The TL431 is a three-terminal programmable shunt regulator diode. This monolithic IC voltage reference operates as a low temperature coefficient zener which is programmable from Vref to 36V with two external resistors. They exhibit a wide operating current range of 1.0mA to 100mA with a typical dynamic impedance of 0.22 ohms.
The characteristics of these references make them excellent replacements for zener diodes in many applications such as digital voltmeters, power supplies, and op-amp circuitry. The 2.5V reference makes it convenient to obtain a stable reference from 5.0V logic supplies, and since the TL431 operates as a shunt regulator, it can be used as either a positive or negative voltage reference.