Low voltage shutdown with regulator using microcontroller

Question

I'm using a lithium polymer battery with an LDO regulator. I want the regulator to shut down when the battery voltage gets too low. I'd prefer to monitor the battery voltage using a microcontroller that's already in my circuit rather than adding a separate voltage supervisor or comparator to control the regulator's Enable pin.

The problem is that the microcontroller is powered by the regulator's output VDD, so before the regulator is enabled, the microcontroller is off and its GPIO pin can't control the Enable pin. The battery's on/off switch is an SPDT switch and not momentary. Is there a solution to this that would "cost less" than adding a voltage supervisor/comparator to control the regulator's enable pin? And not adding a momentary switch? I know this won't work:

Answer 1

A supervisory/comparator or circuit is required if you want to do LDO enable control. One other option are LDO's with undervoltage lockouts which essentially have a comparator built in. Like the one in this question.

LDO with low-voltage cutoff?

Your right, you can't use a microcontroler to do undervoltage control, because it needs voltage to run. You need an external circuit. Another option is to use a pmosfet to do the control, which doesn't require a comparator.

Answer 2

I like this problem and it's kinda common to see something like it where an enable has to be constructed from a set of input signals.

Obviously your microcontroller can establish a PWR_Run signal from it's GPIO output, but it can't do that all the time. You should decide under what other conditions you want your LDO to turn on and stay on, and then build an or gate out of diodes and resistors.

My first thought would be to sense the rising edge of the input to the LDO as it's connected to VIN using a capacitor and an RC filter to let it turn on for a moment as the switch is connected and setting up the GPIO pin. As VIN charges the LDO input it will also charge EN starting everything up. Then before the resistor pulls the value of EN down to below the turnon threshold the microcontroller can assert GPIO keeping it high until the switch is turned off.

^{simulate this circuit – Schematic created using CircuitLab}