> -----Original Message-----
> From: Tal [SMTP:tal@ZAPTA.COM]
> Sent: Thursday, May 01, 2003 5:45 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [PIC]:[EE]: How to build a cheap charge pump
>
> Hello,
>
> I am looking for a cost sensitive (that is, cheap in very high volume
> manufacturing) DC/DC converter to create a low current, not regulated
> bias voltage in a circuit that already contains a PIC.
>
> The circuit should output 11 to 17 VDC, non reugulated and load current
> of no more than 3 to 5ma. The PIC is running on 4 to 5VDC but an higher
> voltage of 5 to 6 volt may also be available in the circuit.
>
> The circuit can use PIC outputs if needed (e.g. a continious square wave
> from a PWM) but should not require continious software operation since
> the PIC is already running a tight timing software.
>
> Any help/ideas/pointers will be greatly appreciated.
>
Using a classic voltage multiplier ladder from the PWM output may not be
very satisfactory in this case.  You reqire 11-17 volts at a maximum of 5mA.
This would require at least a voltage trippler.  The current drawn from the
PIC is going to be an average of three times your 5mA, but the peak current
will be higher, likely above the 20mA pin limit.  You could simply drive a
push-pull complimentry transistor output to boost the current, but parts
count will start rising.

You may want to look at a simple zener clamped boost switcher.  Drive the
transistor from the PWM output and adjust the duty cycle until you achive
your maximum current without dropping under the zener voltage.  This is
quite inefficient as the zener will be disipating most of the power at small
loads, but may be ok for your application.

Inductor
---OOOO-------------------
           |     |    |
           |    _|_  _|_
          /     ___  /_\ Zener
---\/\/\-|       |    |
          \>     |    |
___________|_____|____|___


Regards

Mike


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