Is It logic Level, Or What?!?

There's a lot of confusion, and even among "the professionals", a lot of controversy regarding the true definition of Logic Level.

Figure 1 is an excerpt from the DMG3402L Diodes Inc. datasheet:

DMG3402L MOSFET Datasheet excerpt
Figure 1

So...based on the Product Summary, is a DMG3402L MOSFET, Logic Level, or not?!? The datasheet never states that it is, but clearly we can deduce the following:

  • At 4A it's not! The Gate voltage needs to be at least 10V, to guarantee operation at 4A.
  • At 3A it's 5V Logic Level, but it's not guaranteed to work at 3.3V levels.
  • At 2A, its good even down to 3.3V levels! And, in fact, even down to 2.5V levels!!

So, it depends on what you intend to drive with it.

MOSFET datasheets will, often, state that a particular device is Logic Level. But, if that designation is absent, it can mean one of two things. The manufacture neglected to mention the fact, OR it's NOT a Logic Level device. In other words, it seems to be the case that the only clarity on the subject comes when a device is actually declared to be Logic Level [in other words, the datasheet says so]. Otherwise, you can't really be sure, and are left to make your own determination.

And, even when a datasheet does claim Logic Level, that, typically, only refers to 5 volt logic. If you're looking for a MOSFET to work in, for instance, a 3.3V environment, don't rely on the "Logic Level" tag! In other words, even though that datasheet says "Logic Level", that doesn't mean the MOSFET will work at voltages less than 5V.

So, what do you look for, on a MOSFET datasheet, to fathom whether it can be considered Logic Level or not? More on that later...

Here's an example of the kind of Box Free thinking I'm promoting: Most would argue that an IRF520 is NOT Logic Level. But, even an IRF520 can be considered [5V] Logic Level, if the Drain current is limited to around 400mA [higher if greater channel voltage drop can be tolerated]. And, it truly does seem silly to use an IRF520 to only drive 400mA, but hey, if all you have on hand is an IRF520, and you're in a pinch, then why not?!?

MacGyver would [see caveat, below]!

I wouldn't use an IRF520 at 3.3V, though, because the lowest voltage on the datasheet's transfer curve graphs is 4.5V, and the worst case gate threshold voltage is 4V. So, at 3.3V, the thing might not even be ON!!

Now, I'm not one of those Bad A$$ engineers that design Switch Mode anything! Which is why the following insights, recently offered to me by one of those Bad A$$ engineers, was news to me.

He told me that:

  • The output from typical logic devices, such as TTL, CMOS, MCU, etc, can't deliver enough current to properly PWM drive a MOSFET—at least not at the high speeds typically associated with efficient Switch Mode designs.
  • That for switching speeds less than 100nS, a gate driver is an imperative for high speed PWM, as gate drive currents of 0.1A, to as high as 2A, are needed to overcome the [relatively] high input capacitance of a typical high power MOSFET gate. Not even an MCU output [such as on an Arduino] can muster up currents like that—expecially not at Logic Level voltages!
  • And, generally, it's better to not use a Logic Level device for any kind of high speed switching because they're less robust in such an environment. Besides, a gate driver will supply voltages sufficient to drive a non-Logic Level device, so there's no need for Logic Level.

So, Bottom Line: If you're using MOSFETs to do the kind of stuff I normally do, like lazily turning things ON and OFF, or using MOSFETs to drive high-side MOSFETs that lazily turn things ON and OFF, then all the stuff I said, about busting out of that techno-snob box, is true. But, that IRF520 should never be considered Logic Level if you plan to use it in something like an efficient BOOST or BUCK switching design. In fact, don't even consider using an IRF520 for any of that esoteric stuff. There are far better, more modern MOSFETs on the market!