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Tl431 working principle _tl431 functional block diagram
Tl431 is a three-terminal adjustable shunt voltage reference manufactured by Texas Instruments. It offers excellent thermal stability and can be configured to provide an output voltage ranging from 2.5V (Vref) up to 36V using just two external resistors. With a typical dynamic impedance of 0.2Ω, it is widely used as a low-cost alternative to Zener diodes in various applications such as digital voltmeters, operational amplifier circuits, adjustable power supplies, and switching power supply designs.
Key features of the Tl431 include:
- Programmable output voltage up to 36V
- Voltage reference accuracy of ±0.4% at 25°C (for TL431B)
- Low dynamic output impedance, typically 0.22Ω
- Load current range from 1.0mA to 100mA
- Temperature coefficient of 50 ppm/°C
- Full temperature range compensation
- Low output noise voltage
The internal functional block diagram of the Tl431 consists of several key components: an error amplifier (A), an internal 2.5V reference voltage source (Uref), an NPN transistor (VT) for current regulation, and a protection diode (VD) that prevents damage from reverse polarity.
In terms of operation, the Tl431 functions similarly to an adjustable Zener diode. The output voltage is set through an external resistor divider network. When the output voltage increases, the sampled voltage (UREF) also rises, causing the internal comparator to trigger the NPN transistor into conduction. This lowers the output voltage back to its desired level. Conversely, if the output voltage drops, the comparator turns off the transistor, allowing the output to rise again. This continuous feedback loop ensures stable output voltage.
Additionally, the Tl431 includes a current-limiting resistor (R3) connected to the cathode (K) terminal. As the input voltage increases, the output voltage also rises, leading to more current flowing through the device. This increases the voltage drop across R3, which helps maintain the output voltage at a stable level. This mechanism allows the Tl431 to effectively regulate voltage in a wide range of applications.