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Think I once saw a circuit where a spare pulsing PIC pin, like one of the clock pins, was used to drive a charge pump using a few capacitors, diodes and maybe a transistor to get a negative rail. I need a -3V rail at 1mA to drive my LCD.

Alternatively what about a voltage doubler to get twice Vdd?

Anyone got such circuits?

Hello Echase

I'm afraid that I have no experience with these such circuits. However there is most likely an IC that will perform the task you require.

Plenty of ICs to do this but hoping for something simpler/lower power as only need 1miiliamp.

Am going to try to make a voltage doubler using 2 diodes and 2 capacitor driven by CLKOUT. Can be used to provide a doubled Vdd or a –ve rail depending on which way round the diodes are.

Similar to http://sound.westhost.com/project95.htm where the PIC is used instead of the 555 IC.

Anyone else tried anything similar?

I got it working. I have -2V at several milliamps available with a 4V supply on the PIC. It would be more than -2V at 5V input.

It uses two 0.1uF capacitors and two skottky diodes. With normal diodes it gives about -1.5V

Hello Edward

Thats great news thanks for letting us know. If you wouldnt mind sharing your circuitry then you could post a picture up onto our forums for other customers.

Rather than a pic take the circuit in this link http://sound.westhost.com/project95.htm and remove R1, R2 and C6 and all the left hand of the diagram (as the PIC does the left hand bit) Change the capacitors C4 and C5 to 0.1uF and feed C4 from PIC pin CLKOUT.

R2 and C6 are optional and provide extra smoothing if you need it, otherwise some of the pulsing from the CLKOUT could get through to your load.

If you want a doubler to give say +8V from a 5V supply just reverse the diodes.

If you need more current than my couple of mA increase the capacitance of C4 and C5 but I fear that the high values in the link may overload the PIC at the sort of frequencies a PIC works at. They are only using 17kHz. Check PIC supply current to see what sort of currents are being absorbed and remember that more than 20mA from CLKOUT is not a good idea.

I used a 125kHz internal PIC clock which gives 32KHz at CLKOUT. The circuit should work at any frequency but I suspect voltage will drop off a bit if it gets above a certain frequency and I don’t know what that frequency is.

Just about any diodes will work but IN4148 diodes are cheapest. Or if you want 0.5V more output use Shottky diodes such as IN5817/5818/5819

I found that C4 =0.1u and C5=.47u gives less ripple on the output.

If you are driving a FET off the PIC the gate drive usually needs to be >5V so a 8V supply from this circuit could be just the thing so get the extra voltage. E.g. put an external pull-up resistor to 8V from the FET gate and use the PIC to pull it down to 0V to turn it off. Tristating the output turns FET on. Also need a 4.7V zener to stop more than that back feeding into the PIC. Or say you are using a 3V supply that’s not enough for even logic level FETs so this circuit increase the voltage.

There is also a similar circuit that ‘triples’ the voltage using 3 diodes and 3 capacitors if you need more voltage, but at less current.