At 02:54 PM 1/12/2014, you wrote:
>On Sun, Jan 12, 2014 at 1:43 PM, Spehro Pefhany <speff@interlog.com> wrote=
:
> > All the SOT-23 parts tend to have the same pinout.
> >
> > The IRLML6344TRPbF is a 30V part .. Rds(on) 29m ohm with 4.5V drive
> > and 37m ohm max with 2.5V drive. Only CAD 19.7 cents if you buy
> > $10 worth, so less than half the price of the SSM3K333R for better
> > performance. About double the gate charge, so not everything is free.
> >
> > I tend to pick parts that have wide applicability in ones-y twos-y
> > situations so that the same P/N can be used in the future- so being
> > able to drive it full on with a 3V supply can help with that.
>
>Hi Spehro,
>
>Interesting, I wonder what I chose at some point in my Digikey search
>that hid that one. Sometimes I find that having such a huge selection
>can be a tricky when trying to drill down quickly through results.
>
>Based on my (albeit rudimentary) understanding of how everything
>works, it does look like your new part would be able to accommodate
>PWM at a few hundred Hz, with my load (under 0.2A). Am I correct?
>
>I'm trying to make sure I'm not attempting to solve a spacing problem
>and creating a problem where the transistor will blow instead.
>
>Assuming that the part meets my requirements, would a gate resistor of
>51R still be appropriate?

I would guess it will be more than fine given reasonable conditions, but
I'm not going to do the calculations for you. Here's an excellent
app note that will help calculating switching losses. The conduction
losses are just ton/(ton+toff) * I(on) ^2 * Rds(on). Sum the two and
you have the total losses.

http://www.onsemi.com/pub_link/Collateral/AND9083-D.PDF

Remember an SOT-23 FET is not going to be as forgiving as a TO-220 part
if it goes linear, so don't forget the gate pull-down R.

--sp



>Thanks!
>
>Josh
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