx. 5 volts on the base of TR3. TR2 robs
base current from TR1 and the circuit settles in this state, holding VCC1
at nominally 4.5 volts.
The circuit is not a good regulator since it relies on the TR2 VB-E
drop as its feedback reference. Furthermore, its current capability is
dependent on the VB voltage, since this defines the base current available
through R2. When VB is lower than 6 volts, the VCC1 standby voltage may
fall below its nominal value to compensate. These effects are not large
enough to take it outside the 3-5 volt operating range.
The circuit has a low quiescent current, so that it is itself draining
the minimum of power from the supply. Under worst case conditions, with VB
at 11 volts, the current drawn by the regulator would be approx. 10
microamp, split equally between the R3,R4 divider chain and collector
current through TR2.
The C3 capacitor decouples VCC1, and when the supply is removed
supports the VCC1 rail until the supply is re-connected. The capacitor
will hold VCC1 above 3 volts for 2-3 minutes in this case, but only if the
circuit is in standby mode during this time (i.e. Organiser off).
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3.5 VCC1/VCC2 REGULATOR
To switch the system into active mode, the control IC on the System
board pulls the ON_B signal low. When this happens, TR15 is switched on
through its base resistor R23, and the VCC2 rail is powered on to a voltage
of VCC1 minus 0.2 volts (TR15 VC-E drop). VCC2 powers the main regulator
op-amp (OPA1) and the regulator voltage reference (RD1), and the main
VCC1/VCC2 regulator is switched on. In turn, this now supplies the VCC1
rail and regulates it to 5 volts +/- 5 percent. While the main regulator
is being activated, the VCC1 and VCC2 rails are supported by C3 and the
standby regulator. In active mode, the current demand from the main
regulator is built up from three elements :-
1. The System board requires up to 25 milliamp from VCC1 when the
processor is fully active.
2. The LCD drivers on the system board take approx. 1 milliamp from
the VCC2 rail (supplied from VCC1 via TR15).
3. The regulator op-amp and reference source take approx. 1 milliamp
from the VCC2 rail. (this is effectively the power supply
quiescent current when in active mode).
The LM324 op-amp is a quad device having four individual op-amp
circuits. These are used in various ways in the circuit. The voltage
reference diode (type 9491BJ) is a temperature-compensated bandgap device
with a voltage of 1.22 volts +/- 2 percent. This voltage is used in all
regulators as a feedback reference.
The VCC1/VCC2 regulator is formed from R5,R6,R7,R18,TR5,TR6,TR4 and
the op-amp pins 8,9,10. TR5 and TR6 are PNP pass transistors arranged in
tandem, to supply the VCC1 rail from either VB or directly from VBAT. PNP
transistors are used to minimise the regulator drop-out voltage (i.e. a
VB-E drop is not lost as with NPN pass elements). The tandem arrangement
is so that the drop across D1 is not lost when the battery is the power
source. The regulator will operate from whichever is the higher of VB and
VBAT. The divider chain R5,R18,R6 monitors the VCC1 voltage, and is
arranged so that 1.22 volts is present at the op-amp input when VCC1 is 5
volts. The op-amp compares this to the reference voltage, and its output
controls the pass transistors via TR4. R7 limits the pass transistor base
current. If VCC1 is lower than 5 volts, then TR4 is switched on to draw
more current through the pass transistor. Conversely, if VCC1 is higher
than 5 volts, TR4 is switched off. In practice, the circuit settles at a
VCC1 of 5 volts supplying a current equal to the demand.
The regulator functions down to a supply voltage of approx. 5.3
volts. Below this the VCC1 rail will follow the supply voltage at approx.
0.3 volt below it. In this case TR4 will be switched hard on, since the
op-amp input from the divider chain will be below the reference voltage.
At a point where VCC1 is approx. 2 percent below its nominal regulated
voltage, the op-amp section pins 12,13,14 will switch to set the LOWBAT
signal high to the System board.
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3.6 SVCC REGULATOR
The SVCC rail is used to supply external devices in the three slots.
It is switched on and off by the PACON_B signal from the system board, and
when on, regulates to 5 volts +/- 5 percent. It also supplies the SVPP
rail through a forward diode. The load on the regulator is dependent on
devices plugged in. Maximum load is 170 mA, set essentially by the rating
of the pass transistors, which is 1 watt. For a worst case supply voltage
of 11 volts and an output voltage of 5 volts, this rating is met when 170
mA is drawn.
The regulator design is similar to the VCC1/VCC2 regulator above. TR9
and TR10 are PNP pass transistors from the VB and VBAT rails. R8 and R9
form the feedback divider chain to the op-amp pin 6, and are compared to
the reference voltage. TR7 controls the pass transistors, with base
current being limited by R10.
The PACON_B signal switches the regulator on and off. When PACON_B is
high, TR8 is switched off and base current from the regulator is blocked.
Note that in this state TR7 will be hard on since SVCC is below 5 volts,
but nevertheless no base current is available to the pass transistors.
When PACON_B is low, TR8 is switched on and the regulator can operate. If
there is no load, (i.e. no devices are plugged in) then the base current
through the pass transistors will be minimal, and so the quiescent current
of this regulator is negligible.
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3.7 V_LCD
V_LCD is the negative rail used by the LCD drivers on the System
board. It is adjusted by the thumbwheel variable resistor to set contrast
on the display. Current consumption is 100-300 microamp dependent on the
voltage set, between +1.2 and -3 volts.
V_LCD is generated directly from the OSC signal from the system board.
When the Organiser is on, this gives a 1024 Hz square wave signal from the
control IC. R20 and the VR1 variable resistor limit the current, and then
drive the series capacitor C8. When OSC is high, charge is pushed from C8
and is drained to ground through the forward diode of D3. When OSC goes
low, charge is pulled to C8 making the C8/D3 track negative. The reverse
diode of D3 transfers charge from the storage capacitor C1. With no load,
the voltage at C1 would be the inverse of the OSC voltage less the D3 diode
drop. Note that the maximum positive voltage on the V_LCD rail will be
+1.2 volts, since it is tied to ground through the two diode drops of D3.
This form of voltage inverter is not precise and can only supply a low
current. In this application, however, the circuit is adjusted by the
operator, and the voltage set is not important as long as the contrast
range is sufficient.
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3.8 VOLTAGE PUMP
The voltage pump is used by the system when programming datapacks.
Two stages are required to transfer the voltage required to the SVCC rail.
Firstly the pump is used to charge up a storage capacitor to 35 volts.
This is then regulated to 21 volts and switched to the SVCC rail under
program control. The storage capacitor contains enough charge to programme
an Eprom byte. This sequence is repeated for each byte to be programmed.
The voltage pump is formed from TR11,TR12,R16,R17,L1,D5,ZN1 and C6.
The inductor L1 is the energy transfer element and C6 is the charge storage
capacitor. Pumping is controlled by the PULSE signal from the System
board, and the zener diode ZN1 provides feedback to the processor by
setting the READY line high when C6 is charged sufficiently for the next
byte programming operation.
In steady-state conditions (before pump operation), the PULSE line
will be low, with the darlington pair TR11 and TR12 switched off. C6 will
be charged to the VB voltage, less a diode drop across D5. The READY line
will be low since C6 is well below the 35 volt switching voltage. Note
also that TR14 should be off (i.e. the 21 volt regulator is not switched
on) and so their is no load on the C6 rail.
To operate the pump, the processor enables the PULSE signal from the
control IC. PULSE then oscillates at 32768 Hz frequency. When PULSE is
high, TR11 and TR12 are switched on, with TR12 in saturation bringing its
collector voltage to 0.3 volts. Current is now drawn through the inductor
to the V=L dI/dt law, (where V is the voltage across the inductor
VB-Vcollector, L is the 1 mH inductance and t is the time from Pulse being
set high.). After 15 microsec, PULSE goes low and TR12 is switched off.
Current continues to flow through the inductor, raising the voltage at the
TR12 collector until it can flow through the forward diode D5 to the
storage capacitor C6. The inductor now discharges into the capacitor under
its V=L dI/dt law (where V is now the reverse voltage between the TR12
collector and VB, set by the current C6 voltage plus the D5 diode drop).
This process is repeated on every cycle of PULSE, and the C6 voltage
steadily rises. When C6 reaches the zener voltage of 33 volts, it starts
to pull the READY line up, and this will be seen as high by the processor
when a further 2 volts are added. (During this sequence the ready line is
pulled low by the 47k resistor from the ACOUT signal on the System board,
so leakage through the zener is drained). On reading ready high, the
processor should disable PULSE and can proceed with programming a datapack
byte.
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3.9 SVPP REGULATOR
The SVPP regulator is used to regulate the C6 storage capacitor
voltage to 21 volts and to switch this onto the SVPP rail to the slots. It
is only used during datapack programming and in conjunction with the
voltage pump above.
The regulator is formed from TR13,TR14,R11,R12,R13,R14,D4 and the
op-amp section pins 1,2 and 3. In operation it is similar to the VCC1/VCC2
regulator, with the divider chain R11/R12 forming the feedback loop to set
the op-amp input at 1.22 volts when the SVPP rail is at 21 volts. The
regulator is switched on and off by a combination of the PACON_B, OE_B and
ALARM signals from the system board. To switch on, PACON_B must be low
(i.e. SVCC on), and both ALARM and OE_B must be high. The software ensures
that this condition only arises when the SVPP regulator is required, and
this imposes conditions on the use of the buzzer when the slots are
switched on. If PACON_B is high, base current from the TR14 pass
transistor is blocked by TR8. If either ALARM or OE_B are low, then the
base of TR13 is pulled low to switch it off irrespective of the state of
the op-amp output at pin1. (When the Organiser is on, the op-amp output is
normally high since SVPP is normally lower than the 21 volt regulating
voltage.)
When the regulator is switched on, it draws current from the C6
storage capacitor, and the voltage at C6 falls as it loses charge. The
capacitor has sufficient charge to maintain 21 volts at the regulator
output for 50 msec under a SVPP load of 50 milliamp. The software must
limit switch-on time to this maximum, switch off the regulator and re-pump
the capacitor if it requires any further datapack bytes to be programmed.
The SVPP rail can be in one of three states :-
- OV if the PACON_B is high and the slots are switched off
- 4.5 volts when PACON_B is low, fed by the forward diode D2 from
the SVCC rail
- 21 volts when the SVPP regulator is switched on.
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3.10 REMOTE SWITCH ON
The AC_B signal from the top slot can be used to switch the Organiser
on. The AC_B line is pulled high to VCC1 by the internal 47k resistor R24.
If an external device pulls this input low, then TR16 is switched on, and
the AC signal to the system board goes high to switch the system on. Note
that this will only happen if the ON_B signal to the emitter of TR16 is
high (i.e. Organiser off). As soon as ON_B goes low, the AC_B input is
disconnected an has no effect.
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3.11 BUZZER
The piezoelectric buzzer element is driven from the ALARM signal,
through the 1k resistor R15.