Defining reference signal for PID controller
The analog way to do it is to use any voltage reference circuit to get a reference signal from the 13V side. The easiest idea to go for is a zener voltage, maybe even an adjustable one (like the LM431). You don't need to power anything from this zener voltage, you just need to produce an error signal. Therefore your buck converter is still useful (to actually deliver power to a load).







simulate this circuit &ndash; Schematic created using CircuitLab