MOSFET answer

The spec on those is for 2.8V and at 7.5A.

This stuff is quite difficult to search. MOSFETs are one of those components that's just exploding in options, and the parametric searches often give you Rds(on) without telling you the Vgs for that parameter. Many of the most attractive newer options (especially the newer packages) turn out to be will-o'-the-wisp fata morgana parts which are unavailable in small quantities.

Best regards, Spehro Pefhany

Hello Spehro,

It's also important to rely on guaranteed data and for Rdson at a certain Vgs that would be the "max" column. I have seen many designs where people just took the graph. Worked fine initially. Then another batch arrived and it was like popcorn.

Quite a few devices that are marketed as logic level are too marginal for my taste.

Exactly! That was my concern.

IRF3708 is specified as Rds(on) max 29 milliohms with 2.8 V on the gate. I think that will do, for driving from 3-volt microcontrollers and the like.

Of course. Fortunately there are more and more that are guaranteed to have a certain Rds at 2.5V or even 1.8V. Unfortunately, that usually goes along with a low breakdown voltage (I've seen as low as 8V, so the D-S might break down before the gate). Still, the latter provides a clue for searching, which will help until the people designing the parametric searches get a clue and make it possible to search for what you need.

Best regards, Spehro Pefhany

Just keep in mind that you have to slosh around Cgd in excess of 2000pF with devices like this. Assuming that the Rdson of the high device on a modern uC port can be above 25ohms it can take a long time to drag the FET gate through the last few hundred mV up there. It'll be at the flat portion of the RC curve. IOW the area where you normally would want to be out of the linear range of a FET but might not be in this case.

Some uCs don't like driving capacitive loads of that magnitude.