The TL431 is a widely used three-terminal adjustable shunt reference component known for its excellent thermal stability. Its output voltage can be precisely set between 2.5V and 36V using just two external resistors (as shown in Figure 2). With a typical dynamic impedance of 0.2Ω, the TL431 is often employed as a replacement for Zener diodes in various applications, including digital voltmeters, operational amplifier circuits, adjustable voltage supplies, and switching power supplies.
To better understand the functionality of the TL431, consider the functional block diagram presented in Figure 1. Although this schematic is not the exact internal structure of the TL431, it provides valuable insights into how the device operates. The VI terminal, connected to a 2.5V internal reference voltage, feeds into the inverting input of an operational amplifier. For the output at the REF terminal (non-inverting input) to stabilize near 2.5V, a precise current flows through the transistor, varying between 1mA and 100mA depending on the voltage at the REF terminal. This modular representation is highly useful for conceptualizing and analyzing the circuit.
Now, let’s explore some practical applications of the TL431. In the basic circuit configuration (Figure 2), the TL431 achieves voltage regulation by introducing feedback to the REF terminal. The resistors R1 and R2 determine the feedback ratio, allowing the output voltage (Vo) to be adjusted between 2.5V and 36V. Specifically, when R1 equals R2, the output stabilizes at 5V. For instance, when using a 12V or 24V input, the TL431 ensures a precise 5V output, making it a reliable choice for high-end devices.
Referring to Figure 3 and Figure 4, modifications of this basic circuit yield even more practical solutions. The high-current shunt regulator circuit in Figure 3 demonstrates the TL431's capability to handle substantial loads while maintaining stability. Meanwhile, Figure 4 illustrates a precision 5V regulated power supply using the TL431. By setting R0 to 1.5kΩ and R1/R2 to 10kΩ, the circuit consistently delivers a 5V output regardless of the input voltage (tested with both 12V and 24V inputs). Additionally, the circuit exhibits minimal ripple and high precision, ensuring stable performance under varying load conditions. When the load resistance exceeds 2kΩ, the output remains constant; however, below this threshold, the cathode current requirement may not be satisfied.
Overall, the TL431 is an indispensable component in modern electronics, offering flexibility, reliability, and cost-effectiveness. Whether you’re designing a simple voltage regulator or a complex power supply system, the TL431 proves to be a versatile tool for engineers worldwide. Its compatibility with other semiconductor manufacturers like NXP and Motorola further underscores its widespread adoption across industries.
High-precision Regulated DC Power Supply Circuit Using TL431
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