Teensy 4 power management

[mpasternacki]

Hi,

I'm designing a battery-powered Teensy 4 project. It will run off 18650 Li-Ion and I hope to get away with using a single cell and not having to worry as much about battery management and all the "don't let the batteries explode" that goes with multiple cells.

If I cut the VUSB/+5V trace, is +5V used anywhere except LDO input? The info card says that it accepts as low as 3.6V , which means I can feed it directly from the cell (and avoid discharging it below 3.6V, which should improve battery life), but will the voltage be used anywhere else?

I will need +5V for powering WS28xx LEDs, so I'll need a boost converter anyway, but boosting voltage just to feed it back to LDO seems like a waste of watts. If the Vin voltage is used only to feed LDO, my plan would be:

- Use boost converter to power 5V section (WS28xx and a few other pieces that prefer higher voltage)
- Feed cell voltage directly to Teensy as Vin; use Teensy's 3.3V pins to power other 3.3V chips (staying within the 150mA limit)
- Use boosted 5V to check 3.3V voltage, alert and/or shutdown when it gets below 3.3V (or use 5V to check battery voltage directly and provide early warning and monitor 3.3V as another safety layer)

Does that make sense?

Another option would be a boost converter for 5V, a buck converter for 3.3V, and powering Teensy via 3.3V pins, monitoring the 3.3V regulator (difference between input and output; if the buck converter goes 100% duty, battery's dead). This would be more complex (and I'd need to make sure the 3.3V is stable enough), but then I could get more than 150mA of 3.3V for my elements, extra 0.3V worth of cell capacity, and it seems like monitoring would be simpler (just check 3.3V regulator's dropoff). Not sure how hard would it be to make it stable enough to power Teensy, though.

If the LDO is necessary to stabilize 3.3V and I want to utilize battery capacity below 3.6V, I'd either need a buck/boost 3.6V converter (maybe gain a few mAh from the conversion, but lose on complexity), or just a boost 3.6V converter that would kick in once battery gets below that 3.6.

Actually, if I feed cell voltage directly to Teensy's LDO, maybe it's worth it to add a boost converter (+ warning signal when it kicks in) as a "fuel reserve"?

What are your thoughts?
Thanks,
-- M

 

[RKESSING]

Based on the specs for the 3.3V LDO on the Teensy 4, you can probably get as low as 3.45 volts @ 100mA and still be safe as long as the ambient temperature is reasonable. I'm working on a project that will embed the Teensy 4 in different system modules and have set the buck converter that provides power to them to 3.45 volts for now.

Anyway, feeding the VIN directly from the battery and using the built-in 3.3 volt LDO would be the safest bet since it can tolerate both the charge voltage of the single LION cell as well as the nominal 3.6 volt running voltage. If you are concerned about transient voltages you can place a 1/2 watt Zener diode across the +V input as a simple transient shunt to prevent anything unexpected getting into the Teensy.

You can directly measure the battery with the Teensy as long as you use a voltage divider to reduce the voltage, I would recommend a simple 20K/20K divider to make the math simple.

Running switchers with inputs close to outputs can be problematic. It is usually best (and for many switchers, necessary) to have a minimum voltage difference between the input and the output since buck and boost converters have different configurations, even when using a switching regulator that can do either job. You would really need to use two cells in series and go with a buck topology to do it correctly, but then you get into the problems with multi-cell charging.

The 150mV dropout on the built-in 3.3V regulator is at the top end of the temperature range running at 200 mA load. So it makes sense to try the simple approach of feeding the cell directly to the +VIN and monitoring the voltage with the Teensy using a voltage divider. Once you see how the battery discharge curve works for your application, this approach might work for you just fine.

 

[JPB1]

Hello,

wondering to compare with Arietta G25 here :
https://www.acmesystems.it/power_consumption
Only 85mA full CPU and 128*256MBytes of RAM ... from Sd card too and USB 2 high speed host support..

Wondering to get Teensy 4 with USB high speed host support for SD and WIFI at the same time and low power mode, rtos .....and open source to improve...

Best Regards
Jp