-----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
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