Electronic – Using BSS84 P-channel MOSFET

Note that this is not a power converter but effectively a chip that uses a PFET to implement active rectification. As such, the current is intended to flow from the battery to the load when the switch is on. The FET is being used like a diode, making use of the inherent body diode of the part. The chip then turns on the FET to make the diode look more ideal when it should already be on. The chip will turn off the FET quickly when is sees SENSE go higher than Vin. If it didn't, the "diode" would conduct backwards.

The advantage of such a design is that the effective diode has very low forward drop, which is useful when it's in series with a battery since less of the battery power will be wasted. The drawback is that the overall diode has slow reverse recovery time, since the chip has to sense the reverse voltage and then actively shut off the FET. In this case the diode is used for power ORing, so a few µs reverse on time won't matter much.

"Why was everyone saying this would be so difficult? Am i missing anything here?"

Does it bother you at all that the BSS84 used in your model has a leakage spec that is about 2X the 50nA you are trying to switch?

Single MOSFETs are designed for low Rds for die size, not so much for switching extremely low currents (although tcrosley found one that will almost do what you want). To switch low currents like this, one would use a low leakage analog switch. For example something like DG2012 might work.

Remember that all models are wrong, and it is up to you to figure out if it is right enough to be useful.