Hi... It's not the first time I open a thread about this topic but i need some help in this new design. I've begin this journey a couple years ago and used many different circuit and now i'd like to put everything toghether.
Okay so... I'm trying to design a fast LED driver. The LED that i want to drive has a Vf of about 3.5V and a max current of 600mA. When in pulsed operation it can withstand a lot more (over 2A).
First thing i tried was using a LT1210 CFB op amp as a current source/sink. It works well (except for input offsets that needed compensation) but it's somewhat slow for my purposes.
When driven with an op amp current sink with a low side n-FET i could achieve very good rising edges (7-8ns measured with fast photodiode) but very poor falling edges (100ns). Obviously the single n-FET is not able to pass current both ways...
So i started looking around and found that i could use very fast GaN FET driver directly connected to the LED to achieve very fast rise/fall edges.
I've made a prototype that works really well (using a LMG1020) achieving sub 5-ns rise-fall. The problem here is that i've got no control on the luminous output other than phisically swapping the current limiting resistor.
So i'd like to tie all this togheter designing an op amp based current source/sink. I'm probably gonna use something like this:
or a difference op amp (like a current source it's built in "the art of electronics").
But i've got a problem choosing the right output stage. Let's say i'd like to have 3A peak current with 5ns rise/fall edges.
Should i parallel fast op amps with current sharing resistors (like a ths3491) ?
Could this be done with a BJT or FET push-pull? And in this case, how can i choose the right parts?
Thank you in advance.