Breakout board for LTC3388: https://www.analog.com/media/en/technical-documentation/data-sheets/338813fa.pdf
The purpose of this board is to convert high voltage (up to 20 volts DC) and typically weak harvested current into much lower voltage but also much greater current. If it works as intended, then it should harvest a great deal more electrons per unit of time than a simple solar-cell-plus-diode configuration, which, in a charging scenario, also drops the input voltage to the voltage of the battery or supercap being charged, but without the benefit of increased current.
In addition, by starting with higher voltage (typically from stringing a number of solar cells in series), in theory the LTC3388 is apt to more quickly skirt the cold-start bootstrap problem that might occur when harvesting from a single solar cell (or multiple solar cells in parallel) under poor indoor lighting conditions.
The four solder pads on the back of the board are for configuring one of four possible target voltages for the breakout board to produce as its output voltage. For LTC3388-1, the four possible voltages are: 1.2V, 1.5V, 1.8V, or 2.5V. For LTC3388-3, the four possible voltages are: 2.8V, 3.0V, 3.3V, or 5.0V. See Table 1 on page 12 of the above linked datasheet for the details on how to configure a particular voltage.
Fortunately, the LTC3388 appears to include an undervoltage-lockout (UVLO) circuit, so in theory that should allow the LTC3388 to work even with solar panels that have mere microamps of output. As long as there's a net gain of microamps into the system (i.e. current enters faster than it leaks out), the UVLO should allow charge to accumulate on a capacitor at the input until it reaches a voltage that triggers the LTC3388 to burst convert what's available and then go back to sleep.
|Qty||Value||Device||Package||Parts||Description||MF||MPN||Aliexpress or Ebay link|
|1||buck energy harvester||10-MSOP||main control IC||Linear Technology||LTC3388||https://www.digikey.com/product-detail/en/linear-technology-...|
|1||1uF||ceramic capacitor||0805||X7R||50v surface mount||https://www.digikey.com/product-detail/en/CL21B105KBFNNNG/12...|
|1||4.7uF||ceramic capacitor||1206||X7R||50v surface mount||https://www.digikey.com/product-detail/en/kemet/C1206C475K5R...|
|1||10uF||ceramic capacitor||1206||X7R||50V surface mount||https://www.digikey.com/product-detail/en/samsung-electro-me...|
|1||100uH||inductor||shielded SMT inductor||Coilcraft||LPS5030-223MRC||https://www.mouser.com/ProductDetail/?qs=QQJxVsr8EGaw%252Bi6...|