On Jun 18, 2011, at 4:11 PM, IVP wrote:

> I'm not sure that a PIC pin has the drive capability to switch a FET
> fast and hard enough.

You (we) could do the math.  Gate capacitance is usually on the =20
datasheet, and we know that a PIC output is good for 20+mA...

My general impression is that a PIC output is likely to be pretty good =20
at driving all but the highest power MOSFETs (which may have =20
particularly high gate capacitance.)

(2n7000 datasheet says 50pF max.  25mA PIC output implies an output =20
impedance of about 200 ohms (less than that, actually, since the =20
current isn't limited to 25mA)  That's an RC time constant of 10 ns, =20
which seems like it ought to be pretty safe.  You might also take into =20
account the consequences of switching the FET slowly and having it sit =20
in it's linear region during the switching time.   On a big FET =20
conducting 10s of amps, and REALLY counting on being fully on or off, =20
the consequences might be severe. On a 2n7000 class FET driving some =20
LEDs at less than half an amp; probably not so much.)

BillW

--=20
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