When there is no load, the voltage at reset pin and mosfet's source is VCC unless the voltage is below 3.08V giving 0V at reset pin and mosfet's source. But when there is load, the reset pin is always 0V whatever voltage is at VCC.
What is wrong with my circuit?
There was a similar question supply-voltage supervision circuit (follow up) with no answers.
I 've updated the circuit based on your suggestions but the problem remains.
I 've used also pull up and pull down resistors at mosfet gate with same results.
I have this electronic load. When I start my power supply with 5V and the load adjusted to 0 mA, it works normally: Output is almost 5V (4.95V).
Then I adjust my load to 100mA and start decreasing the dc power to 3.0V. When voltage is about 3.1V output (load) is turned off (0V 0mA) as it should.
Then I start increasing the VCC voltage to 5V but the output (at load) is never recovered. It remains 0V. It recovers only if I adjust my load current to 0mA.
MAX809T reset pin remains grounded while increasing VCC. I can not understand why. When I test it without the mosfet if VCC<3.08 RESET = GND and if VCC>3.15 RESET = VCC.
I 've tried with AO3400 mosfet with same results.
After changing my load to a simple resistor of 22Ω my circuit works as expected. I don't know how my electronic load works but testing my circuit with it caused unexpected results. Thank you all for the help.
The circuit does not work with a real load. I 've modified it like the following image, but still does not work.
When mosfet gate is not connected (floating) I can measure the voltage at transistor collector = VCC if reset is LOW and 0V if RESET = HIGH = VCC. But when I connect the mosfet gate to transistor collector, the voltage at collector is always equal to VCC.
I 've tried also a 2N7000 mosfet in place of the transistor with same results.