Solar charging and autonomous power supply for MCU

[Levan]

Hi All,
I'd like to create a PCB for my school project that captures energy from a small solar panel using SP1040. The harvested energy will then be used to charge a LiFePO4 battery (3.6~3.7V) through the LD6924 battery charger. Additionally, the voltage will be regulated to supply power to the MCU VDD. My ultimate goal is to construct a completely autonomous outdoor wireless module powered by solar energy. I've consulted various reference designs and product datasheets to develop the following schematics. I would greatly appreciate it if you could review it and provide feedback on potential improvements or identify any mistakes in the implementation.
Best regards,
Levan

[Peter BENSCH]

Quick cross-check:

• the L6924D is designed for Li-Ion/Li-Polymer batteries (4.1V/4.2V), not for LiFePO4. To make matters worse, you have set the end-of-charge voltage to the highest value of 4.2V with Voprg to GND, which would definitely kill the LiFePO4.
• you also have a 33pF capacitor (C12, C13) at the input and output of the LD39050, which is definitely not necessary. An MLCC with 1µF would be sufficient there (see data sheet of the L6924D, section 6).

Unfortunately, I cannot currently recommend a LiFePO4 charger.

Hope this helps anyway?

Regards
/Peter

[Levan]

Thank you for your feedback and guidance. If I substitute LiFePo4 with LiPo in this schematic, would it be fine? Regarding the dropout on LD39050 from 4.2V to 3.3V, should thermal vias be enough or i will need some heat sinks?
Thanks

[Peter BENSCH]

Yes, L6924D can charge LiPo, whereby the LiPo is charged to 100% with the current setting of 4.2V. If you want to fully utilise its possible lifetime (which also charges the battery to slightly below 100%), you should set the battery-saving voltage of 4.1V.

With the LD39050 and your application you will burn a maximum of 900mV (800mV at 4.1V), which can still be some power loss depending on the battery capacity and the output power of the PV panel. As a rule, however, you should be able to manage with thermal vias, as they are also included in the STEVAL-ISV012V1, because the maximum current in the LD39050 is limited anyway.

Good luck!
/Peter