AVR-Chat

> Steve - thank you for your info on these two.  What are some
> applications you've used these in?  I'm a newbie, just starting to
> learn, and I appreciate these hardware insights and experiences.
> Would you ever drive the 2804 directly from a uC output pin?  If you
> can discuss the use of the driver a bit, that would be great.  What
> do/would you say to the boss that says "we can't afford that extra
> part"?  Thanks for any further insight you can add to your already
> helpful post.  Info on things that work well for others, especially
> low power/drain components that "do the job" well, are always useful.
>
> Thanks !!
>
> Charlie


Charlie -

How you drive a FET depends on a couple of things. The biggest is whether
it is high-side or low-side. If it is N-channel high-side, or P-channel
low-side, you almost always need some sort of driver. If it is P-channel
high-side, then the issue is whether or not the voltage being controlled
is higher or lower than Vcc of the micro. If the voltage being controlled
is same or lower than Vcc, then you possibly CAN drive it directly from
the micro; otherwise, some interface is needed (though it might be pretty
simple). If it is N-channel low-side, then you very likely CAN drive it
direct from the micro.

All of this is then modified by the question of switching speed. FETs take
a LOT of gate current (sometimes AMPERES) to switch fast. Fast switch
means less power dissipation and more switching cycles per second. If you
application is pretty low speed (40KHz switcher, for example) and load
currents are not huge, then  direct drive from the micro could be just
fine (as bounded by the cases described in the previous paragraph). But,
if you need fast switching, then a good gate driver is in order,

Another limiting factor is the gate threshold of the FET and the Vcc of
the micro. You need the gate threshold to be no bigger in magnitude than
Vcc/2. Thus, driving a FET with a threshold of 2.4V from a 3.3V micro is
asking for big problems. These problems will happen at temperature
extremes and will be most noticeable with higher load currents (as the FET
won't be fully saturated and will have excessive power dissipation).

Hope this helps,

Jim Wagner
Oregon Research Electronics