Introduction to TL431

The TL431 is a versatile and widely used precision adjustable shunt regulator. It is designed to provide a stable reference voltage and can be used in a variety of applications, such as power supplies, battery chargers, and voltage monitors. In this comprehensive guide, we will explore the fundamentals of TL431 circuits, their applications, and how to design and troubleshoot them.

What is a TL431?

The TL431 is a three-terminal adjustable shunt regulator with a reference voltage of 2.5V. It consists of a temperature-compensated bandgap reference, an error amplifier, and a programmable reference divider. The device can be used to regulate voltages between 2.5V and 36V with high accuracy and low temperature drift.

TL431 Key Features

• Adjustable output voltage from 2.5V to 36V
• Low temperature drift: 50ppm/°C typical
• High output current: up to 100mA
• Low dynamic impedance: 0.2Ω typical
• Wide operating temperature range: -40°C to +125°C
• Available in various packages: TO-92, SOT-23, and SMD

TL431 Pinout and Internal Structure

TL431 Pinout

The TL431 has three terminals:

1. Cathode (K): This is the reference input terminal. It is connected to the negative side of the reference voltage divider.
2. Reference (REF): This is the reference output terminal. It provides a stable 2.5V reference voltage.
3. Anode (A): This is the output terminal. It is connected to the load and the positive side of the reference voltage divider.

TL431 Internal Structure

The internal structure of the TL431 consists of the following components:

• Bandgap reference: Provides a stable 2.5V reference voltage
• Error amplifier: Compares the reference voltage with the voltage at the REF pin and adjusts the output accordingly
• Programmable reference divider: Allows the output voltage to be adjusted by setting the ratio of the external resistors connected to the REF and Anode pins

TL431 Basic Circuits

TL431 as a Voltage Reference

One of the most common applications of the TL431 is as a voltage reference. To use the TL431 as a voltage reference, follow these steps:

1. Connect the Cathode pin to ground
2. Connect the REF pin to the Anode pin through a resistor (R1)
3. Connect a capacitor (C1) between the REF pin and ground for stability
4. The reference voltage (2.5V) will be available at the REF pin

TL431 as an Adjustable Voltage Regulator

The TL431 can also be used as an adjustable voltage regulator. To set the output voltage, follow these steps:

1. Connect the Cathode pin to ground
2. Connect the REF pin to a voltage divider (R1 and R2) between the Anode pin and ground
3. Connect a capacitor (C1) between the REF pin and ground for stability
4. The output voltage (Vout) can be calculated using the following formula:

Vout = 2.5V × (1 + R1 / R2)

Advanced TL431 Circuits

TL431 as a Current Limiter

The TL431 can be used to create a current-limited power supply. To implement a current limiter using the TL431, follow these steps:

1. Connect the Cathode pin to the negative side of the input voltage
2. Connect the REF pin to a voltage divider (R1 and R2) between the Anode pin and the negative side of the input voltage
3. Connect a current sense resistor (Rsense) between the Anode pin and the positive side of the input voltage
4. Connect a capacitor (C1) between the REF pin and the Cathode pin for stability
5. The maximum output current (Imax) can be calculated using the following formula:

Imax = 2.5V / Rsense

TL431 as an Overvoltage Protection

The TL431 can be used to protect circuits from overvoltage conditions. To implement an overvoltage protection using the TL431, follow these steps:

1. Connect the Cathode pin to ground
2. Connect the REF pin to a voltage divider (R1 and R2) between the input voltage and ground
3. Connect the Anode pin to the gate of a MOSFET or the base of a transistor
4. Connect a capacitor (C1) between the REF pin and ground for stability
5. When the input voltage exceeds the threshold set by the voltage divider, the TL431 will turn on the MOSFET or transistor, effectively shorting the input voltage to ground

TL431 Design Considerations

When designing circuits using the TL431, keep the following considerations in mind:

1. Stability: Always use a capacitor between the REF pin and ground (or Cathode pin) to ensure stability and prevent oscillations.
2. Input Voltage Range: Ensure that the input voltage is within the specified range (2.5V to 36V) to avoid damaging the device.
3. Output Current: The TL431 can sink up to 100mA of current. If higher currents are required, use a transistor or MOSFET to amplify the output current.
4. Temperature Drift: Although the TL431 has a low temperature drift, it is essential to consider the effects of temperature variations on the overall circuit performance.
5. Noise Immunity: In noisy environments, use proper filtering and decoupling techniques to minimize the impact of noise on the TL431’s performance.

TL431 Troubleshooting Tips

If you encounter issues while working with TL431 circuits, consider the following troubleshooting tips:

1. Check the pinout: Ensure that the TL431 is correctly connected according to the pinout diagram.
2. Verify the input voltage: Make sure that the input voltage is within the specified range (2.5V to 36V).
3. Check the reference voltage: Measure the voltage at the REF pin to ensure that it is close to 2.5V.
4. Inspect the voltage divider: Verify that the resistor values in the voltage divider are correct and that the connections are secure.
5. Check for stability: If the output voltage is oscillating, ensure that the capacitor between the REF pin and ground (or Cathode pin) is properly connected and has the correct value.
6. Verify the load: Make sure that the load connected to the TL431 circuit is within the specified current limit and does not exceed the device’s maximum rating.

TL431 Applications

The TL431 is used in a wide range of applications, including:

1. Power supplies
2. Battery chargers
3. Voltage monitors
4. Overcurrent protection
5. Undervoltage lockout
6. Temperature sensors
7. LED drivers
8. Linear regulators
9. Switching regulators
10. Programmable voltage sources

Frequently Asked Questions (FAQ)

1. Q: What is the reference voltage of the TL431?
   A: The reference voltage of the TL431 is 2.5V.

2. Q: How can I adjust the output voltage of a TL431 circuit?
   A: To adjust the output voltage, use a voltage divider connected between the Anode pin and ground. The REF pin should be connected to the center tap of the voltage divider.

3. Q: What is the maximum input voltage for the TL431?
   A: The maximum input voltage for the TL431 is 36V.

4. Q: How much current can the TL431 sink?
   A: The TL431 can sink up to 100mA of current.

5. Q: Why is a capacitor connected between the REF pin and ground in TL431 circuits?
   A: The capacitor between the REF pin and ground (or Cathode pin) is used to ensure stability and prevent oscillations in the TL431 circuit.

Conclusion

The TL431 is a versatile and widely used precision adjustable shunt regulator that offers a stable reference voltage and numerous applications in analog circuit design. By understanding the fundamentals of TL431 circuits, their pinout, internal structure, and basic and advanced configurations, you can effectively incorporate this device into your designs.

When working with TL431 circuits, keep in mind the design considerations, such as stability, input voltage range, output current, temperature drift, and noise immunity. If you encounter issues, refer to the troubleshooting tips provided in this guide.

With its wide range of applications, from power supplies and battery chargers to voltage monitors and programmable voltage sources, the TL431 is an essential component in any analog designer’s toolkit. By mastering the use of TL431 circuits, you can create more efficient, accurate, and reliable designs.