I'm using the LTC3108 step-up converter to charge a supercapacitor from a thermoelectric generator. The output voltage drives a 1.8V regulator (S-1318) which is enabled once the voltage reaches 4V (MIC2779 is used to enable at 4V and disable at 3.3V). When enabled, the regulator drives a microcontroller and sensors.
If I directly power the board using a 4.1V external input applied directly across the output capacitor, the regulator functions as expected. However, when the input is coming from the step-up converter input, the regulated output increases to about 3.8V. It is not clear to me at all why this is happening, I'm not really sure where to look. I don't see any obvious coupling between the two. The output voltage is pretty steady (see plot) so I wouldn't think it is due to high frequency interference from the converter operation.
Any ideas on where to look?
Here is a schematic, top copper layer, and board CAD rendering (PDF in link, images below). The board is a four layer design stacked from top to bottom as Signal, GND, Vout, Signal.
A bit about the test setup: in order to test board functionality, the input is not actually a thermoelectric device, but rather a power supply. A voltage divider is used to supply a ~60mV input.
I think this has something to do with whether or not there is a substantial load. When the MCU is asleep, the voltage increases as per the previous graph. When it is awake, the output is a stable 1.8V. I actually noticed that the output regulation seems to work even while asleep when I had the programmer adaptor connected (see picture). I did this to facilitate measurement (instead of holding a lead to the test point) and noticed it was gone. I imagine the additional resistance in the wiring was enough of a load to stabilize the voltage, there are no other components (see photo below).
So it seems when the capacitor is fully charged (the step-up converter should want to hold it at around 4.1V) maybe the converter is still extracting energy and that charge is building up in, e.g., C12 on the regulator output? I'm not really sure how that would be possible, but it seems like having a large resistor on the output of the regulator would solve that issue. This sounds like a hack though.
When unloaded (to induce the 1.8V signal failure mode) the Vstore remains near 0V, maybe getting up to 0.5V max. Maybe somehow charge is instead accumulating elsewhere (like the 1.8V line). Also of note from the manual:
An internal shunt regulator limits the maximum voltage on VAUX to 5.25V typical. It shunts to GND any excess current into VAUX when there is no load on the converter or the input source is generating more power than is required by the load.
Which is interesting: where is this charge really going if the device is floating? If it's generating electricity but doesn't have anything to do with it, is charge just accumulating on the board then?
With no input to the boost regulator, but a charged Vout capacitor, connecting and disconnecting the programmer cable (my "load") causes the system to toggle between the proper 1.8V regulation (connected) and the floating to 3.8V failure mode (disconnected). The regulator datasheet doesn't indicate this should happen, although load regulation is only rated down to 1µA current (the MCU sleep should draw 300nA). Maybe this is just what the regulator is going to do with such as small load. o_O'
(At this point I'm just rambling and thinking out loud:) In the edit 5 tests, the connector wasn't being used to measure the voltage, the test point was, so it's not clear there's really any additional "loading". Unless the programming board is acting as a larger ground plane, disrupting capacitive behavior of the board's ground plane and signals, or discharging static buildup, something mysterious is still going on...