Latching control via active-high battery supply / regulator shutdown pin?

Question

I'm looking to supply an MCU (3.3 V) from battery via voltage regulator: a pushbutton should enable the supply, the MCU then should keep the supply enabled until it wants to shut itself down. But the pushbutton should be dual-purpose, i.e. it should also work as a normal button readable by the MCU when it is powered.

I found STs STNS01 which integrates an LDO (3.1 V), a Lithium battery charger (4.2 V), and power path switches (supply or battery) and features a shutdown pin (active-high, but <4 V, with internal 500k pull-down) that disconnects the battery resulting in microamp quiescent current.

I don't get how the shutdown would be used without resorting to a voltage divider referenced to the battery voltage (~600 k pull-up on SD to 4.2 V should result in 1.6 V). For one, this would result in current (~ 4.3 uA), but also I can't think of a way to decouple the MCU/pushbutton from the battery voltage, i.e. when the regulator is shut down, the high-level on SD would cause currento to flow into the MCU supply via the protection diodes.

Using separate IO to control SD and read the pushbutton would be OK. Also, I'd think this could be done with a few transistors... but I don't see it :-/

Answer 1

Using the shutdown pin you would need to add some diodes and a MOSFET to do what you want. However, as you have already guessed, the pull-down in the LDO adds to your quiescent current when it is off.

^{simulate this circuit – Schematic created using CircuitLab}

A better solution would be to add in a P-MOSFET and shut off the regulator entirely.

^{simulate this circuit}

How that ties into the charging part though I am not sure.