The TL431 is a three-terminal adjustable precision shunt voltage regulator integrated circuit. With the use of an external voltage divider, a TL431 can regulate voltages ranging from 2.5 to 36 V, at currents up 100 mA. The typical initial deviation of reference voltage from the nominal 2.495 V level is measured in millivolts, the maximum worst-case deviation is measured in tens of millivolts. The circuit can control power transistors directly; combinations of the TL431 with power MOS transistors are used in high efficiency, very low dropout linear regulators. The TL431 is the de facto industry standard error amplifier circuit for switched-mode power supplies with optoelectronic coupling of the input and output networks. Texas Instruments introduced the TL431 in 1977. In the 21st century, the original TL431 remains in production along with a multitude of clones and derivatives (TL432, ATL431, KA431, LM431, TS431, 142ЕН19 and others). These functionally similar circuits may differ considerably in die size and layout, precision and speed characteristics, minimal operating currents and safe operating areas. Construction and operation The TL431 is a three-terminal bipolar transistor switch, functionally equivalent to an ideal n-type transistor with a stable 2.5 V switching threshold and no apparent hysteresis. "Base", "collector" and "emitter" of this "transistor" are traditionally called reference (R or REF), cathode (C) and anode (A). The positive control voltage, VREF, is applied between reference input and the anode; the output current, ICA, flows from the cathode to the anode. On a functional level the TL431 contains a 2.5 V voltage reference, and an open-loop operational amplifier that compares the input control voltage with the reference. This, however, is merely an abstraction: both functions are inextricably linked inside the TL431's front end. There is no physical 2.5 V source: the actual internal reference is provided by a 1.2 V Widlar bandgap (transistors T3, T4, T5), driven by the input emitter followers T1, T6. This enables correct operation even when the cathode-anode voltage drops below 2.5 V, down to around 2.0 V minimum. The differential amplifier is made of two current sources (T8, T9); the positive difference of their currents sinks into the base of T10. The output open collector transistor, T11, can sink currents up to 100 mA, and is protected from polarity reversal with a reverse diode. The circuit does not provide protection against excessive current or overheating. When VREF is safely below the 2.5 V threshold (point A