I'm fooling around with a PMOS, a Fairchild FDD6637, and pondering whether it would be possible to use Rdson as a sort of shunt-resistance for measuring the voltage drop, to get the amount of current flowing through drain and source.
The circuit will be using 2A at most. Here I'm thinking that if there's a short circuit somewhere, and the current rushes, I want to be notified when the current goes above 3A.
With Vgs = -4.5V, Rdson is 20mohm. At 2A, the Vds-drop will be 40mV. At 3A, the drop is 60mV.
So I had this idea: feed the drop into an opamp and pre-amp it to a more feasible level. Exactly how it's supposed to be coupled is unknown at the moment (I'm just playing with the idea), but let's say I couple the opamp to convert 60mV drop to, say, 4V (Vcc is 5V). Then let's say I deliver these 4V to another opamp (comparator) where one of the reference inputs is set to a fixed 3.9V, making it trip when the 4V arrives. That way I'll get a signal telling me there's a current rush.
Question: Am I thinking correct, or am I attacking this problem from the wrong direction?