> ...=A0use a PNP with a LDO to keep the
> switcher output one junction drop above the LDO output.

You can refine this in some cases by using divider to ground to
forward bias the PNP slightly,  so as to get even less than a Vbe drop
if desired - eg some LDOs have a much lower than 1 Vbe dropout and
this saves "wasting" excess voltage.
In some cases (eg 3V3 supply with 0.1v dropout) you may get say 5% to
10% more regulator efficiency this way.

I've used the PNP trick as above in a commercial design* -
"independently invented" so it's probably been thought up by many
people. I've never used  the divider refinement commercially - and
you'd usually not need to wring that sort of performance from a
system. If you wanted to fine tune it a single opamp section would
allow choosing any preset headroom voltage desired.


 Russell

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