I use SMPS all the time. The trick is to use a series resister (about 10 ohms) with an electrolytic cap (about 220 uf) to ground. Separate this filtered power supply leg from the rest of the circuit. Use it only for the PIC. Create other filters for the rest of your circuits, and they will operate perfect. A 220 uf cap at 6 Vdc is small. Adjust the resister value according to your load. Example: 300 mv p-p ripple is 150 mv above or below your 5 Vdc. 150 mv / .02 A = 7.5 ohm resister. 150 mv * .02 = .003 watts. So a 1/4W or a 0805 SM resister will work. Your 5 Vdc will be 4.85 Vdc which is great for most 5 Vdc applications. This is why most 5 Vdc circuits are 5.1 Vdc. If you are using a SMPS them you can compensate for the loss in the filter. You must have less than 10 mv p-p Ripple on most analog circuits and less than 50 mv p-p ripple for most digital circuits. Gordon Varney BSEE ----- Original Message ----- From: Dan Tye To: Sent: Thursday, August 12, 1999 8:54 AM Subject: Switching Power Supply for PIC > 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. > > +++++++++++++++++++++++++++++++++++++++ > ++ Dan Tye > ++ Chief Engineer > ++ Tel-Tron Technologies Corporation > ++ 220 Fentress Boulevard > ++ Daytona Beach, FL 32114 > ++ Phone: 904-255-1921 Fax: 904-258-3782 > +++++++++++++++++++++++++++++++++++++++