The TL431 is a three-terminal adjustable shunt regulator that is widely used in Electronics. As this is a three terminal device and looks exactly like a transistor, it also requires less PCB space. It has three pins, Anode, Cathode and Reference pin. For accurate shunt stability the current into the cathode pin must remain >1mA. The Reference pin or Adjust pin is used to set the reference voltage using voltage dividers. 

Advantages of TL431 over the Zener diode is that the output voltage can be set to any value between its reference voltage (approximately 2.5 V) to maximum of 36 V and requires only two external resistors. Active output circuitry is another advantage that provides a very sharp turn-on characteristic where the Zener diodes fail in many applications, for example - onboard regulation, adjustable power supplies, and switching power supplies. 

In a nutshell, TL431  can be used  as a single voltage reference, error amplifier, voltage clamper comparator with integrated reference.

TL431 pinout:

Package Contains

1x TL431A Programmable Shunt Regulator 

TL431 Specifications:

• Adjustable output voltage VREF (2.5V) - 36V
• Operation From −40°C to 125°C
• 0.2-Ω Typical Output Impedance
• Sink-CurrentCapability:1 mA to 100 mA

TL431 Applications:

• Zener DIODE Replacement
• Adjustable Voltage and Current Referencing
• Secondary Side Regulation in Fly-back SMPS
• Voltage Monitoring
• Comparator
• As a reference voltage generator

TL431 Dimension and footprint:

Example Projects on TL431:

100mA Constant Current Generator

Additional Resources:

TL431 Datasheet