ATL431LI consists of an internal reference and amplifier that outputs a sink current based on the difference between the reference pin and the virtual internal pin. The sink current is produced by the internal Darlington pair, shown in the above schematic (Figure 21). A Darlington pair is used in order for this device to be able to sink a maximum current of 15 mA.
When operated with enough voltage headroom (≥ 2.5 V) and cathode current (IKA), ATL431LI forces the reference pin to 2.5 V. However, the reference pin can not be left floating, as it needs IREF ≥ 0.4 µA ( see Specifications). This is because the reference pin is driven into an NPN, which needs base current in order operate properly.
When feedback is applied from the Cathode and Reference pins, ATL431LI 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 needed in the above feedback situation must be applied to this device in open loop, servo or error amplifying implementations in order for it to be in the proper linear region giving ATL431LI enough gain.
Unlike many linear regulators, ATL431LI 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 13 can be used as a guide to assist in choosing the correct capacitor to maintain stability.