pwm fan controller - mosfet sizing help needed

[TerminalJack505]

Glad to see you got it figured out.

The voltage regulator looks fine. 

I liked the other MOSFET better than this new one.  The problem with this new one is it has a poorer worse-case V_gs(th).  It is 5V and the other one is 2.5V or 3V if I remember correctly.  Since you will be driving it at only 5V it may not be as efficient.  When using a MOSFET as a switch you typically want to drive the gate at least a couple of volts over the worse-case V_gs(th).

[heychris]

Interesting..  I'm not quite sure what the impact is of the Vgs.  The one I tried  is rated identically to the one in the maxim eval kit.
MAXIMUM RATINGS
Parameter Symbol Value Unit
Drain?to?Source Voltage VDSS 30 V
Gate?to?Source Voltage VGS ±20 V

Looking this up did cause me to realize that the one I tried was not a logic level gate.  That seems like it could be important.  right?

Maybe something like this?

http://www.onsemi.com/pub_link/Collateral/NTD5867NL-D.PDF

[TerminalJack505]

That one looks good.  Look at firgure 1 in the datasheet.  This will give you an idea of how Vgs affects the maximum current.  A 5V Vgs is more than adequate to drive this particular load.