Long story short: I'm using an Arduino Nano33 IoT in a project which is powered by a 16-cell NiMH pack. When fully charged the pack measures 23,1V. The Nano33 IoT max supply voltage is 21V.
At some point when designing the PCB the battery specs changed form a lower voltage pack and I simply forgot to add in a regulator for the Nano. I now need to bodge a together a fix, suitable for a very space constrained application.
So close, yet so far …
I'm imagining three possibilities:
Try and somehow cram an actual switching regulator in there somewhere
Use a couple of diodes in series to drop the voltage a tiny bit. I'm estimating that this will waste about 200-400mW. That feels really stupid (which it is, obviously), but for this application it's not really a huge deal, compared to the 30 watts of LED strips that this contraption is controlling. It's just meant to last for an hour before recharging.
The datasheet for the Nano's onboard regulator recommends a voltage input range up to 21V, but the absolute maximum rating is 28V. That's a bit of margin - and being NiMH, the battery voltage will quickly drop below 21V. I'll test and see what happens if I just feed the poor thing the raw battery voltage. BUT: if I find that it seems to work and not overheat, can I consider it safe to use? Or are there any possible non-obvious long term consequences of using the MPM3610 in this middle ground between recommended and maximum input voltage?
Which of these options is the least bad? Regulator feels overly big and complex for such a small voltage difference, which is only relevant at full charge anyway. Diode dropper feels wasteful, though that doesn't really matter too much here. Raw battery voltage feels dangerous, even if it's well below the absolute maximum rating.
Side question: the 7V difference between recommended and maximum sounds like a lot. Is this just to allow for a bit of ripple and weirdness on a nominal 21V supply? I must admit I rarely consider the absolute maximum ratings of regulators, I'm usually not so dumb that I end up there.