>>> Looking for a suitable one of these.  Application will be to have 
>>> a PIC output pin drive an LED display column (up to 200mA).

> Interestingly, the LED display example on piclist James pointed me 
> at in
> reply to MY question had N-channel MOSFETs in that position (yeah, 
> even
> with common anode LEDs.)  I guess you can more-or-less guarantee 
> Vsource
> (LEDs being non-resistive), so you can ensure enough Vgs for that to 
> work?
>
> http://www.piclist.com/techref/io/led/8x7s8pin.htm

As you infer, that's a rather "naughty" way to use those FETs.
The end result is probably acceptable in many cases but is not 
"tightly designable" .
It will work, of course, but it leads to a rather undefined LED state.

The basic design is quite clever. With one I/O pin low and one high 
and the rest floating only one LED is driven. No problem with that. 
BUT the FET is driven by the difference between its gate - driven to 
Vdd (presumably 5 volts) and its source which is "on top" of the 
relevant LED. The FET needs Vth minimum to turn on and somewhat more 
than this to provide LED current. How much more than this will vary 
with FET, batch, temperature, current and more.

Iled ~= (Vdd - Vled - Vgs)/ 220r

But Vled will vary with current, batch, temperature etc

A more controlled result could be produced using the same scheme with 
the FET drains grounded and common cathode LEDs or possibly common 
anode LEDs still and a bit of head scratching. I'll leave that to 
others as i should be working.



            RM 

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