RESEARCH: Behavior of QX5252F (and probably CL0116)

Intro

The QX5252F (and it's brother CL0116) are a joule-thief type LED driver that can also use a solar cell to charge a 1.2V rechargeable battery (use YX8018 if you want 2.4V). Here I share my findings to try and figure out how this IC works.

Solar Cell Characterization

First off here is the IV & PV curve of the (shoddy) solar cell I made up. The test was done on a hot summer day with clear skies, so results are rough and don't use an exact 1000W/m² lamp.
As you can see peak power (~390mW) occurs at ~1.7V (~230mA).

QX5252F Tests

Circuit

I used the exact same circuit as shown in the datasheet which you can see here:

L = 100uH

Initially I tried setting the inductor (L) to 100uH, interestingly this limited the battery current to ~40mA. This might be relevant to table on pg3 of datasheet, though this table shows how you can set LED current by using different inductor values.

L = 20uH

I then lowered the inductor to 20uH, this time current was not limited and the battery got a much better charge. Also the battery I used had a capacity of 1200mWhr and the QX5252F managed to charge the battery to 925mWhr (77%) for the day.

SBAT to VBAT Diode Drop

From further tests I concluded a few of things:

1. The battery is charged directly by the solar-cell via a Schottky diode, hence the voltage drop varies with current. What this means is that at a low charging current you have a higher efficiency and at a high charging current you see a lower efficiency; for example with above data the peak efficiency (98.1%) occurred at a current of 0.01mA, while the lowest efficiency (83.8%) occurred at 136.44mA, also the overall efficiency for the day was 86.9% which is pretty close to the datasheet value of 90%
   
2. The QX5252F does not have maximum power point tracking (MPPT). Interestingly enough the peak power (230mW) for the 20uH test occurs at Vsolar-cell ~= 1.7V which if you look at the PV curve (different light conditions) is also the peak power voltage. I think this is more to do with me getting lucky with the solar-cell arrangement, as when I used the same solar-cell on a YX8018 while trying to charge a 2.4V battery the circuit would peak at 10mA before steadily dropping to 1mA (see graph below, terrible charging efficiency).
   
3. Strangely the inductor value seems to set a charging current limit for the battery, I am not sure how this works as I thought charging the battery occurred via the schottky diode. Also the oscilloscope did not show any switching DCDC converter behavior when charging the battery (light hitting solar-cell). 
   
4. When the battery is discharging the operational frequency of the QX5252F is ~133kHz. This is when the joule thief part of the IC springs into action.
   

Conclusion

The QX5252F is a pretty nifty IC which makes building a simple solar harvesting circuit very easy. A few small downsides is that:

• You are limited to a single 1.2V battery, though you might get away with using a YX8018 and a higher Voc solar cell
• You have to choose solar-cell that has a Voc of at least 2.4V (2x1.2V) for it to work properly
• As you would expect it does not have MPPT, not a biggie at this price point

Also the inductor sets the peak battery charging current (not expected) as well as the peak LED current (expected). I might have had my data logging circuit wrong, so will have to redo this step in the future