Dan wrote

> Greetings All!  I am looking again for sage advice from some veteran
> PICsters.  I was wondering if anyone has experience powering a PIC with
> a switching power supply.  In my design, I am using a 17C752 and I want
> to supply Vcc with a 24V to 5V step down 150kHz switching regulator.  In
> my initial testing, I find that the average output of the switcher is
> pretty good at about 5.02V throughout a range of loads from 20mA to
> 500mA, but noise on the output can be up to 300mV.  Question is, will
> the PIC have trouble with this noise magnitude at 150kHz.
>
> I have thought about cleaning up the output with an LDO, but I'd rather
> not go to the trouble and expense if I can help it.
>
Switching PSUs are much easier to use these days than in times past
basically because I.Cs have been developed that simplify the design
values.
There are two ways to minimize noise on the output. You can
filter and/or cancel.
To aid in keeping the filter components as small as possible the
higher the switching frequency the better, Maxim do a number of
300kHz to 1 Mhz switching regulators. I often use 300kHz based
switchers.
R/C or L/C filtering can be used, the inductor based L/C type
offer the best Q or roll off  but at greater expense but I can
testify this approach is the best by far.
 Cancellation can be achieved through active components such as the
internal transistors of some voltage regulators (up to 70dB in a 7805
from recollection) BUT not LDOs, they are designed for use with
batteries and have very poor ripple rejection.
This is a brief coverage of a very involved subject, there are many
app notes covering it ( techniques, component selection etc).
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Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand
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