Powering T3.2 or T3.6 from a single LiFePO4 battery
Hi all,
I'm looking at the schematics, trying to figure out how to supply a Teensy 3.2 or 3.6 from a single LiFePO4 battery. Some random things to start with:
Now there are a few ways to connect the battery:
The simplest solution might be: Cut VUSB and connect the Teensy USB connector's 5V (VUSB) to the battery charger's input, so that I don't need the charger's USB port at all. Then I can connect the battery output to the 3.3V pin and hope for the best. A series diode might make things safer, but it limits the usable battery voltage range and usable capacity.
Any thoughts on this? What's a simple, reliable way of supplying the Teensy from a LiFePO4 cell?
Regards
Christoph
Hi all,
I'm looking at the schematics, trying to figure out how to supply a Teensy 3.2 or 3.6 from a single LiFePO4 battery. Some random things to start with:
- The battery will have its own charger circuit with an extra USB connector (see https://hackaday.io/project/18041-lifepo4wered18650). I can get ground, D-, D+ and battery voltage from a header on the charger circuit. There is a pair of pads to connect a non-USB source.
- Since D+ and D- are required for communiation with a host, there will be a USB cable plugged into the Teensy. D+ and D- are not broken out for the Teensy's device port (the bottom D+ and D- pads are for the host port) so I can't connect them to the battery charger's header.
- LiFePo4 batteries are charged to about 3.6 V and discharged down to some 2.5 V or less.
- Depending on how I connect the battery, I'll have to subtract voltage drop for Teensy's regulator (330 mV typ. @ 500 mA) and at least one diode (probably another 0.3 V) on the way.
- The main microcontroller will operate down to 1.71 V.
- I'll need an SD card. I think they will be happy with as low as 2.7 V.
- I don't know the boot loader chip's voltage range.
- Whatever other devices I decide to connect later has to cope with the resulting supply voltage situation.
Now there are a few ways to connect the battery:
- VBAT on the Teensy is only for the RTC, right? So I'd ignore that pin right away.
- VIN pin, cut VUSB: There will be up to 0.6 V voltage drop (regulator plus diode). That doesn't sound good.
- 3.3V pin, cut VUSB:voltage might be too high when the battery is charged to 3.6 V since there's zero added voltage drop.
- 3.3V pin + series diode, cut VUSB: With a series diode I could get about 3.3V from a battery that's charged to 3.6 V. Might be unsafe with low currents, though, since diode drop will be lower in that case.
- 3.3V pin with up/down converter, cut VUSB: seems like a lot of effort for this.
- 3.3V pin with another low dropout linear regulator, cut VUSB: might be better than just a diode
- VIN pin, cut VUSB, remove and short on-board diode: might fail/damage the teensy while removing the existing diode. Creating a short there is probably also hard.
The simplest solution might be: Cut VUSB and connect the Teensy USB connector's 5V (VUSB) to the battery charger's input, so that I don't need the charger's USB port at all. Then I can connect the battery output to the 3.3V pin and hope for the best. A series diode might make things safer, but it limits the usable battery voltage range and usable capacity.
Any thoughts on this? What's a simple, reliable way of supplying the Teensy from a LiFePO4 cell?
Regards
Christoph
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