When thinking into powering my wireless headphones project, I automatically thought of Li-Ion batteries with a USB powered charger built in, pretty snazzy huh?, well not actually as most small electronic devices built in the past 5 years have done the exact same thing, but as it is a good way of doing things and works very well I decided to do it.
1) MCP73812T-420I/OT Charger Chip
Initially when looking for a charge chip, I consulted a schematic of a Olimex Leonardo clone that I have and know that used a Li-Ion battery charger. It turned out to be the MCP73812T 420I OT from Microchip, in a SOT-23 package. I’ve been very happy with this chip charging salvaged US18650 batteries. As Farnell already had the Eagle schematic files it was very easy incorporating it into my design and I used a DPDT switch to switch between USB and battery power. However when looking at the Olimex board and finding the chip, I realised the size of the SOT-23 and realised that there was no possible way of me hand soldering that, so I scrapped the idea and looked for another chip.
2) TP4056 Charger Chip
The other charger which I then considered using was the TP4056 which I already have in a little board off ebay. This was bought to charge the laptop cells at a higher current (1A) than the Olimex board could provide. This IC is in a SOP-8 package so very easy to solder, and I could just drop it into my design after making an eagle library for it. I created the eagle files for it here. However, there was another, easier way to charger the battery, straight from the Bluetooth chip.
3) QVC3860 internal charger
Just after finishing the eagle library for the TP4056, I was trying to find out more about the Bluetooth module itself as it had no documentation provided. I finally found the datasheet for the controller chip, the QVC3860 and looking at this made me realise that the controller chip had a Li-Ion charger built in, with the charger power supply on pin 25- VPOW. After further consulting the module’s schematic, I saw that VPOW was broken out on the CHG pin of the module-board for use. This is extremely helpful as it means I can connect the USB power directly to this and there is no need to add any other chips or switches.
After so much strife, ’twas so simple…
breadboardtronics 