Thread Starter


Joined Feb 23, 2007
Hey guys,

I'm kind of stuck on a circuit I'm designing. Mostly due to my lack of experience with FETs.

I have a square wave output from a MAXIM IC (MAX3131) that goes from ~2V to 0V. This signal is used to drive an IR LED. The output is connected to the cathode of the LED so that when the voltage drops to 0V, the LED turns on. In order to boost the intensity of the LED, I have to increase the current. However, the MAXIM IC can't handle the amount of current that I plan to use (~800mA with two IR LEDs). See the attached pictures (yellow signal is the output to the LED cathode).

Therefore, I want to use a MOSFET as a switch in order to turn the LED on and off. However, I seem to be having trouble picking a FET type. A P-channel JFET would do exactly what I want...but the ones I see on Digi-key do not let a high current through (drain current @ VGS = 0). I then tried searching for depletion mode, p-channel MOSFETs, but I don't see a lot of those.

Any tips on what MOSFET I can use, or alternative solutions? Thanks!



Joined Jul 31, 2011
I've just replied to your thread on this subject in the Projects forum, but see here that you've got some additional info: two leds at 400mA each (sounds dangerous to me!).

Well, I think I'd steer clear of mosfets and use bipolar. However, we'll be needing a fair bit of base drive at these current levels, so this might complicate things. I have to go now, so I'll revisit tomorrow...