Dan Tye 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.

Noise in switching power supplies (SPS) is a result of poor output
filtering. Basically a SPS is a high frequency power oscillator, so it
will appear at the output.  Increasing the value of the output capacitor
can reduce the noise but doesn't eliminate it completely since a bulk
capacitor doesn't answer fast for the high frequency noise. The best
solution is to incorporate a PI filter with inductor or resistor, tunned
to the noise frequency. A C-L-C circuit is inexpensive and can block
almost completely the noise if the SPS frequency is fixed.  Some SPS
frequency varies according with the load, so they are more difficult to
filter.  There are some formulas around to calculate PI C-L-C filters
for power supplies. At 150kHz it will requires a coil that can be done
home made over a small toroid.  Some microcontrollers don't care about
300mV noise, but it is not recommended to have it at the VCC.