I have a problem where I'd need to get 5v at just a few mA out of a hv supply. It might be as low as 96v and perhaps as high as 200v, maybe even 300v for some applications.
Now a zener can do it, but not very efficiently. This is a battery bank and it would take 1W out of a 200v battery to provide 5mA. So I'd like a buck switching reg.
This looks a bit tricky though. I didn't see any regs which like 200v for a source voltage! Unless I'm missing one somewhere. Actually, I have need to do this as both +5v from battery ground (positive regulator) and also as the battery + voltage minus 5v (negative regulator). The need for a negative reg seems to lower my options somewhat.
One thought was that the buck could power itself with the 5v it generates. Or if necessary maybe it could generate 12v, power itself off that, and then a second reg makes 5v for the load. That brings up a question of how it gets started, and the zener may be the solution there. After it is generating output it can switch off the zener.
Does this sound practical? I'm effectively asking for zero dropout here- the switcher reg supply voltage will be the same as its output voltage- can a switcher DO this?