Feature DescriptionTL43xx consists

Feature Description
TL43xx 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 the internal Darlington pair, shown
in the above schematic (Figure 24). A Darlington pair is used in order for this device to be able to sink a
maximum current of 100 mA.
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.
When feedback is applied from the Cathode and Reference pins, TL43xx 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 TL43xx enough gain.
Unlike many linear regulators, TL43xx 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.