Supercapacitors and ESR

[just4user]

But in the circuit I sent a link for, I used it in low-side.
If the dropout you guys know about is in the datasheet, I'd really appreciate it if you tell me where exactly that's hidden.

[T3sl4co1l]

What circuit?  The post you quoted was (I believe) in context of a high-side or in-line device, where Vgs(on) does matter (or a much more complicated solution is required).

Tim

[Smokey]

If you are using the ESP32 wireless, make sure you disable that when you go into shutdown.  It has a much higher current draw when transmitting.

[ArdWar]

But in the circuit I sent a link for, I used it in low-side.
If the dropout you guys know about is in the datasheet, I'd really appreciate it if you tell me where exactly that's hidden.

It won't be on the datasheet explicitly since the dropout is related to circuit implementation. For simple current source/sink, a BJT need to drop at least a V_BE although it's more common to use 2*V_BE, a MOSFET need to drop at least a V_GS(TH), a linear regulator need to drop at least a V_REF.
You can get lower dropout by using amplified current sense and/or lower reference voltage, but that won't be "simple" anymore. Some switching converter optimized for constant current (LED drivers) also have very low V_REF (usually something like 0.2V) specifically to reduce dropout.

In my opinion, I don't think dropout is even a problem here since you're using it to charge a capacitor, where the current will taper off to zero and the dropout will also be zero.

[just4user]

Yeah, I think the only problem could be that once VDS goes lower than VGS, it'll be in ohmic mode i.e. act just like a resistor. Now the bigger the FET, the higher the threshold and the sooner we encounter the ohmic mode. Put another way, instead of charging the cap @1A from 0v to 3.4v (=5-1.6) and then taper off the current (as in the example I linked to before), it'll charge at the current only from 0v to 1v (=5-4) and start tapering off thereafter. Not a voltage dropout per se but worth keeping in mind.

[jwet]

You could use a PMOS and avoid some of this but take on some other funny issues in a different direction.  The road goes on forever.

Like I said before, there are several ways to skin this cat... as there usually are.  This is why good engineers demand such incredible dough!

Keep it simple.

[just4user]

Dishonest mechanics and stupid doctors demand even more though. Engineering is quite underpaid IMO.