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The TL431 is one of the most widely used adjustable precision shunt regulators in power electronics and analog circuit design. This article will discuss the TL431 pin configuration, working principle, circuit diagrams, specifications, features, application examples, performance curves, practical uses, comparison with TLV431, and more.
The TL431 is a three-terminal adjustable precision shunt regulator designed to provide stable and accurate voltage control. It operates like an enhanced Zener diode but offers much better precision and temperature stability. The device includes an internal 2.5 V reference and an error amplifier, allowing it to maintain a constant reference voltage with tight tolerance.
The TL431 has three pins: Reference (REF), Anode (A), and Cathode (K). When the voltage at the REF pin reaches 2.5 V, the device begins conducting from cathode to anode. By using two external resistors, the output voltage can be adjusted from approximately 2.5 V up to 36 V. As a shunt regulator, it controls voltage by increasing or decreasing current flow to maintain regulation.
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| Pin# | Pin Name | Type | Description | Typical Voltage |
| 1 | REF (Reference) | Input | Sets the output regulation voltage. When REF reaches 2.5 V (internal reference), the device starts conducting between cathode and anode. External resistor divider is connected here. | 2.5 V (internal reference) |
| 2 | ANODE | Ground / Negative | Connected to ground (in most applications). Current flows from cathode to anode when regulating. | 0 V (usually) |
| 3 | CATHODE | Output | Acts like the regulated terminal. Connected to the supply through a resistor. The device sinks current here to maintain set voltage. | Adjustable (2.5 V to 36 V typical range) |
The circuit shows the TL431 used as a shunt voltage regulator. The input voltage is connected through a series resistor to the cathode (K). This resistor limits the current flowing into the device. The anode (A) is connected to ground, while the reference (REF) pin monitors the output voltage through a feedback connection.
Inside the TL431, there is a precise 2.5 V internal reference and an internal transistor. When the voltage at the REF pin reaches 2.5 V, the internal transistor turns on. This allows current (IKA) to flow from the cathode to the anode. As more current is drawn, the voltage at the cathode (VKA) is pulled down, stabilizing the output voltage.
If the output voltage tries to rise above the set value, the TL431 sinks more current to ground, reducing the voltage. If the output voltage drops, the device reduces current flow. This continuous adjustment keeps the voltage stable. In simple terms, the TL431 acts like an adjustable precision Zener diode that maintains a constant output voltage.
| Parameter | Typical Value |
| Reference Voltage (Vref) | 2.495 V (typ.) |
| Reference Voltage Tolerance | ±0.5% (B grade) ±1% (A grade) ±2% (Standard) |
| Adjustable Output Voltage | 2.5 V to 36 V |
| Cathode Voltage (VKA) | Up to 36 V |
| Sink Current (IKA) | 1 mA to 100 mA |
| Minimum Operating Current | ≈ 1 mA |
| Typical Output Impedance | 0.2 Ω |
| Temperature Range | 0°C to 70°C (C) −40°C to 85°C (I) −40°C to 125°C (Q) |
| Typical Temperature Drift (TL43xB) | 6 mV (Commercial) 14 mV (Industrial/Automotive) |
| Output Noise | Low |
The TL431 includes a built-in 2.5 V precision reference that provides stable and accurate voltage regulation. This internal reference allows the device to maintain consistent performance across different operating conditions, making it suitable for feedback and voltage control applications.
The output voltage can be programmed from 2.5 V up to 36 V using two external resistors. This flexibility allows designers to use the TL431 in a wide range of circuits, including power supplies, battery chargers, and overvoltage protection systems.
The TL431 is available in different accuracy grades, including ±0.5%, ±1%, and ±2%. This enables designers to choose the appropriate precision level depending on the requirements of their application, especially in sensitive analog or reference circuits.
Depending on the version selected, the device can operate from 0°C to 70°C (commercial), −40°C to 85°C (industrial), or −40°C to 125°C (automotive). This makes it suitable for consumer, industrial, and automotive environments.
The TL431 can sink currents from 1 mA up to 100 mA, allowing it to handle regulation tasks in both low-power and moderate-power circuits. This current control capability is essential for maintaining stable output voltage under varying load conditions.
With a typical output impedance of around 0.2 Ω, the TL431 provides strong voltage stability and fast response to load changes. This helps improve regulation accuracy in switching power supplies and feedback systems.
The device produces relatively low noise during operation, which makes it suitable for precision reference circuits and analog applications where signal stability is important.
The TL431 functions like an adjustable Zener diode but offers much better precision and flexibility. This makes it a preferred choice over traditional Zener diodes in regulated power supply and control circuits.
This circuit shows the TL431 used as an adjustable shunt voltage regulator with feedback control. The supply voltage (Vsup) is connected to the output node (Vout) through a series resistor (Rsup). This resistor limits the current flowing into the TL431 and provides the necessary current for regulation. The cathode is connected to Vout, while the anode is grounded.
The output voltage is set using the resistor divider R1 and R2. These resistors feed a portion of Vout back to the REF pin. When the voltage at the REF pin reaches the internal 2.5 V reference, the TL431 begins to conduct and sinks current from the cathode to the anode. By increasing or decreasing this sink current, the device keeps Vout stable at the desired level.
If Vout rises above the set value, the TL431 sinks more current, pulling the voltage down. If Vout drops, it sinks less current, allowing the voltage to rise again. This feedback action maintains a regulated and stable output voltage. The right side of the diagram shows how the regulated voltage can be used to drive a load or another control stage.
The graph shows the output response of the TL431 when the input voltage changes suddenly. The black waveform represents the input voltage (Vin), which switches between low and high levels. The red and blue curves represent the cathode voltage (VKA) under different series resistor values (Rsup = 10 kΩ and 1 kΩ), which affect the cathode current.
When Vin rises, the TL431 reacts by sinking current to regulate the voltage. With a lower resistor value (1 kΩ), more cathode current is available, so the device responds faster and the output settles more quickly (blue curve). With a higher resistor value (10 kΩ), the available current is lower, so the response is slightly slower and shows more deviation before stabilizing (red curve).
This curve demonstrates that the TL431’s regulation speed and stability depend on the cathode current. Higher cathode current improves transient response and voltage regulation performance.
• Switching power supply (SMPS) feedback control
• Adjustable voltage reference circuits
• Overvoltage protection circuits
• Battery charger regulation
• Linear power supply regulation
• Precision reference for ADC/DAC systems
• Voltage monitoring circuits
• LED driver regulation, etc.
| Parameter | TL431 | TLV431 |
| Type | Adjustable precision shunt regulator | Low-voltage adjustable shunt regulator |
| Internal Reference Voltage (Vref) | 2.495 V (typ.) | 1.24 V (typ.) |
| Minimum Operating Voltage | ≈ 2.5 V | ≈ 1.24 V |
| Adjustable Output Voltage Range | 2.5 V to 36 V | 1.24 V to 18 V (typ.) |
| Maximum Cathode Voltage | 36 V | 18 V (typ.) |
| Sink Current Range | 1 mA to 100 mA | 100 µA to 100 mA |
| Minimum Cathode Current | ~1 mA | ~100 µA |
| Accuracy Options | ±0.5%, ±1%, ±2% | ±0.5%, ±1%, ±2% (varies by grade) |
| Typical Applications | SMPS feedback, power supplies, voltage reference | Low-voltage systems, battery-powered devices |
| Best For | Standard 3.3 V, 5 V, 12 V regulation systems | 1.8 V, 2.5 V, low-voltage logic systems |
| Power Consumption | Higher (due to higher minimum current) | Lower (better for low-power designs) |
The TL431 is available in several through-hole and surface-mount packages to support different PCB design requirements. The most common mechanical packages include:
| Package Name | Mounting Style | Typical Pin Count | Common Use Case |
| TO-92 | Through-hole | 3 pins | Prototyping, general power supply circuits |
| SOT-23-3 | Surface-mount (SMD) | 3 pins | Compact consumer electronics |
| SOT-23-5 | Surface-mount (SMD) | 5 pins | High-density PCB layouts |
| SOT-89 | Surface-mount (SMD) | 3 pins | Medium-current regulation designs |
| SC-70 | Surface-mount (SMD) | 3 pins | Space-limited portable devices |
| SOIC-8 | Surface-mount (SMD) | 8 pins | Industrial and precision applications |
| PDIP-8 | Through-hole | 8 pins | Legacy systems and development boards |
| TSSOP | Surface-mount (SMD) | 8 pins | Slim embedded system boards |
With TL431 adjustable output from 2.5 V to 36 V, multiple accuracy grades, strong sink-current capability, and low dynamic impedance, it serves as a reliable solution for feedback regulation and reference generation. Its performance characteristics, such as stable internal reference and good transient response, make it suitable for both linear and switching power supply designs. Additionally, the availability of various package types ensures compatibility with different PCB layouts and system requirements.
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