Re: Micropower charge pump for 3-6v @ 20uA

On Wed, 07 Nov 2007 18:04:44 GMT, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

John Larkin wrote:
On Wed, 07 Nov 2007 15:39:45 GMT, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

John Larkin wrote:
On Tue, 06 Nov 2007 21:30:51 GMT, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

John Larkin wrote:
On Tue, 06 Nov 2007 10:35:22 -0800, Martin Brown
<|||newspam|||@nezumi.demon.co.uk> wrote:

I suspect this is a tall order.

But can anyone see a simple way to build a micropower charge pump for
a hobby electronics PIC based device drawing approx 20uA at a voltage
between 2.5 to 6v. The aim is to run it off a single cheap 1.5v
battery rather than a pair of them or an expensive 3v cell. Efficiency
of 20-50% would be OK - 40-100uA drain on the cell. Regulation need
not be good - but its output mustn't go above 6v. Its for a PIC based
sidereal clock for astronomy.

The linear LTC1751 looks like it might come close but the data***
says it is only good for 2v and up input voltage. And if the solution
is bigger than the battery then it may as well use a pair of cells.
OTOH It draws so little current.

Thanks for any enlightenment.

Regards,
Martin Brown
Use a PIC port pin to drive an NPN that in turn drives the inductor of
a boost converter. Let your software pulse it as needed. If you have a
spare adc input, feed back on that, and you're now in the DSP power
supply business!

You could also do a schottky charge pump/C-W multiplier directly off a
port or clock pin, given the efficiency you're willing to accept.
That's not bad... a 4-diode multiplier stack, two sot-23 dual
schottkies and some caps, piggybacked on top the +1.5 rail.

But, as Jim would put it, how does it start?

OK, be pickey.

This version self-starts:

http://img402.imageshack.us/my.php?image=picdcik3.jpg

The boost voltage isn't well defined, so if the pic can't control it
maybe a shunt regulator, possibly a couple of led's, would help.

I don't know PICs but I would imagine they need something above 2V for a
substantial amount of time (maybe msecs, don't know) to be able to start
anything meaningful in there. Might be a wee problem if the battery is a
well used 1.5V alkaline.

This should boost to well over 2 volts at startup. It would peak at 3
volts except for the diode loss.


But it would first require the PIC to properly start.

No, it doesn't. The LC just behaves that way at powerup. The pic just
has to pump it now and then to keep the cap from discharging.

John

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