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PICList Thread
'Re[6]: Clarifying PIC control of DC-DC converter'
1996\05\17@151522 by Mike Riendeau

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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

1996\05\17@173639 by Jonathan King

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At 03:15 PM 5/17/96 -0400, you wrote:
>
>Johnathan King wrote:
>d



>> 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.
>
At that power level, I guess if it works, it works.  One other thought was
if this was for a home project( instead of production), I would definately
add a driver chip, if nothing else to protect the PIC from the day that the
MOSFET died and took its driver with it.  The thought of losing my expensive
ceramic PIC instead of a $1 chip ruin my day :(


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