I am using a lithium ion battery to power a circuit board at 5V. I am using the LM2623 boost voltage regulator, which works fine when the battery is hooked up to the circuit board. I would like to add a soft latch switching circuit between the battery and the voltage regulator.
I have successfully replicated this soft latch circuit on a breadboard. I am able to use the soft latch switch with the battery as the input voltage to light up an LED. There are no issues whatsoever with latching. I am also able to use the soft latch switch with the 5V output from the voltage regulator as the latch's input voltage.
The P-MOSFET I am using is the IRF9510, which is rated up to 100V and 4A. The voltage regulator doesn't ever pull more than 2A to handle its switching circuit.
The N-MOSFET I'm using is the ZVNL110A.
Works:
battery –> latch switch –> LED
battery –> voltage regulator –> latch switch –> LED
Doesn't work:
battery –> latch switch –> voltage regulator
This non-working set-up has strange behavior at the output of the voltage regulator. Upon clicking the button in the switch circuit, the voltage out of the regulator will spike to about 1.3V and then slowly decay down to 0V. This decay takes at least a minute or two, which has me thinking that this issue has something to do with the fact that the load is capacitive and that is why the switch won't latch. I am wondering whether this is a result of the switching circuit within the voltage regulator circuit, or due to the input/output capacitors to the regulator circuit. For all I know, it could be malevolent fairies who don't want my circuit to latch.
I don't think the problem has to do with the P-MOSFET being unable to handle the current that the voltage regulator wants to pull from the battery, and I know it isn't the battery's discharge rate that is somehow limiting the current into the circuit. I also don't believe it is the N-MOSFET's current limitation in the switching circuit causing the problem, because I don't believe that component ever receives an excessive amount of current or voltage.
I am a relative n00b when it comes to electronics, so any help would be much appreciated. I can provide more info if needed.
—Edit—
I tried monitoring the voltage at the gate of the P-MOSFET to get to the bottom of this. When I have 3.7V coming into the latch circuit, the gate reads 3.65V. If the voltage regulator is not attached, clicking the button will bring the gate voltage down to 0 V. If the voltage regulator is attached, however, clicking and holding the button only brings the gate voltage to 3.32V.
Best Answer
I'm not an expert, but I think your problem is your weak 1M pullup R3. Until your soft latch switches on, the load circuit will be acting as a pull down through that 100k and it might keep the gate low.