Power Supply : How to keep SMPS always above the post-regulator that follows it
You have a "floating" voltage (the delta across your post-regulator) that you want to convert into a ground-referenced voltage to use as feedback for your pre-regulator.

One way to do this is using a current mirror.







simulate this circuit &ndash; Schematic created using CircuitLab

The general idea is that whatever voltage you have from IN to OUT on U2 will cause a certain amount of current to flow through Q2. Q1 will create a copy of this same current, and if R1 has the same value as R2, you'll get the same voltage drop across it.

If U1 is designed to hold its FB pin at, say, 1.25 V, then its output will rise until it is 1.25 V plus Q2's VBE above U2's OUT voltage. For best results, Q1 and Q2 should be a matched pair in a common package.