OK, thanks. I guess then it could be solved by doing a divider on the driv= e from the CPLD, or just use something like a Si4788CY to solve it. Lower = RDSon with that package as well. "Thomas C. Sefranek" wrote: = -----Original Message----- From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of alan smith Sent: Monday, September 25, 2006 3:10 PM To: Microcontroller discussion list - Public. Subject: [EE] high side FET....remind me on this OK, I've got a P channel FET (not my design, just took it over) that is using 3.3V to switch another 3.3V supply (always on turning on the rest of the board). Part is a FDC638P. Looking at the data sheet, Vgs is the voltage differential to turn on the gate, right? So pretty much....you can't have the same voltage on both the gate and drain to turn it on, just wont work. Now if I had 5V (or anything more than 4.1V) there, with a resistor to ground to keep it turned off, that should work. Huh? The gate needs only be .8 volts less than the 3.3 volts feeding the source. To turn it on. I does not hurt to be brought right to ground (0V.) [-3.3Vgs] You may want a resistor from the gate to the source to turn it off. (NOT to ground!) You got things the other way "round". * | __O Thomas C. Sefranek WA1RHP@ARRL.NET |_-\<,_ Amateur Radio Operator: WA1RHP (*)/ (*) Bicycle mobile on 145.41MHz PL74.4 ARRL Instructor, Technical Specialist, VE Contact. http://hamradio.cmcorp.com/inventory/Inventory.html http://www.harvardrepeater.org -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist = --------------------------------- Yahoo! Messenger with Voice. Make PC-to-Phone Calls to the US (and 30+ coun= tries) for 2=A2/min or less. -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist