Johnathan King wrote:

> Much can be done with the PIC output, but if you are looking for a
> high switching frequency to allow better response/smaller
> inductor, you will have real problems with a PIC

I probably wouldn't use a switching frequency above 20KHZ with
a PIC anyway.

>However someone trying to the mythical max of 2W dissipation in the
>transistor will find themselves in for an unplesant surprise
>based on your assumptions.

The 2W was just stated verbatim as a max Pdiss from the data sheet.
I should have mentioned the linear derating factor of 25mW per
degC that you have to subtract from this for every degree above
a 25degC ambient that the package is at during normal operation.
Thanks for calling me on this one.

>Try using the gate charge graphs  in the IR datasheet, adjusted for
>the voltage and current conditions you are using.

 Thanks again John for the points on my sloppy research.  I assumed
the PIC was going to provide the same drive as the 74HC00 nand gate
that IR used in there app. note.  They were also switching resistive
loads.

> Trying to switch extra current will quickly cause problems with
> these switching times.

 Is this extra current greater than the design peak current? I guess
I'm asking if you should use, as the bottom line, a MOSFET driver for
this application.

                                 Mike