Designing a modern LiPo/Li-ion power solution: Selecting an I²C Switching Charge Controller.
Previously on this topic, we looked at the boost converter and inductor selection behind the Adafruit Powerboost 1000C and our own Chargeboost 2000. Since then, we have been testing and evaluating our chip selection to verify our initial calculations. At the same time, we’ve been searching for the best lithium-ion/polymer charger IC to pair with our boost converter. This has turned out to be much more difficult than selecting the boost converter, mainly due to the worldwide chip shortage we are all facing. Read more about that topic here. On top of that, charger IC’s with the high power ratings needed are in high demand for newer portable electronics like tablets and smartphones. Challenges aside - we are diligently working to find the best solution.
When we look at the specification needed for the charger IC, they are a bit different than the boost converter. Some of the specifications we are looking for are:
4-6V input voltage (compatible with most USB-A ports)
3-5A maximum input current (for fast charging with larger battery packs)
6-9A maximum battery discharge current (this is one of the most critical specifications when paired with the boost converter)
Intelligent power path management (this allows the charger to operate seamlessly between charge/ discharge mode, eliminating power drops when charging)
We initially tested the BQ24070 by Texas Instruments, however, it lacked some of the critical features listed above. Most importantly, it has a maximum discharge current of 3-4A which was not enough for the 6-9A peak switching current of the boost converter. The BQ24070 quickly overheated and burned out in testing, making it unsuitable for this application.
Our next pick was the MP2624 by Monolithic Power Solutions, featuring a maximum discharge current of up to 11A! The MP2624 is a bit more complicated to use than the BQ24070 as it’s programmed with a host controller using I²C communication rather than being a standalone solution. We were able to test and verify the board function by connecting it to a PC but the chip itself, unfortunately, cannot store these settings internally. This means that anytime the board is powered up or restarted, it would need to be programmed via an onboard or external microcontroller. In testing, the MP2624 works quite well so we are currently figuring out a cost-effective solution for the onboard microcontroller. Alternatively, we are in communication with MPS to see if there is a comparable solution that does not require a host controller.
MP2624 Charger IC Evaluation Board
MP2624 11A Maximum Discharge Rate
BQ24070 4A Max Discharge Rate
The other major challenge with our design is thermal regulation. We tested the MP2624 with the MP3422 boost converter we selected last time, but the boost converter quickly exceeded its maximum working temperature of 125°C (257°F) when outputting 5V @ 3A. Some of this heat was due to the MP2344 being slightly underpowered (See “Designing a modern LiPo/Li-ion power solution: High Current at 5V for DIY Projects.” for details). We then tested the MP3423 boost converter with a max switching current of up to 9A and this combination appeared to dissipate heat more efficiently. That said, the final PCB will still need to be designed with heat dissipation in mind. The small breakout board we are currently using for the boost converter is not optimal for the amount of heat generated.
MP3422 Maximum Working Temperature of 125°C
MP3423 with 9A Switching Current Limit
MP2624 + MP3423 Reaches Temperatures Near ~200°F at 5V 3A Output
MP2624 + MP3423 Test Layout
We hope to have more updates in the coming weeks with some concrete solutions using the MP2624 (or comparable) in tangent with the MP3423. Our next steps are to figure out a solution for the I²C programming and then to design a custom PCB layout for thermal regulation. Stay tuned for more updates on the Chargeboost project!
As always, please feel free to comment with any questions, corrections, or suggestions!
References:
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