@Mishka Thank you very much for that link. Gosh, it sure would have been awesome to have such an ultra low power wake-on radio such as that described there. Unfortunately, I'm still getting no reply to even my second email attempt at contacting the company. Maybe they'll reply later, but for now I'm going to assume they are closed for business during the Caronavirus attack.
Fortunately, Figure 5 in the paper you linked shows an equivalent transistor layout for the voltage detector. It lacks a BOM with part numbers, but I'll nonetheless take a quick run at trying to simulate it in LTSpice--maybe I'll get lucky. If you were in my shoes, exactly which simulated transistors/mosfets would you be trying?
As for alternatives to the UB20M, the nearest I could find is this:
https://www.ablic.com/en/doc/datasheet/photo_ic/S5470_E.pdf
which, admittedly, isn't as nice because it is an ultra low current detector rather than a low voltage detector. Its quiescent current is higher than the UB20m, but it appears to be still quite low in absolute terms. What the S5470 does have that the UB20M lacks though is that the s5470 is well stocked at Digikey and similar places.
Have you run across any other parts that might fit the UB20M role?
Edit: I put Figure 5 into LTSpice. I could get it to generate the ~100mv reference voltage, but it doesn't appear to switch anything nor "detect" and then switch anything either. So, maybe there is more to the circuit that what they are showing. Given the circumstances of not being able to acquire their UB20M, it's a bit of a let down.
