AcHmed99, thanks again for your help.
In a previous post I said "You can look at the circuit in the schematic I referenced in my first post." Sorry, I didn't reference it there.
This is the reference to it.
I think I've understood the reduced performance problems of the the OPA2170 now and see why I need a real RRIO opamp.
Do you already have a board made up?
Yes, I jumped the gun a bit with getting the PCB made. I thought I'd finished but found I had to make some changes.
I suppose Dave has his reasons for using opamps versus a dedicated current sense IC, price I guess but considering you will now need at least one RRIO amp plus 0.1% resistors you might as well just use a decent high-side current sense IC.
If I understand what "a dedicated current sense IC" is, I think Dave has used one. It's the
INA219 (U5) on his schematic. This is the high-side current sensor read by the micro controller which is intended to measure the actual output current, which will be displayed. The OpAmp circuit (U12) is a less accurate high-side current sensor, across the same shunt resistors. It feeds one side of the the current limit comparator U3B. The other side is set by the micro controller as the current limit. The follower OpAmp part of U12 is only there to avoid current which varies with the output flowing through R17, R18 and R33, as this would prevent being able to zero out the output current display.
I don't understand why Dave is not doing the comparison in software, using the current measured by the IN219, but I guess he has his reasons.
ADA4091
I've had a look at this. I can see from it's specs that it is a much better opamp than the others I've been looking at but it looks to me to be overkill and is twice the price of the
LM7322. The current limiting circuit is not trying to be very accurate. It just needs to impose a rough limit. The output current measurement wants to be as accurate as possible across the whole current range (10µA to 1A). Hence the use of the INA219.
I would be grateful if you could take a look at the specs for the LM7322 in the light of what it is for and let me know if you think it is inadequate.
Thanks.
Paul