TLVH431 (or TL431) is a transconductance device. The current gain is enabled when the Op-amp input is above the reference voltage 1.24V (TL431 has 2.5V).
The need to bias the bandgap reference requires a voltage drop which I show with a Vb-e multiplier, and a 80uA current source. The reference input will not work unless the bandgap has bias.
These voltage reference circuits are common and I tinker with a fair range of them, so this circuit gets lots of changes.
