EVBlog #262 Problem with power switch circuit variation

[boanerxe]

Hello everybody!

I'm currently working on a circuit which I'm trying to switch on using a button. The main idea is that when button is pressed, the power source gives power to a PIC microcontroller which writes '1' to PWR_CTL, causing the circuit to remain ON. I can use BTN_IN to detect when the button is pressed again for two seconds, and then, write '0' to PWR_CTL and power off the circuit.

The problem is: When the circuit is off, I have about 1.7 V in V_IN, which means to have 1V in the regulator output. This is causing several problems in the rest of the circuit, since it is not completely powered off (in addition I have an undesiderable consumption).

Can anybody suggest a simple soluition to keep the power source completely off?

Thank you!!

[klr5205]

Stronger pull resistors all around perhaps?  Could be that one or both of the bottom fets are leaking enough to drop ~500mV across R10 and turn it partially on.

All of those fets have very low threshold voltages. What do you measure at each fet's gate when the circuit is "off" ?


EDIT: You could also use your microcontroller to control the shutdown pin of the regulator, rather than having your own pass fet. The idea would be similar to how you use the PWR_CTL net now, where to mcu wakes up before the button is released and writes a "1" to the shutdown pin to latch power.

[boanerxe]

Hi klr5205. That leak current you're talking about was one of my assumptions, but I could not get any changes by tunning the R10 values.
Maybe changing N-channel MOSFETs by NPN could have some effect?  Although I think that solution would be less energy efficient...

The solution of controlling the shutdown pin of the source has the same problem I have know, since it is considering as "low" value less than 15% of the input voltage. If my reference is V_IN I'm having the same problem with those 1.7V.