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PICList Thread
'PIC circuit running on penlight batteries'
1997\12\11@021544 by wterreb

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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.

This sounds like a fairly simple problem to solve, but I would still
be interested to hear how other people would tackle the same problem.
The full  requirements to be met are:

1. Stable 5V supply over the entire battery life
2.  Software ON/OFF control of the supply voltage
3. Automatic wake up of the micro every 10 minutes or so.

I was planning to use a MAX860 to double the voltage and then just
use a low dropout fixed 5V regulator like the LM2936-5 to regulate it
down to 5V.  My circuit only draws about 20mA, so the efficiency of
the MAX860 should be more than 90%.   The MAX860 also provides a
shutdown function, which should make it possible for the micro to shut
down its own supply.

How do the rest of you guys out there that makes battery operated
equipment ensure a solid 5V over the entire life of the batteries?
Is this proposed method that I want to use an acceptable way of doing
this, or is there a better way to skin the cat?

Another question : What is the best way to switch the 5V supply to
other parts of the circuit.   I suppose it would be a MOSFET, but
what I really want to know is the part number of a good general
purpose MOSFET that is widely available.  Every time I try to order
one from my databook, I get told that I need to buy a minimum of
10 000 and that it would take 8 weeks to deliver.  This is very
frustrating when you only need a couple for prototyping.  Where do
you guys get your MOSFETS from?  (Especially you fellow South
Africans)


Rgds
Werner
FOR SALE: Parachute. Only used once, never opened, small stain.

--
Werner Terreblanche     users.iafrica.com/w/we/wernerte/index.htm
spam_OUTwterrebTakeThisOuTspamplessey.co.za (work)  OR  .....wernerteKILLspamspam@spam@iafrica.com  (home)
Plessey SA, PO Box 30451,Tokai 7966, Cape Town, South Africa
or at home : Suite 251, PostNet X5061, Stellenbosch, 7599
Tel +27 21 7102251  Fax +27 21 7102886  Home +27 21 8872196
------------------------------------------------------------

1997\12\11@053735 by bam-mon

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Werner Terreblanche wrote:
{Quote hidden}

You could try some of MAXIMs IC, they have got a lot of power-supply and
power-distribution circuits,  they also offer FREE samples. That's
better than odering 10 000 MOSFETs, hey ?

keep on the good work,
  kind regards
        R. Monsees
--
        BAMBERG & MONSEES GbR
 Systeme f|r Wissenschaft und Technik
   Am Postmoor 36 * D-28719 Bremen
Fon +49-421-646775 * Fax +49-421-646785

1997\12\11@054811 by wterreb

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> You could try some of MAXIMs IC, they have got a lot of power-supply and
> power-distribution circuits,  they also offer FREE samples. That's
> better than odering 10 000 MOSFETs, hey ?

I am considering Maxim IC's.  They have great products, but the
trouble with Maxim parts is that that you usually have no problems
getting samples but when you start ordering for production you find
that they can not deliver.  I was in a very bad situation once with
Maxim A/D that I implemented in design, but then ran into stock
problems when I had to purchase the productions units.  Eventually I
got so frustrated that I just changed the design for a more common
A/D from another supplier.  After that I have been very wary of
single source items from Maxim.  Maybe things have changed since
then.  Do you never experience any problems getting stock of Maxim
products?

Rgds
Werner
FOR SALE: Parachute. Only used once, never opened, small stain.

--
Werner Terreblanche     users.iafrica.com/w/we/wernerte/index.htm
EraseMEwterrebspam_OUTspamTakeThisOuTplessey.co.za (work)  OR  wernertespamspam_OUTiafrica.com  (home)
Plessey SA, PO Box 30451,Tokai 7966, Cape Town, South Africa
or at home : Suite 251, PostNet X5061, Stellenbosch, 7599
Tel +27 21 7102251  Fax +27 21 7102886  Home +27 21 8872196
------------------------------------------------------------

1997\12\11@090817 by Mike Watson

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In message  <@spam@EC6B3C2FBBKILLspamspamfs7.plessey.co.za> KILLspamwterrebKILLspamspamplessey.co.za writes:
> 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.
>
> This sounds like a fairly simple problem to solve, but I would still
> be interested to hear how other people would tackle the same problem.
> The full  requirements to be met are:
>
> 1. Stable 5V supply over the entire battery life
[snips]

Werner,

Is it necessary to have a stable 5V supply? The PIC will run over
the whole useable battery life (6V -> 3.2V).

Just a thought.

Regards,

Mike Watson

1997\12\11@113458 by Matt Bonner

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Werner Terreblanche wrote:

> I was planning to use a MAX860 to double the voltage and then just
> use a low dropout fixed 5V regulator like the LM2936-5 to regulate it
> down to 5V.  My circuit only draws about 20mA, so the efficiency of
> the MAX860 should be more than 90%.   The MAX860 also provides a
> shutdown function, which should make it possible for the micro to shut
> down its own supply.
I'm not familiar with the MAX860 in particular, but doublers and
inverters tend to generate switching noise.
>
> How do the rest of you guys out there that makes battery operated
> equipment ensure a solid 5V over the entire life of the batteries?
> Is this proposed method that I want to use an acceptable way of doing
> this, or is there a better way to skin the cat?
National's low drop-out regulators are a good choice.  They make a
variety of devices so look for one with low temperature drift and good
line regulation. To avoid voltage drift problems, look at making your
analog circuitry ratiometric - e.g.: bridge transducer, voltage divider
reference (buffered and filtered).
>
> Another question : What is the best way to switch the 5V supply to
> other parts of the circuit.   I suppose it would be a MOSFET, but
> what I really want to know is the part number of a good general
> purpose MOSFET that is widely available.
National also makes a 5 pin regulator (LP2957AIT) that can have its
output switched on and off.  I'm just evaluating these now.  What has
worked for us in the past is a VFET (voltage switched FET) driving the
base of a PNP medium power transistor (the PNP acting like a switch).
If you over-saturate the transitor, the emitter-collector drop can be as
low as 300mV.  This is a constant voltage drop, whereas a FET is more
resitive in nature - your drop will be load-dependant.

--Matt

1997\12\11@161934 by Dwayne Reid

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>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   <RemoveMEdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, Alberta, CANADA
(403) 489-3199 voice     (403) 487-6397 fax

1997\12\12@023803 by wterreb

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Dear Tom and all the others that offered advice

 Thank you for all the advice.  Your idea about the way to read the
pressure transducer looks very interesting.  My current prototipe
Altimeter/Variometer uses a MPX5100A pressure transducer with a 13
bit A/D.  It can display altitude to a 1 meter resolution with great
accuracy.  The advantage of the MPX5100A is that it is already
temperature compensated and linearized.  I have gone into great pain
to implement lookup tables to make this instrument as accurate as
possible, and do not really want to change that part of the design at
this stage.  I'll keep your advice in mind for a next generation
model.

All I want to change at this stage is to use four penlight batteries
instead of a 9V battery.  You say that it should not be necessary to
use the voltage step up converter.  I assume then that the discharge curve of
the
four penlights in series would drop very  slowly initially from 6V down to
5V, and once it goes below 5V it means that the batteries are almost
discharged anyway and could be thrown away?

Thanks once again to you and all the other people that offered
advice.

Rgds
Werner

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