I am trying to create a super simple Low Battery alert monitor, by utilizing an open-loop comparator. LBO is active-low - it's connected to a TTL input pin of a uC, running from same Vbatt voltage. The circuit's running from two series AA cells so I get 3.0V when it's fresh, and I want to create an alert when battery voltage (Vbatt) falls below ~2.15V. Zener diode is Vz=1.8V @ 50uA.
I calculated the voltage divider R43/R46 so that it'll give that 1.8V threshold voltage when Vbatt reaches 2.15V, and indeed it does so. So whenever Vbatt>2.15 I get higher voltage on V+ than V-, but for some strange reason the comparator does not output its VCC, (I measure 0V on its output pin) only when Vbatt reaches 3.3V (used a programmable power supply) and so V+=2.75, V-=2.11V I detect that the comparator is switching to output VCC.
Why does this simple circuit doesn't work? Why does this comparator needs >0.6V difference between V+/V- to switch its output to high? Is there some parameter I am missing? Can I solve this issue without changing the circuit (only resistor values)?
U5 comparator is Diodes AP331A chose it because it can run from single supply down to 2.0V which matches the whole idea.
D1 zener is DDZ9678