# Picking SMT Logic Level FET

I need a logic level SMT FET that can handle 1.5A (safety factor included, will probably only see 800mA) that I can use with my Pro Mini.

1. If I want the FET, which is acting as a switch, to be closed but I want to keep the Arduino in sleep to save power, can I just set the default output of the pin to be HIGH? I don't want it on all the time with a pullup, just on when the Arduino is on but still sleeping.

2. If I’m turning on a FET with a digital IO pin (3.3V) from the Pro Mini, is a FET with a Vgs (th) of 3V low enough or should I really be using a FET with a Vgs (th) of 2.5V?

3. I found a FET that seems to fit my needs but I’m confused by some of the datasheet vales. It says:

VGS(TH) = 3V MAX
4.5 V Rated for Low Voltage Gate Drive
Drive Voltage (Max Rds On, Min Rds On) 4.5V, 10V

Does the drive voltage mean the FET is fully on? How can VGS(TH) = 3 max when the drive voltage is 10V?

1. Yup.

2. Use a MOSFET with Rds(on) spec at 3.0v or below. The Vgs(th) doesn’t tell you when it’s fully on, just when it first starts to conduct a little. You want some head room - the gate acts like a capacitor, so changes more slowly as the voltage approaches the drive voltage (that’s why it’s spec’ed at 4.5 and 10v - those are standard when the drive voltage is 5v and 12v respectively, and why the voltages quoted for Rds(on) are generally a little bit below the common logic levels), and you generally want it to reach a voltage where it can be considered fully on as quickly as possible (especially when PWMing it), because during that time it may be conducting a significant amount of current, but because Rds(on) will be higher, more heat will be generated in the FET. For high speed PWM, especially of heavy loads, one often uses a “gate driver” IC, which is designed to be able to dump a brief pulse of current into the gate (ie, a few amps) to minimize the switching time. That pulse of current is also why, when directly switching the gate, it is common to put a small resistor in series with the gate, as the pulse of current would briefly exceed the max current an I/O pin is rated at? That said, with small FETs, you can generally get away without it (I have had parts running like that, even with PWM, for literally years with no problems).

According to the data sheet it will work fine, though indeed the VGS, TH at up to 3.0V is unusually high for such a part. I’m more used to see <2V as maximum there.

Mind that 1.5A will be pretty much the limit for this part, unless you have a big copper pour attached to it that can help with the heat sinking. At 1.5A it dissipates almost 250mW.

AO3416 is really nice - total overkill, but amazing for a SOT-23. SI2302 would probably do the trick, and can be had dirt cheap in bulk on ebay/etc

The ones that I use on these boards I sell are all nice parts: https://www.tindie.com/products/drazzy/simple-sot-23-mosfet-six-pack-logic-level/

Thanks for all the responses, I will take a look at the ones you suggested. I was thinking about the IRLR8743TRPBF, can someone help confirm that this would work for an Arduino for 5V I/O? Probably way overkill but I assume I won't need a giant ground pour, a heat sink, or cooling because they are so efficient.

VGS = 4.5V
ID = 20A
Rds on = 3.9mOhm

You can crank the Queen Marry’s electric motors with that.

Those are massive overkill, but yeah they'll work. I usually default the IRF3708 as my excessively beefy fet, since it's also okay at 3.3v logic level drive.

One thing to be aware of - the beefier the fet, the higher the gate charge, and hence the slower the switching. This can be relevant if you're trying to PWM the load, particularly at higher frequencies. And the more you need to think about that small resistor in series with the gate if you're driving it directly from an I/O pin.

freebird4446:
a giant ground pour, a heat sink, or cooling

At 250 mW you don't need those - the PCB and traces themselves do the job just fine, and as you said most of the time you're running at half that current so 1/4 the dissipation.

If you have a ground plane on the top that also acts as heat sink (usually you'll have the source connected to GND).

wvmarle:
According to the data sheet it will work fine, though indeed the VGS, TH at up to 3.0V is unusually high for such a part. I’m more used to see <2V as maximum there.

If the Vthr is 3V is cannot even remotely be used at 3.3V Vthr is the switch-off point, not the switch-on point.

Always go by the Rds(on) spec, that’s shows the switching voltage(s) its spec’d for.

Thanks, forgot about gate capacitance @DrAzzy.

DrAzzy:
Those are massive overkill, but yeah they'll work. I usually default the IRF3708 as my excessively beefy fet, since it's also okay at 3.3v logic level drive.

Would you mind sharing your default logic level FET choices for various power levels?

See parts to the left of the title block:

Also see:
AO4484
AO4409
AO4419
AO4614B