Bill Kuncicky wrote:
> I have to say that I am amazed to learn that the caps on the regulator
> (input to ground and output to ground) can be that large!  I have
> always assumed that the specs of the manufacturer had to be followed
> pretty closely!

Many regulators don't like really large caps on their output.  I think
people here were assuming you were using a 7805, which tend to be forgiving
but some versions don't want more than a few uF if I remember correctly.
Adding a larger cap to the output of a regulator can be a bad thing.  Always
stick to what the data sheet says.  If someone told you to do otherwise,
they were just plain wrong.

Think about it a bit too.  Large bulk caps are going to respond slowly,
which the circuit in the regulator can do too.  This is why you need large
caps on the regulator input.  That's where the regulator will get the extra
current from for the frequencies its circuitry can respond to.  However, the
circuit has high frequency limitations.  So the caps you add need to make up
for that.  Since the frequencies these caps need to repond to are high, the
caps themselves don't need to be that large.  If you think about it, a
regulator is a control problem, with all the associated instabilities as you
try to approach the edge of the circuit's speed.  A brick outhouse like the
7805 will be more stable over a range of conditions, but some LDOs are
unstable except with a narrow output capacitance range with a specific ESR.
Always do what the data sheet says.

All this has nothing to do with the bypass cap at the PIC.  The PIC can vary
it's current draw at fairly high frequencies, which can cause significant
voltage excursions at the other end of a wire from a regulator, due to the
wire's inductance and resistance.  That's why bypass caps need to be as
close as possible to the pins they are bypassing.


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