The TL431A is a three-terminal adjustable shunt regulator that offers excellent temperature stability and high precision. It is often referred to as an "adjustable Zener diode" because it behaves like a Zener diode with a programmable breakdown voltage. By using two external resistors, the output voltage can be set to any value between 2.5V and 36V.

Specifications

• Reference Voltage (Vref): 2.495V (Typical)
• Reference Voltage Tolerance: ± 1% (A-grade)
• Adjustable Output Voltage: Vref to 36V
• Sink Current Capability: 1.0mA to 100mA
• Dynamic Impedance: 0.2 Ohm (Typical)
• Operating Temperature: -40°C to +125°C (depending on variant)
• Package: TO-92 (Through-hole)

Key Features

• Programmable Output: Easily set the regulated voltage using a simple voltage divider.
• Sharp Turn-On Characteristics: Provides a much more precise "knee" than a standard Zener diode, making it ideal for high-stability applications.
• Low Output Noise: Ensures a clean voltage reference for sensitive analog-to-digital converters (ADCs).
• High Temperature Stability: Maintains a consistent reference voltage across a wide range of environmental conditions.

Common Applications

• Shunt Regulator: Providing a precise voltage rail for low-power circuits.
• Voltage Monitoring: Acting as a threshold detector for battery undervoltage or overvoltage alarms.
• Isolated Feedback: Working alongside an optocoupler (like the PC817) to provide regulated feedback in Switch Mode Power Supplies (SMPS).
• Constant Current Source: When configured with a transistor, it can provide highly stable current for LEDs or sensor bridges.

Usage Tips

• Setting the Voltage: The output voltage (Vout) at the Cathode is calculated using:
  Vout = Vref X (1 + (R1/R2))
  Where R1 is between Cathode and REF, and R2 is between REF and Anode.
• Minimum Current: Ensure the device has at least 1mA of "keep-alive" current flowing through the cathode to maintain regulation.
• Stability: If using a bypass capacitor directly on the output, be aware that certain values (typically between 0.01uF and 1uF) can cause the regulator to oscillate. Use a value 10uF for high stability.
• Orientation: In the TO-92 package, with the flat face toward you, the pins are usually Ref, Anode, Cathode (left to right), but always verify with the specific manufacturer's datasheet as some variations exist.

Datasheet