I have the following circuit, part of my LM317 digitally adjustable power supply.
The problem that I have is correctly controlling the LM317 at U2: if I pull it's adjust pin to the ground, the output will go to 1.25 V. So far so good.
The problem comes when I connect the output of the opamp to the adjust pin, as in the schematics. For some reason when the output of the op-amp goes below 0.76V the output of the LM317 regulator will stay at 2.06V, even if the op-amp goes to 0V.
When the op-amp goes above 0.76V, the output of the LM317 is as expected.
So, why the op-amp does not sink the adjust pin correctly bellow 0.74V ? Am I missing something?