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I've always been confused when picking a FET to be used for logic level signals. There are so many factors to look at for FETs, I get overwhelmed.

In this case my supply voltage will be a 3 alkaline batteries in series so my supply will range from 3.8 to 3.2 and max current seen will be under 1A. My PIC processor output logic high is Vcc-.7 so the logic output will range from 3.1 to 2.5V. My typical go to FET would be the AO3416 or the AOSS32136C, but they are all out of stock. I was thinking of using the IRLML6244TRPBF but unsure if it will work with 3.1-2.5V. What characteristics would tell me if this FET would be a good choice and does anyone have any "go-to" logic level FETs?

https://www.mouser.com/ProductDetail/?qs=9%252BKlkBgLFf1HkY%2F2U%252BIhLQ%3D%3D

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    \$\begingroup\$ As far as logic-level or not goes, the only thing you need to worry about is the gate-source voltage to achieved the rated RDson. \$\endgroup\$
    – DKNguyen
    Dec 27, 2021 at 6:45
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    \$\begingroup\$ In my experience designers tend to re-use whatever they’ve had success with in other circuits- perhaps less so for signal FETs but very often for more complex parts. I tend to use 2N7002 for the simple reason that I have a couple of reels on hand, but when I get new BOMs from establishment clients I’ll often see the same parts specified again and again. In the current climate though, you may find that you have to use whatever you can get your hands on. \$\endgroup\$
    – Frog
    Dec 27, 2021 at 7:02

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You want Rds(on) to be specified at your lowest gate voltage. And the specification must be adequate for your purpose.

If the drain current is 1 A in your application, as you say, and you can tolerate 50 mV of voltage drop, then the max Rds(on) is 0.05 V / 1 A = 50 mOhm.

So you are looking for a FET which has an Rds(on) of 50 mOhm or less at 2.5 V. However, I would be tempted to relax your criterion a bit. The Vout high is probably not going to actually be 2.5 V if the only load is a FET gate. So I think 50 mOhm with Vgs of 3 V would be OK.

Also, if you can tolerate more voltage drop at 1 A, then you can use a higher Rds(on). I just made up 50 mV to show how the calculation works.

In any event, the IRLML6244TRPBF should work fine.

Hope this helps.

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