I thought I had this sorted but I was wrong...
I have an LM3671 3.3V regulator which has an EN pin which...
... enables the regulator when presented with >1.0 V
... shutsdown the regulator when presented with <0.4 V
I thought that if I pulsed 3.5V onto the EN pin to switch on the regulator, and also had a potential divider of 100k (lower) and 620k (upper) with 3.5V at the top, with center-tap also going to the EN pin that with the regulator pulsed on, the potential divider would hold the regulator on as it was above the 'off' voltage of 0.4V (at anything above 2.9 V, the potential divider will have a center value of above 0.4 V). What is this 'no man's land' between 0.4 V and 1.0 V ?
To pulse the regulator on initially I have a REED switch which connects the 3.5 V from the battery-in (regulator Vin) to the regulator EN. What I would like is for the regulator to continue to run producing 3.3 V at Vout until the battery voltage drops to about 2.8-2.9 V, and then the EN pin will have 'something' to tell it to shutdown...
Any help would be gratefully received. Was up until 530 am trying to solve this!