that's exactly what I did, using a MAX619 chip, just needs two .1uF caps.
This was to get 5v from two AA cells though, but the principle is the
same.  A button press activates it, then a pin on the pic holds it on
until it decides to turn it off.  Needs a bit of thinking about or the pic
will think the button is permanently pressed if you just drive the
same line.  Nothing a few transistors can't fix.

Martin


PICLIST@MITVMA.MIT.EDU

> This may not fit into your design, but there are low power switching
> regulators which require fewer components than a typical switching design,
> and feature shut-down modes which reduce current draw to the uA range.  A
> button could supply enough power to the shutdown to start it up, and the
> uC could shut it down as long as the button is not pressed.
>
> You might try this one, for instance...
> http://dbserv.maxim-ic.com/quick_view2.cfm?pdf_num=1155&Fam=DCDC_All&TREE=PowerSupplies.asp&HP=PowerSupplies.cfm
>
> -Adam
>
> Germain Morbe wrote:
> >
> > Hello,
> >
> > has anyone a LOW COST / LOW CURRENT idea that could help solving the
> > following alimentation problem?
> >
> > A 16C505 or 12C508 shall be used as an ecoder for a 12V battery powered 3
> > button handheld transmitter. Because of the needed code completion feature
> > after button release, the power must be present permanently / resp. as long
> > as the PIC needs it.
> >
> > The two approaches i see are:
> >
> > a) use a very low power 5V regulator which is comparable expensive.  That
> > way the PIC could wakeup from sleep at button activation for as long time as
> > needed to complete a message after release of the button.
> >
> > b) use a low cost 7805 regulator or a zener which draws to much current to
> > be allways on. Thus, the buttons need to switch the power directly and the
> > PIC needs to hold the power in the ON-state via some transistors. After
> > completion it could cut itself and also the stabilizing circuit from power
> > by releasing the transistors.
> >
> > Is there a third possibility i dont think of?  I feel that from the two
> > mentioned either has its drawbacks in a low cost radio control? What would
> > the gurus advice?
> >
> > Germain Morbe
> >
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