I've created a dual source energy harvesting board to store energy from small energy sources, like a 3V 15mA solar panel, into a 3F 2.7V supercap and to power a small wireless node (3.3V, 30mA), but I am having trouble finding the cause of a current leak.
The voltage regulator LTC3125 (U1) gets the energy from the solar panel and steps the voltage up to 2.65V to charge the 3F 2.7V supercap.
The supercap is the input for the 3.3V voltage stepup regulator LTC3525D-3.3 (U3) and its output (3.3V) goes to the load (wireless node).
This regulator get shut down when the voltage of the supercap goes below 1.1V and starts if the supercap goes above 2.3V. This is done by the non-inverting op-amp with additional hystheresis, the MAX9015 (U2), that drives the shutdown pin of the LTC3525.
The op-amp is powered by two sources: if the sun shines, it is powered by the 2.1V regulated voltage from the LTC3105 (LDO, pin2). If no sun and supercap >1.1V, the power comes from the LTC3525D-3.3 output. If no sun and supercap <=1.1, then the opamp is off and so is the LTC3525. Usually the supercap is >1.1V.
My problem is the following.
I let the supercap discharge thanks to a 1k resistor load on the 3.3V output of LTC3525.
At 1.1V the op-amp drives the SHDN pin of LTC325 low (0.00V) and the latter goes off (0.00V output) and the current draw from the supercap is about 10uA. With a power supply I simulate a solar panel and charge the supercap with 30mA@3V.
After a few seconds I turn off the power supply, the supercap stops charging, the current draw from it is 10uA for 1 second, then suddenly it goes to 2.3mA (1k res as load) and the output of LTC3525 is about 0.4V if supercap @1.4V or 1V if supercap @ 1.8V.
On the SHDN pin of LTC3525 I measure max 0.2V (polling every 5ms), average 0.04V. the LTC3525 shuts down if SHDN <0.4V and start if >1V. I added the pulldown resistor with a low value (20K) on the op-amp output, so that it does not float.
The current leak stays there until the supercap goes to about 1.2V. If I turn on the power supply a few secs and shut it down, the problem repeats.
If I leave the board off a few hours and without the supercap, then turn of the power supply just a couple of secs, there is no current leak. If I leave the psu on for 10 more secs, current leak starts again.
Holding with ceramic tweezers a resistor in parallel with R8 (pull-down on op-amp output) but going to on other GND pin (supercap GND), while the leak is happendin, it stops it. If I connect that resistor before the current leak starts, it doesn't solve the problem.
Maybe the output of the op-amp is floating really fast, that I don't see it with the multimeter and it is turning off and on the LTC3525 very quickly? If so, how do I prevent the floating, if there is no power source anywhere in the circuit?
Or maybe the cause is something else?
Thanks for your help