Electronic – Basic comparator strange problem

batteriescomparatordiodeszener

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)?

schematic

simulate this circuit – Schematic created using CircuitLab

Info:

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

Best Answer

I think you are outside the input common mode voltage range. The data sheet gives (V+) - 1.5 max.