Kind peers,
First, I think I know the answer to this. I am just looking for buddy-checks.
Suppose I am designing protection for a cell-phone ... a 1-cell application. I am protecting to I(max) of 2Amp, for both charge and discharge. So, normally, currents no higher than that will pass through the MOSFET pair.
I believe I should be selecting the MOSFET based on the R_on rating. I would be thinking in terms of what Delta-V my application could tolerate at maximum, normal currents. When I pick that MOSFET pair, I wind up with something in the low 10's of milliOhms, combined resistance. This is familiar territory.
===> Here's the question: Do I still need to worry about the I_max rating of the transistors? It seems that every transistor I tentatively select automatically has an I(max-rating) way above what I need.
Since I am protecting (for example) at a 2A threshold, we might see a snivet of surge current until the protection kicks-in. But the steady-state rating is still much higher than I need. I think if the steady-state current rating is about 2x my protection threshold, I'm OK. (Usually it's much higher).
All I can figure is that, for over-current faults, my transistor must be able to handle excess current for the time it takes to qualify the fault (10-20ms).
Any other considerations for the I(max-rating) on a protection transistor?
Why do I ask? If a transistor with a lower (but still generous) I-max rating is cheaper, I think I should go that way, all other things being equal. I don't want to overlook something.
Most gratefully,
MABanak