AVR-Chat

If you are building a push-pull, H-bridge style driver, look up "shoot
through." That is when you turn the high side off, and the low side
on, or vice-versa. The drivers don't turn off instantly, so you need a
little delay. Without it, you effectively short out the power supply
every time it switches.

Rather than all our speculating, just let us know what you are
driving, voltages, etc. Unless it's an anti-gravity lawnmower, there
is probably some info around about doing it.

Regards,
  JJ

On Wed, Jun 18, 2008 at 4:08 PM,  <wagnerj@proaxis.com> wrote:
>> 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
>
> 



-- 
Regards,
 JJ

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