# What should I do if my MCU output is higher voltage than the Max Gate V on my N Channel MOSFET?

The V(GS) Gate to Source Threshold Voltage for a MOSFET I would like to use is listed as Min 1.5, Typ 1.9, and Max 2.3V. I am working with a Teensy3.1 that outputs 3V on the PWM pin. Do I need to get this voltage in proper range for this MOSFET, and how should I do this?

The datasheet for the MOSFET can be found here: CSD18534KCS 60V.

Not to worry. The gate threshold voltage is simply the level required to make sure the device switches. In this case, in can be as high as 2.3 volts, so your 3 volt PWM output is just about right.

This represents a minimum gate voltage that you need to apply. For the maximum, look at page 1 of the data sheet, "Maximum Ratings". The second item is Vgs, gate-source voltage. At +/- 20 volts, you have nothing to worry about.

• And "switches" is specified for a drain current of 250 µA, which is probably not enough in many cases. – CL. Aug 14 '15 at 8:05

However, the problem you are likely to find is ... not enough gate-source voltage. As the datasheet shows, the Abs MaX rating for Vgs is +/-20V so you are in no danger of destroying the FET from overvoltage.

However, if you look at the ON resistance, RDS(on) you will see it specified for Vgs=4.5 and 10V. What it will do at Vgs=3V is less clearly specified, but you can get some idea from the Threshold and Transconductance measurements.

Loosely, Vgs(th) is the threshold where the transconductance starts to apply, so if we take Vgs(th) as 2.3V and actual Vgs as 3V, the device is operating at 0.7V above the threshold.

Now the transconductance is given as (typical) 100A/V at Vds=30V, and it may be lower (and that is likely if Vgs(th) is on the high side)., But for 0.7V above the threshold, that would allow it to conduct 70A (at Vds=30V, which is where the datasheet specifies the transconductance) for an effective ON-resistance of 0.43 ohm.

So this gives us one (pessimistic) datapoint of about 0.4 ohm ON-resistance with only 3V Vgs.

The graphs may give us more information - let's look at Figure 3 : Transfer Characteristics.

This shows typical current at Vds=5V. At Vgs=3V. these currents range from 20 to over 30A depending on temperature, or an effective resistance of 0.25 ohms downwards - again these are typical figures, but closer to the 0.4ohm guesstimate above than the 0.013 ohm worst-case at Vgs=0.5V. (NB we don't know the Vgs(th) of the device measured for these graphs. If it was the typical 1.9V, that would bring the two estimates closer into line)

See also Figure 7 : "On-state resistance vs Vgs" : at Vgs=3V it is right off the top of the graph.

So use budgetary figures of 0.4 ohms and 0.25 ohms for the ON-state resistance, noting that these are both "typical" rather than worst-case figures, and see what the power dissipation and voltage drop are, at your maximum switching current.

And if the power dissipation is too high for your application - or even close - then you probably need to increase the PWM drive voltage above 3V. Or find a FET with more appropriate specifications, such as Rds(on) specified at Vgs=2.5V or 3V.