First, thanks for your response Owen.
I have resolved it in another way.
The problem is that voltage falls proportionally in both inputs so, even putting a divider, they never get to match.
You´ll need a more stable internal reference.
Thanks to another partner of the forum, if we put in IDAC we have a "fixed" (through a external fixed resistor) voltage reference source. We can make a more stable Vref which the condition is given.
I have tested and work "better". The current source drops too but more slower.
VLOW : Resistor divider to 2.9V
VREF : 2.8V
When Vdd(3.1V) drops below 2.7V I have an interruption.