I wanted to add to my power supply circuit an undervoltage lockout based on this but with different components. The circuit looks like this:


simulate this circuit – Schematic created using CircuitLab

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.


1 Answer 1


You have M2 connected incorrectly.

The article you linked shows this (left) and yours is on the right: -

enter image description here

Basically source and drain are inadvertently swapped.

Link to a comparator circuit that also could be used (with attention to detail): https://electronics.stackexchange.com/a/516481/20218

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    \$\begingroup\$ well, that's embarassing :) \$\endgroup\$
    – Status Re
    Sep 28 at 7:49
  • \$\begingroup\$ Almost every problem solved on this site is usually embarrassing for someone but, please don't be embarrassed because it happens to us all from time to time. \$\endgroup\$
    – Andy aka
    Sep 28 at 7:51
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    \$\begingroup\$ I have turned the Pmos and the circuit works so thanks, but now the voltage needs to up to at least 3.6V in order for the output to return at normal level. It also draw quite a bit of current unitl it stabilizes. Tried with 1M resistor with the point of return is at 3.5V. I guess it could be lower but what concerns me is this current draw of 0.5A that I can see on the power supply. Is this somehow normal? \$\endgroup\$
    – Status Re
    Sep 28 at 11:57
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    \$\begingroup\$ This is the trouble with comments; sometimes you just can't put enough information in them to ask a decent follow-up question. However, the solution is simple; ask a brand-new question and, highlight the nodes you are talking about and where the 0.5 amps is flowing. No, it's not normal @StatusRe \$\endgroup\$
    – Andy aka
    Sep 28 at 12:39

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