Thanks for the replies, excuse the newbie questions, and apologies for
hi-jacking this thread.  I hope the original poster also got their questions
answered.  I originally had used 100Khz but saw some distortion in the DS
waveform and backed it off to 80Khz where it looked happier :).  The
datasheet says typical 25nC but the plotted data later in the datasheet
shows 12nC or less for the Vds (13.8v) that I am using.  I am using this
circuit to manage the current to a T.E.C. and which is typically 3A.

I could back off the frequency more if you think I am pushing the pic pin
although I tuned the LC ripple filter for 80Khz.

The datasheet is here:
http://www.irf.com/product-info/datasheets/data/irlz34ns.pdf

Any suggestions would be greatly appreciated.

Gregory





> -----Original Message-----
> From: Olin Lathrop [mailto:olin_piclist@EMBEDINC.COM]
> Sent: Wednesday, April 23, 2003 5:16 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: PIC driving a mosFET...driver chip or a FET?
>
> > I am switching at 80khz.  So it sounds that without the resistor there
> > is a risk of over loading the pic but with the resistor there is a
> > chance of overheating the mosfet. :)
>
> That means you only have 6.25uS per transition.  No wonder the FET was
> getting hot.  It was spending most of its time in the region between full
> on and full off where it will dissipate power.
>
> At this frequency, you need a much faster (higher current) gate drive than
> what a PIC pin can provide.  The FET should be in transition only a small
> portion of the time, let's say under 1uS.  For sake of example, let's say
> the total gate charge is 50nC.  50nC / 500nS = 100mA.  That's a lot more
> than you can expect from a PIC pin, let alone one with a resistor in
> series with it.
>
>
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