I'm interested in using some logic level (Vgs = 5v) MOSFETs, but I'm a bit stuck on which specifications are important.

Is the only thing that really matters the Rds (on) at Vgs = 5v? Besides the amperage/voltage rating, since I won't be going anywhere near those.

For example, take these two MOSFETs:
FDP5800
(Datasheet)
Rds (on) = 4.5 milliohms at Vgs = 4.5v

FDP8440
(Datasheet)
Rds (on) = 1.7 milliohms at Vgs = 4.5v

It looks like the FDP8440 is a better MOSFET because of a lower on-resistance, but it is not even labeled as a logic-level MOSFET. The FDP5800 is labelled as one, and has worse characteristics at Vgs = 4.5v. Am I missing anything here? If I had the choice between these two, would it be wiser to use the FDP8440 as a logic level MOSFET?

 

The term "logic level MOSFET" is a marketing tool and not a real specification. What really matters are the specs at Vgs = 4.5V

So either part would work for you. Either pick the cheaper one or the one with the larger BVDSS. Essentially there is no difference in RDS(on).

 

Well, depends on what you're using it for.

If its going to be used in a high current application then the choice of package and the corresponding thermal resistoance is critical to consider. You need to calculate the power dissipation in the MOSFET and multiply this by the thermal resitance to get the temperature rise.

If its going to be used in a switching application then the switching speed may be important to you. Actually its the gate capacitance that is the key, and designers usually use the "gate charge Qg" specification as a figure of merit. Lower Qg means faster switching with less current drive needed to by the gate driver.

 

The output high voltage of an old-fashioned TTL logic part does not go anywhere near +4.5V.
The term "logic level" applies only to modern Cmos logic.