Dear PIC Listers,

I already sent Mike the following files, but I thought that
maybe some others might also be interested in receiving
a copy of the circuit and waveform.

If you want a copy, PLEASE request it by sending *ME* an
email , NOT by replying to the PIC List! My email address is:

tom_mcgahee@mailsnare.com

Make the SUBJECT line: BOOST555 so I can easily locate
your request amidst all my other email. If you have
SWCADIII from LTC I can also provide an (asc) schematic
file that you can run through their simulator.
You can get the FREE SWCADIII at this site:

http://www.linear-tech.com/software/

********* Reply to Mike

I have a GIF file with a circuit I designed to do this and a GIF
file showing a 'scope plot of the output.

This circuit will take unregulated +12V and boost it to more than
+21V. When the load is drawing 22 ma the ripple is .3v

This should be more than adequate for driving such applications.
Note that when providing +21V at X ma the current drawn by
the circuit has an average current draw of about 2X ma from
the +12V supply.

A PIC pin cannot provide enough current drive, and because
it is running on a +5 volt supply it could not provide
adequate voltage drive without having a three transistor
push-pull driver stage replacing the 555.

In the end the stand-alone 555 circuit is probably cheaper anyway.


Fr. Tom McGahee

----- Original Message -----
From: "Mike Hord" <gaidinmd@HOTMAIL.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Monday, April 12, 2004 9:58 AM
Subject: [PIC:] 13V/5V variable supply


> I have a device which uses a 13V programming level; normally 5V
> for operation.  I need to figure out a way to quickly raise the
5V
> to 13V, then program the device.  Generating the programming
> waveforms may not be a problem; I'll cross that bridge when I
> come to it.
>
> I'm using an 18F458 (since that's what I had on hand ;-)), and
> considering using the ECCP module to make a charge pump, or
> boost regulator, or something, to make it run.
>
> I'll have access to 12V, so I only need to boost it a little.
The
> problem comes in switching the supply voltage over quickly.
> The device (a Melexis programmable hall effect sensor) draws
> about 10 mA at 13V, which is more than a charge pump is
> easily able to provide.
>
> Anyone have a masterful idea?  This is an entirely new realm
> to me.
>
> Mike H.
>
>
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