I was reading the data sheet for the LT8301 and laid on eyes on one of their example schematics.
The input range (Vin) in the example states 2.7V-36V, and even gives measurements for when Vin = 5V. However, looking at the Sumida 12387-T041 inductor (T1) shows that the application voltage range on the primary size is 18V-42V.
Presumably allowing less than 18V (i.e. Vin < 18V) enter the transformer is fine, otherwise Linear wouldn't give this example schematic.
So this begs the question: how is the lower voltage limit calculated, and at what point does the transformer no longer functions (output power on the secondary becomes insignificant (I realize this is a bit ambiguous, but I can't figure out how to word it differently))?