What would happen if the input voltage to a boost regulator exceeded the desired output voltage?
If the input voltage on a 12V-output boost regulator increased to more than about 12.5V, the output voltage would rise and track the input, less about a diode drop. Better regulation would result from adding a low dropout linear regulator to the output of an adjustable-output boost regulator, which has its output voltage set to 12.5-12.7V. The low-dropout regulator, of an appropriate current rating, can be used to drop the output to 12V, even when the boost regulator input is too high.
