Momentary Power Switch Troubles

[ParkingLotLust]

Hey all,

I have built the following circuit on a PCB, and I'm having some trouble getting it to work.

I've double-checked connections and values, and I've only assembled the relevant section on the PCB.

The latch circuit is driven from a MAX1555, and eventually feeds 2 LDO regulators (I currently have a red LED hooked up to the output for testing). The output of the MAX1555 is ~4.2v, and I am using this dual MOSFET (Vgs is ~1v, should be fine).

When I power the circuit, the switch is "on", which is fine. When I press the button to turn it "off" (even holding the button), I can see a quick FLASH where the LED goes off (power turns off) but within 1/10 of a sec, it turns back on by itself.

I've tested the circuit in a sim and it appears to work as expected.

Anyone have experience with this/a similar circuit?
Cheers!

 

[MikeMl]

For that circuit to work, the load must be <<10K to ground. A LED wont do it...

 

[ParkingLotLust]

For that circuit to work, the load must be <<10K to ground. A LED wont do it...

Mike, thanks for your reply.

Is this a result of the 10k pull-up on the gate? If I understand you correctly, the load needs to be less than this 10k in order to shut off.

Since this is a battery powered projet, I dont want to waste current through a parallel load resistor - think I could bump the 10k up to 100k? Then my load just needs to be less than 100k if I understand you.

 

[MikeMl]

I spiced it. If the load is a simple resistor, it works if load resistance < 18K. If you put a LED in series with the load resistor, it never works, regardless of the LED's current limiting resistance value. If you shunt the LED/resistor with a 10K resistor, it works.

I think this was intended to switch a load with an effective resistance of a few Ohms.

With a high impedance load, I had to decrease R3 to make it work with a high impedance load. Here it is: play with it...

 

[ParkingLotLust]

Mike, thanks a bunch for the SPICE file! I've never used LTSpice before but I immediately like it better than the other (Java) sim I was using.

I see what you mean about shunting the LED/resistor. I will play with it from here, thanks again!