POWER SUPPLY BOARD
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.
3.2 POWER SUPPLY REQUIREMENTS
The power supply provides five supply rails to various parts of the system :-
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 ---------------------------------------------------------------------- 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 ----------------------------------------------------------------------
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.
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 :
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.
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).
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 :-
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.
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.
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.
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 d to the V=LdI/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.
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 :-
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.
The piezoelectric buzzer element is driven from the ALARM signal, through the 1k resistor R15.