> > > C12 should really go to Vcc

> > There is nothing wrong with tying it to ground.  I always do it this
way.

> But it does make a difference. See the first three questions (and their
> answers) on this page:
>    http://www.circuitcellar.com/library/eq/156/index.htm
>
> The output impedance of the charge pump is approximately halved by the Vcc
> connection, and this is (or should be) an order or two of magnitude higher
> than the 5V supply impedance, which is negligible in comparison.

I appreciate that Dave wrote the technical note he refers to. I'm afraid I
have to disagree with the second part of it.

- The part about startup is correct. When returned to Vcc the capacitor
rises to Vcc as supply is applied. Startup is slightly shorter.

- I personally do not believe the second part. Provided that supply
decoupling is good, Vcc and ground are separated by zero impedance. While
this will not be quite so in practice, it should be close enough that there
is a negligible difference between the two situations. The difference is not
able to be analytically determined from the information available and is not
a factor of 2.



        Russell McMahon

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