In my quest to reduce the cost and manufacturability of the LiFePO4wered/Pi+, my attention was drawn to the ETA1096, a Chinese boost converter. It is significantly lower cost than the TPS61236P, and it comes in a nice leaded package, so I thought I'd evaluate it to see if I could use it on the LiFePO4wered/Pi+.
I found out about this part while investigating some of my competitors. I found the Geekworm UPS HAT and although the creator tries very hard to hide which parts are used by scraping the surface markings off, some smart people on the internet figured it out anyway. Since the UPS lists 2A output current and the chip's specification lists up to 3A, it looked promising.
What was harder was to get a hold of samples to evaluate. The Chinese distributor refused to ship samples in the mail, and I wasn't going to spend $100+ to have a couple cheap chips shipped by Fedex. In the end I decided that the cheapest way to get a sample part was to buy the Geekworm UPS HAT. :)
Now it might be considered a stupid idea for my business to talk about a competitor here, but honestly the Geekworm UPS HAT is a pretty lousy device compared to the LiFePO4wered/Pi and Pi+. Of course I'm biased, but check out this attempt by an enterprising individual to make it usable and you'll know what I mean. Calling it a UPS is a bit of a stretch in my opinion, the out-of-the-box functionality is really limited. It's something you can make usable if you want a project, but if you just want something that works so you can get on with the project you're already doing, you might want to get a LiFePO4wered/Pi instead. ;)
I decided to hack in my own battery and charger (a #LiFePO4wered/18650 base) so I could focus on the ETA1096 performance without worrying about the rest of the system.
[Just one quick note on the "rest of the system". The charger chip (likely an ETA9741) is an odd part that actually puts 5V back on the input USB port even if the ETA1096 is off, and seems to draw 40-200uA from the battery.]
I used my electronic load and started with a load current of 1A. The ETA1096 seemed to be doing fine, the voltage didn't sag and the chip didn't seem to get warm. I bumped the current to 2A and the whole thing shut off. OK... start over and take smaller steps I guess?
So I went to 1.1A, 1.2A, 1.3A, 1.4A, 1.5A, 1.6A and it shut off again. Start over and let's keep it at 1.5A. After a minute I decided to touch the chip to check how hot it was and I yelped--blazing hot! I don't have access to the thermal camera at the moment so I used my IR thermometer and it reported 107°C! And it's always been conservative compared to the FLIR One since it averages a larger area.
Well, that's... unexpected. I knew it would get warmer than the TPS61236P since it has 40/55 mΩ MOSFETs instead of the TPS61236P's 14/14 mΩ. But that still only translates into 0.3W instead of 0.1W (based on 2.8A switch current estimate due to 3V->5V @ ~90% efficiency guess), not the "blazing hot" I was getting.
I guess I'll forget about the ETA1096 and stick to the "expensive" TPS61236P for now. ;) It bugs me though that I don't quite understand what's going on and cannot explain the amount of heat produced. I seem to be missing something and would love some fresh ideas on what might be happening.