The TL431 is HeroMicro an extremely popular three-terminal adjustable shunt regulator, often hailed as a "programmable precision Zener diode." Thanks to its high accuracy (2%), low temperature drift, low dynamic impedance (0.2Ω), and wide operating voltage range (2.5V - 36V), it has become a cornerstone component in applications like switch-mode power supply (SMPS) feedback loops, linear regulators, voltage references, and over/under-voltage protection circuits.
1. Key Features Overview
Based on the provided document (TL431.pdf), key specifications are:
Reference Voltage (Vref): 2.495V (typical), with ±2% accuracy (i.e., 2.440V - 2.550V).
Adjustable Output Voltage Range: From Vref (≈2.5V) to 36V.
Operating Current Range (IKA): 1mA to 100mA. Regulation accuracy is not guaranteed below 1mA.
Dynamic Output Impedance (ZKA): 0.2Ω (typical), ensuring excellent load regulation.
Temperature Drift (Tempco):
Standard Grade (TL431): 6mV (typ.)
Industrial Grade (TL431I): 14mV (typ.)
Operating Temperature Range:
TL431: -40°C to +85°C
TL431I: -40°C to +125°C
Packages: TO-92, SOT23-3, SOT23-5
2. Pinout and Internal Structure
The TL431 is a three-terminal device. Its pinout (using the common SOT23-3 package as an example) is:
| Pin | Name | Function |
|---|---|---|
| 1 | Cathode (K) | Cathode. This is the output/regulation terminal. |
| 2 | Anode (A) | Anode. Typically connected to ground. |
| 3 | Reference (Ref) | Reference/Control input. |
Internal Structure: The TL431 contains a high-precision 2.5V reference, a high-gain error amplifier, and an NPN output transistor. It operates by comparing the voltage at the Ref pin to its internal 2.5V reference. If they are equal, the output transistor adjusts its conduction to maintain a stable cathode-to-anode voltage (VKA).
3. Critical Design Guidelines
(1) Basic Regulator Circuit (As an Adjustable Voltage Source)This is the most classic application of the TL431. Two external resistors, R1 and R2, form a voltage divider to set the output voltage Vout to any value between 2.5V and 36V.
Circuit Connection:
Anode (A) to ground.
Cathode (K) to the output voltage Vout.
R1 between Vout and Ref.
R2 between Ref and ground.
Voltage Calculation: Vout = Vref * (1 + R1 / R2) where Vref ≈ 2.5V.
Design Considerations:
Ensure Minimum Operating Current: The current through the TL431 (IKA) must be between 1mA and 100mA. This is typically provided by a pull-up resistor (Rpull-up) from Vout to the cathode.
Choose R2 Appropriately: To minimize the effect of the reference input current Iref (2μA typ.) on the divider ratio, R2 should not be too large. A common rule is to make the current through R2 at least 100 times Iref (i.e., > 200μA). For example, R2 = 2.5V / 0.5mA = 5kΩ.
(2) Application in Switch-Mode Power Supplies (Optocoupler Feedback)The TL431 is the standard component for secondary-side feedback in isolated SMPS like flyback converters.
Working Principle:
The Ref pin samples Vout through a resistor network.
If Vout increases, the Ref voltage rises, causing the cathode current IKA to increase.
This increased IKA flows through the LED of an optocoupler, making it brighter.
The phototransistor on the primary side conducts more, sending an error signal to the primary-side PWM controller (e.g., TL494), which reduces the duty cycle to lower Vout, closing the feedback loop.
Key Components:
Optocoupler: Provides galvanic isolation between primary and secondary.
Compensation Network (RC): A capacitor C and resistor R in series, placed between the cathode and Ref, are used to stabilize the feedback loop and prevent oscillation.
(3) As a Precision Voltage ReferenceWhen R1 = 0 and R2 = ∞ (i.e., Ref is directly connected to the cathode), the cathode-to-anode voltage VKA equals the internal reference Vref (≈2.5V). In this configuration, it acts as a 2.5V precision voltage reference.
(4) Over/Under-Voltage Protection (Comparator Mode)The TL431's switching characteristic can be used to build simple voltage monitor circuits.
The voltage to be monitored is divided down and fed to the Ref pin.
When this voltage exceeds 2.5V, the TL431 turns on; when it's below 2.5V, it turns off.
By connecting a pull-up resistor and a load (e.g., a transistor's base or an optocoupler's LED), a digital signal can be generated to trigger a protection action.
(5) PCB Layout Considerations
Feedback Traces: Keep the trace from the output to the R1/R2 divider node as short as possible to minimize noise pickup.
Grounding: Connect the TL431's anode (ground) directly to a clean area of the system's analog or power ground, away from high-current paths that could introduce voltage offsets.
4. Summary
The TL431 is an indispensable component in any analog or power designer's toolkit, thanks to its simplicity, reliability, high performance, and low cost. Whether used as an adjustable regulator, the heart of an isolated feedback loop, or a precision reference, it will perform exceptionally well as long as its fundamental operating principles and design rules (especially ensuring sufficient operating current) are followed.
