TLV431 consists of an internal reference and amplifier that outputs a sink current base on the difference between the reference pin and the virtual internal pin. The sink current is produced by an internal darlington pair.
When operated with enough voltage headroom (≥ 1.24 V) and cathode current (Ika), TLV431 forces the reference pin to 1.24 V. However, the reference pin can not be left floating, as it requires Iref ≥ 0.5 µA (see the Functional Block Diagram). This is because the reference pin is driven into an npn, which requires a base current to operate properly.
When feedback is applied from the Cathode and Reference pins, TLV431 behaves as a Zener diode, regulating to a constant voltage dependent on current being supplied into the cathode. This is due to the internal amplifier and reference entering the proper operating regions. The same amount of current required in the above feedback situation must be applied to this device in open-loop, servo, or error-amplifying implementations for it to be in the proper linear region giving TLV431 enough gain.
Unlike many linear regulators, TLV431 is internally compensated to be stable without an output capacitor between the cathode and anode. However, if it is desired to use an output capacitor Figure 18 can be used as a guide to assist in choosing the correct capacitor to maintain stability.