TL431 is a common adjustable precision reference voltage source/programmable zener regulator, which is widely used in power supply design. Its main function is to act as a precision reference voltage source. It provides a reference voltage of 2.495V (typical value) internally. It has high stability and low temperature drift characteristics. It can also be used as an adjustable zener diode. The output voltage can be set by dividing the voltage of external resistance, and the adjustable range is about 2.5V-36V. TL431 has a load-absorbing (Shunt) structure, similar to the operating mode of zener diode. It can absorb excess current, stabilize the output voltage to the set value, and has switching characteristics. It can be used as a comparator or simple control switch.
TL431 common application scenarios include serving as a precision voltage reference in a switching power supply, which can be used in conjunction with an optocoupler in a feedback control path to adjust the output voltage (e.g. AC-DC, DC-DC modules), or as a reference for a linear regulated power supply to provide a stable reference voltage to a linear regulator or comparator. In overvoltage/undervoltage protection circuit, it can be matched with resistor voltage divider and transistor to start protection when monitoring voltage exceeds or below threshold. TL431 can also be used in simple comparators, which can be used as a low-cost comparator to compare the input voltage with the reference voltage. TL431 can also detect the voltage in the battery charging circuit, detect whether the charging voltage meets the standard and control the charging process switch.
Simplified schematic diagram of TL431
Simplified schematic diagram of TL431
TL431 has the advantages of high precision, good stability (± 0.5% or ± 1% precision version), low cost, easy to obtain, adjustable voltage, high elasticity, simple application circuit and direct replacement of zener diode.
To give you a better understanding of the application of TL431, the following describes the most common circuits of the three TL431, as well as an actual functional circuit built with independent components.
Designing a 2.5V Voltage Reference Circuit with TL431
When you want to design a 2.5V voltage reference circuit with TL431, you actually use its built-in precision reference voltage (typical 2.495V) to output a stable 2.5V voltage. This is one of the most basic and common applications of TL431. TL431 has the advantage of high accuracy of ± 0.5% and good temperature stability. It is suitable for use as the benchmark of ADC/DAC or comparator, and its low cost makes it easy to obtain components.
The three pins of TL431 include Reference, internally connected to 2.495V reference, Cathede (K) connected to output (equivalent to positive terminal), Anode (A) connected to ground (GND). When Ref and Cathede are short-circuited, TL431 will output its built-in reference voltage ≈ 2.5V. If the Ref is not connected to Cathede, but voltage is divided by resistance, TL431 can be adjusted to higher output voltage (e.g. 3.3V, 5V, etc.). However, when obtaining a 2.5V reference output, directly shorting Ref and Cathede is the easiest and most stable way to do this.
Example of a 2.5V voltage reference circuit
Example of a 2.5V voltage reference circuit
First, the first circuit introduced is also the most commonly used 2.5V voltage reference circuit. Directly connect the Ref and cathode of TL431 as the 2.5V voltage reference source. During use, it is only required to calculate the resistance value of R1 according to the actual input voltage and current to ensure that the current is 1mA to 100mA and the power voltage is less than 36V.