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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 only 0.2Ω, the Tl431 is widely used as a replacement for 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: 1.0mA to 100mA
- Temperature coefficient of 50 ppm/°C
- Full temperature compensation over the entire operating range
- Low output noise level
The functional block diagram of the Tl431 consists of several key components:
1. An internal error amplifier with its non-inverting input connected to a sampled voltage from a resistor divider, while the inverting input is tied to a built-in 2.5V reference voltage (Uref).
2. A 2.5V internal reference voltage source.
3. An NPN transistor that regulates the load current.
4. A protection diode that safeguards the device against reverse polarity on the power supply.
How the Tl431 works: The device functions similarly to an adjustable Zener diode. The output voltage is set by an external resistor divider. When the output voltage increases, the sampled voltage (UREF) also rises above the internal 2.5V reference, causing the internal comparator to trigger. This turns on the NPN transistor, which in turn reduces the output voltage. 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 regulation.
In more detail, when the input voltage increases, the output voltage rises, leading to a higher sampling voltage. The internal circuit then adjusts by increasing the current through the device, which causes a larger voltage drop across the current-limiting resistor. This effectively lowers the output voltage back to the desired level, maintaining a stable output. This mechanism ensures precise and reliable voltage regulation in a wide range of electronic systems.