Another technology doesn't quite do what I need.

Got a project that involves, among other things, feeding a more or less constant current, on the order of 2-3 mA, to some components, while both their voltage drop and the supply voltage may vary.

Back when I were a wee lad, there were things called "current regulator diodes," which I never had occasion to use.

And, look!  They still exist.  And are available in SMT.  And currents in the range I want.  And...

Oh.  Minimum forward voltage to ensure supplying that much current is ~2V.  My supply headroom is, worst case, rather less than 1V.

So, if in fact we end up needing current regulation (a subject which is still much open to question), a CRD won't do it.  There are other ways, but they take up more board space.

Ah, well.  Knowing they still exist is useful; I occasionally need a small constant-current source in an application where a CRD would work.