Hi I'm having trouble with the circuit below. The idea is that an 5V input is regulated down to 2.5 V to charge a supercapacitor, C3.
The supercapacitor then powers the load (an MCU). However, the load is disconnected when the supercapacitor is charging. This is to allow the load to be hard reset in case of a brownout (which is likely as the supercap voltage drops).
The disconnection happens because the P channel mosfet gate is taken high when the input voltage is applied, and stops conducting from S to D. The rest of the time, the gate is pulled low through R2. This all works fine, and indeed I've used the same principle when charging a LIPO cell (using a LIPO charging regulator rather than a voltage regulator). While the voltage regulator is absent from the circuit, the mosfet does what I would expect, turning on most of the time unless the gate is brought high. However, when the regulator is in the circuit, I find that current seems to flow from C3 back through the MOSFET to the gate, incidentally illuminating the LED so long as there is charge in C3, keeping the gate high and shutting off current to the load.
I'm not sure whether the problem is with the regulator or the mosfet or what.
The supercapacitor then powers the load (an MCU). However, the load is disconnected when the supercapacitor is charging. This is to allow the load to be hard reset in case of a brownout (which is likely as the supercap voltage drops).
The disconnection happens because the P channel mosfet gate is taken high when the input voltage is applied, and stops conducting from S to D. The rest of the time, the gate is pulled low through R2. This all works fine, and indeed I've used the same principle when charging a LIPO cell (using a LIPO charging regulator rather than a voltage regulator). While the voltage regulator is absent from the circuit, the mosfet does what I would expect, turning on most of the time unless the gate is brought high. However, when the regulator is in the circuit, I find that current seems to flow from C3 back through the MOSFET to the gate, incidentally illuminating the LED so long as there is charge in C3, keeping the gate high and shutting off current to the load.
I'm not sure whether the problem is with the regulator or the mosfet or what.