The TL431 is a versatile three-terminal adjustable shunt reference component known for its excellent thermal stability. Its output voltage can be precisely adjusted between Vref (2.5V) and 36V using just two external resistors (as shown in Figure 2). With a typical dynamic impedance of 0.2Ω, the TL431 can replace traditional Zener diodes in various applications like digital voltmeters, operational amplifier circuits, adjustable voltage supplies, and more. Its functionality can be understood through the simplified functional block diagram provided in Figure 1.
As seen in the diagram, the VI represents an internal 2.5V reference source connected to the inverting input of the operational amplifier. The op amp ensures that the voltage at the REF terminal stays close to VI (2.5V), allowing the transistor to conduct a stable current ranging from 1 to 100mA depending on the REF voltage. Although this isn’t the exact internal structure of the TL431, it serves as a useful conceptual model for understanding how the device operates.
In practical applications, the TL431 can form the basis of a basic circuit or be integrated into more complex designs. For instance, Figure 2 illustrates how the TL431 can regulate output voltage by introducing feedback via resistors R1 and R2. These resistors create a feedback loop that adjusts the output voltage (Vo) dynamically. When Vo increases, the feedback strengthens, increasing the shunting effect of the TL431, thereby reducing Vo back toward stability. The equation for the output voltage becomes Vo = (1 + R1/R2) * Vref. By varying R1 and R2, one can achieve an output voltage anywhere between 2.5V and 36V. Notably, when R1 equals R2, the output stabilizes at 5V.
To ensure proper operation, the current flowing through the cathode must exceed 1mA. While the basic circuit demonstrates the principle well, slight modifications make it applicable to real-world scenarios, such as high-current shunt regulators (Figure 3) or precision 5V regulated power supplies (Figure 4).
In a practical implementation using TL431, setting R0 to 1.5kΩ and both R1 and R2 to 10kΩ yields a stable 5V output. Testing this setup with input voltages of 12V and 24V confirmed consistent 5V outputs, showcasing the TL431’s high precision. Additionally, the circuit performs well under varying load conditions; when the load resistance exceeds 2kΩ, the output remains stable. Below this threshold, however, the cathode current drops, affecting performance.
The circuit boasts minimal ripple and high precision, making it suitable for powering high-end electronic devices. For example, Figure 5 shows a complete schematic of a high-precision regulated DC power supply constructed using the TL431.
Overall, the TL431 proves to be a cost-effective and reliable solution for creating adjustable regulated power supplies, offering flexibility and precision in a variety of applications.
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