# Selection of MOSFET

The datasheet for the LM74700-Q1 states that "The maximum V(gs) that the IC can supply is 13V, so a MOSFET with 15V minimum V(gs) should be selected."

I don't understand for the MOSFET to be on the condition for NMOS is V(gs) >= V(th) which is positive for NMOS.

If the minimum V(gs) is 15V, and the maximum supply can be 13V, how would this work?

For the IC, the charge pump capacitor (external) is connected between anode and Vcap to provide sufficient gate drive voltage to turn the MOSFET on. This seems contradictory

• They mean 15V Vgs(max). Commented Apr 8, 2023 at 17:48
• It makes much more sense now, I guess they meant a minimum of 15V V(gs)[max] Commented Apr 8, 2023 at 17:57
• It is not a minimum of 15V Vgs[max]. It means a maximum of 15V Vgs[max]. Commented Apr 8, 2023 at 20:24

$$\V_{GS}\$$ is the maximum allowed voltage between gate and source. The LM74700-Q1 can supply a maximum of 13V from gate to source of the MOSFET. To avoid damaging the the MOSFET, you need to use one with a $$\V_{GS}\$$ greater than 13V. 15V adds a bit of a safety margin so that nothing bad happens if there's a little ringing on the gate.

You seem to be thinking of the gate-source threshold voltage ($$\V_{GS(th)}\$$). The 13V drive voltage and a $$\V_{GS(th)}\$$ of over 15V wouldn't work - as you correctly noted. The threshold voltage is the point at which the MOSFET begins to conduct.

• So essentially, the 13V is more than enough to drive the NMOS with it being fully operational. In this case, the max V(gs) for the NMOS is 15V, above which it will not work as intended. Am I right to draw this conclusion? Commented Apr 8, 2023 at 17:54
• If you use a MOSFET with a VGS maximum of less than 15V, then the voltage from the driver (13V) will probably damage the MOSFET.
– JRE
Commented Apr 8, 2023 at 18:15

There are two specifications for $$\V_{GS}\$$ in all MOSFET data sheets that are important: -

• The threshold at which the MOSFET starts to conduct called $$\V_{GS(THRESHOLD)}\$$ and,
• The maximum safe operating voltage between gate and source called $$\V_{GS(MAX)}\$$.

You are trying to make comparisons and mixing up the two things.