# MAX809 driving a mosfet

I am building a circuit that stops giving power to the load when the voltage drops below 3.08V. I am using MAX809T (3.08V version) and a SI2302 mosfet ($$\R_{ds}\$$ on 0.056Ω@2.5V)

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.

## Update:

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.

## Update 2:

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.

## Update 3:

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.

• Generally to use an N-MOS as a switch it needs to go on the low side. You can create a high side switch using a P-MOS. Jul 18, 2019 at 14:52
• @hekete I edited my port with the updated the circuit but I have the same problem. Jul 19, 2019 at 7:54
• So is it true that the only situation in which your circuit works is when you gradually adjust the load current from 0 to 100mA? For example a cold start at 5V with full load doesn't work either? Jul 19, 2019 at 9:12
• @hekete Correct. Cold start with load at 100mA does not work too. Jul 19, 2019 at 9:18

Your updated circuit looks okay. The MAX809 has push-pull output on the RESET pin and can source 20mA. I believe 20mA is enough to drive your MOSFET, but double check that.

Since you have a push-pull output, no pull-up/down resistors are needed (though RESET will become floating when Vcc drops below 1V).