I wanted to add to my power supply circuit an undervoltage lockout based on this but with different components. The circuit looks like this:
The circuit doesn't work as expected, M2 should disconnect the load at 3.2 V but it doesn't.
Basically @3.2 V I have (measured at OA1) at pin2 1.79 V and at pin3 1.88 V, pin1 goes high to 1.95 V.
D2 voltage reference LM4041DIM3-1.2 needs, according to the datasheet, 45 μA minimum operating current so if R4+R5 are about 40 kΩ I have:
I=V/R => 3.2 V - 1.2 V/40 kΩ = 50 μA.
The Vgs of FDS4685 is -1 V to -3 V not sure which to account for. I guess I need about -1.3 V since is 1.95 V (gate) - 3.2 V (source).
I have also tried the IRF9310 that has a Vgs of -1.3 V to -2.4 V with same result.
Are both my Pmos choices wrong? Are my calculations wrong?
Also, I could really use some help in calculating R7 resistor value for the hysteresis, this 1.8 MΩ value is just an approximation.