When operated with enough voltage headroom (≥ 2.5 V) and cathode current (IKA), TL431 forces the reference
pin to 2.5 V. However, the reference pin can not be left floating, as it needs IREF ≥ 4 µA (please see Electrical
Characteristics, TL431C, TL432C). This is because the reference pin is driven into an npn, which needs base
current in order operate properly.