>I'm currently designing a product that will have a PIC running of
>four 1.5V penlight cells.   There is also a 12bit A/D and pressure sensor
>on the circuit which both demands very accurate 5V supply.

Hi Werner.

We are currently using a '73A, a LT1298 12 bit a/d, Xicor X25043 watchdog /
pwr up reset / 512 bytes eeprom, 4 ADXL05 acceleration sensors and 4 or 8
digits of LED 7 seg display.  The whole shebang runs off 3-AA cells.  The
display runs from unregulated battery supply with PWM dimming to compensate
for battery voltage.  Segment resistors were picked to give desired
brightness at minimum battery voltage (0.9 Vdc per cell, 2.7V total).  (We
used LED displays because the project is for an underground mine with low
light levels.)  The rest of the electronics runs from a LT1300 DC-DC boost
convertor giving 5 Vdc regulated.  The 5V output is also switched with a
2n4403 PNP transistor to everything EXCEPT the PIC and display drivers
(74hc595).

When the unit goes to sleep, the boost convertor and 5V switch are turned
off and the o/p enable for the display turned off.  That kills the display,
the sensors, the mux, the a/d, and the watchdog.  The PIC and display
drivers get the output of the boost convertor but since it is disabled, they
see the unregulated battery supply less the drop of the catch diode (0.2V
schottky).  Standby current was about 20 uA when the thing was powered up
for the first time 2 days ago but that was without the PIC installed (I
expect that it will go up as the project nears completion.

The important thing is that max unregulated battery voltage was less than
5V.  That let us use a simple boost convertor as opposed to buck-boost.
Running the high current displays directly from the battery kept the rest of
the power supply small.

The 2n4403 saturates very nicely.  I am seeing a VCEsat less than 0.1V at 20
mA load.  Something you may consider is that your sensors and the a/d may be
ratiometric (another project we did using pressure sensors used the same
LT1298 and was insensiteve to VCC variations).

I hope this helps.

Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, Alberta, CANADA
(403) 489-3199 voice     (403) 487-6397 fax