> Driving a large mosfet directly from any micro is a bad idea.
>
> The gate capacitance will take a while to charge, since the on
> resistance of the fets in the micro is significant (>10-100 ohms)

I wouldn't say it's always a bad idea.  It depends on the switching
frequency, for one.  Let's say the total effective gate capacitance is
5nF, and the micro can supply at least 10mA (quite pessimistic
assumptions).  That comes out to a slew rate of 2V/uS, or let's say 3uS to
switch from one state to the other.

In a lot of circuits, 3uS switching transition time may be no big deal.
This means you'd be spending all of your time switching at 167KHz, which
is obviously a bad idea if you're trying to "switch" a load.  But at
10KHz, the switching transition time is only 6%.  That may be quite
acceptable.  Obviously the percentage only gets less as the switching
frequency gets lower.


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