The TL43x-Q1 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 Figure 24. A Darlington pair is used to allow this device to sink a maximum current of 100 mA.
When operated with enough voltage headroom (at least 2.5 V) and cathode current (IKA), the TL43x-Q1 forces the reference pin to 2.5 V. However, the reference pin can not be left floating, as IREF must be at least 4 µA (see Specifications). This is because the reference pin is driven into an NPN, which requires base current to operate properly.
When feedback is applied from the CATHODE and REF pins, the TL43x-Q1 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 the device enough gain.
Unlike many linear regulators, the TL43x-Q1 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 24 can be used as a guide to assist in choosing the correct capacitor to maintain stability.