Hi Robert.

Use two diodes, one ceramic cap and one tantalum cap.
Drive them with some PWM freq. and measure somehow
output voltage. Changing PWM duty cycle will change
output negative voltage as well. PIC PWM module
and some small piece of software will do that trick
for you.

WBR Dmitry.



"Robert E. Griffith" wrote:
>
> Hello,
>
> Does anyone know of a simple, cheap DC-DC negative voltage doubler that can
> double the  8.5 of a MAX232 to at least  12v at 2ma?
>
> Or, do you know of an inverting voltage tripler that would take a +5v
> to  15v (less the V drop across the tripler)?
>
> Details
>
> I got some help on this topic a couple of weeks ago that pointed me in the
> right direction. After researching it on and off for these weeks I have
> learned a lot and have several solutions, but I thought I would pursue this
> one last way before settling on one.
>
> I am creating PIC base LCD circuit that has a +5v regulated supply coming
> in.  The LCD needs a  8v to  12v bias at less than 2ma. The optimum value
> varies from LCD to LCD so it needs to be set with a pot. Since the current
> requirement is so low I can use a simple resistor and pot passive voltage
> divider to adjust the voltage down, if I can just get a voltage equal or
> less (more) than 12v.
>
> I can get  8.5v from the unused half of a MAX232 on the board.  Too bad the
> MAX232 does not drive the output to RS232 s full +-12v spec!
>
> So it seems that all I have to do it double that  8.5v from the MAX232
> to  17v (less the V drop across the doubler) and then drop it down with a
> passive voltage divider.
>
> What is a simple way to double a negative voltage?
>
> I am assuming that since I only have DC to start with that I cannot build a
> simple discrete solution.
>
> The only negative voltage doubler I have found is the LT1045 and it is a
> little more expensive than I would like (about twice the cost of other DC-DC
> Ics I have looked at like the LTC1044) and it requires a lot of external
> components when acting as a negative doubler.
>
> Does anyone know of a simple, inexpensive (<$2 in quantity 100) IC? I have
> mainly looked into IC DC-DC inverters/doublers from Maxim, National, Toko
> and TI.
>
> Thanks,
>
> --BobG
>
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