this post was submitted on 04 Nov 2024
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I don't think people realize how extremely little 50uW is.
For a standard 3.3V microcontroller assuming a 95% efficient voltage regulator will be a current of 14.4uA. Just having the HSI master clock enabled on one of the low power STML0 chips is 15uA. This will literally only the clock. That is 0 sensors, 0 communication, 0 IO, nothing useful at all. For reference, reading SPO2 with a very efficient maxm86161 takes 10uA by itself in ultra low power mode with low accuracy and not counting the max leakage current of 1uA. For full operation, you need about 1000x-10000x that amount for short bursts.
"Oh but it can cHaRgE tHe BaTtErY"
Let's say the device has a standard 100mAh battery (apple watch had a 228mAh or more). At 100% efficiency with absolutely not one millijoule being used by any other electronics (which would never ever happen, it would at the very least need a boost converter), it would take around 277 days to charge up that tiny tiny battery.
Let's take another example of an even smaller battery. To charge one side of the airpods 3rd gen (0.133Wh battery), it would take 110 days per ear
This is one of those free "energy harvesting" fad BS based in nothing but wishing and marketing. It is an interesting learning project for wireless antenna beginners, but that is the extent.
It's more than you think. I work with the MSP430 microcontroller, which is capable of a sleep current of 40nA @ 2V, full active mode at 140uA/MHz with all onboard peripherals turned on. With this you could achieve almost a 20% on-off ratio with a 1MHz clock, or keep it in active mode all the time at ~150kHz, which is sufficient for many embedded sensor applications.
You said it right there in your comment.
Sleep mode, (and other effectively off modes) where it is functionally useless, it can do.
MSP430 can do 140uA/MHz. That is ~7 times the power that this application supplies, and that is not counting any single other chip quiescent current or chip that actively provides useful data. You would have to have a battery anyway or a big cap to provide the needed current for on-states. Or you could run it extremely low frequencies like you said, but those tend to not scale linearly at all with per MHz power ratings. Quiescent currents tend to catch up fast at that scale. I would be extremely doubtful that 150kHz would scale perfectly and wouldn't have already exponentially decayed to around its lowest possible on-state consumption for the chip. I would definitely have to see tests on that.
The smallest of batteries like the VARTA tiny cells in TWS's are infinitely more useful and practical and it would take this application months to fill a single cell, discounting all losses.
The MSP430 is just the chip I happen to use at work, if you're not convinced you could try looking for an actual ultra low power chip, I found the STM32U0 at 70uA/MHz and the STM32U5 at 16uA/MHz in the first result.
Even ignoring selecting a more efficient micro, a smattering of tiny ceramic caps will buy you a few hundred microjoules for bursts. If you're already operating at 2V you can get a 6V rated 100uF cap in a 1210 package - and that's after considering the capacitance drop with DC biasing. Each one of those would buy you 200 microjoules, even just one ought to be plenty to wake up for a few tens of milliseconds every second to get a reading from some onboard peripheral (as an example) then go to sleep again.
For sure, you're not going to be doing any heavy lifting and external peripherals could be tricky, but there are certainly embedded sensor use cases where this could be sufficient.
277 days? Lol, at that point natural battery discharge is a serious enemy in your charing aspirations xD