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Thread: Power handling for a sleeping PIC system
www.piclist.com/techref/power.htm?key=power
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face BY : Wagner Lipnharski email (remove spam text)



Marco Genovesi wrote:
> Not a g.p. 70L05, but there are some very low power 5V regulator
> (National LM2936 and others, Seiko S-812XX family..) with less than
> 10uA standby current. However,  the voltage doubler only for the TX
> isn't a bad idea.
>
> Marco

If you think about power conversion, nothing converts Voltage better
efficiency than 85 to 90%.
Suppose the processor consumes 5mA @ 3V, and the transmitter consumes 20mA
@ 5V.
Converting 3V to 5V at 85% efficiency for 20mA, will drain 5/3 * 0.02 /
0.85 = 40mA, plus the processor's 5mA, total is 45mA from the 3V cell.

Now, the same processor running at 5V can drain probably 8 to 10mA.
Suppose you tie up all to 4 NiMH cells (4.8V), it will drain 28 to 30mA
from the batteries. It means 33% less power consume than the 3V and step up
conversion solution.

Now, if you use 2 simple 1N4004 diodes in series with the VCC to the
processor, it probably will reduce current consume to 6 to 7mA, total
consume to 26 to 27mA.

The point here, is that the processor can reduce its current consume based
on the voltage supplied as VCC, and the 15% loss in conversion can justify
to use a 4.8V battery pack instead of 3V cell.

Suppose you use 2 x 3V cells (6V), even using 1 diode to drop a bit the VCC
to the transmitter, and 3 diodes to drop the VCC to the processor, you
probably will have half the consume from the step up solution.  But either
way, two cells, twice the energy autonomy, so, no much gain.  This,
considering a high efficiency of 85%, capacitive charge pumps does have
numbers worse than that.

The thing to observe; Is the processor able to drive the transmitter with
its logic up to 3V while the transmitter is tied to 5V ?

Wagner Lipnharski - email:  spam_OUTwagnerspamspamRemoveMEustr.net
UST Research Inc. - Development Director
http://www.ustr.net - Orlando Florida 32837
Licensed Consultant Atmel AVR _/_/_/_/_/_/

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