Voltage Regulators & LDOsActive

TL431LP

Manufacturer: Texas Instruments

Figures from the datasheet: Texas Instruments SLVS543, rev. Rev. S, May 2024

Adjustable precision shunt regulator with programmable output voltage from 2.495V to 36V, 1mA to 100mA cathode current capability, and 0.2Ω typical dynamic impedance in a 3-pin TO-92 package.

PackagesTO-92
Supply voltage2.495 – 36 V
Temperature range0 – 70 °C
Official Datasheet (PDF)

Technical Overview

01

Technical Overview

The TL431 is a three-terminal adjustable precision shunt regulator manufactured by Texas Instruments (Document SLVS543S, revised May 2024). Housed in a through-hole TO-92 (LP) package with a REF-ANODE-CATHODE pinout (Pin 1 REF, Pin 2 Anode, Pin 3 Cathode), it provides a sharp turn-on characteristic and acts as an open-collector operational amplifier with a precise 2.495V bandgap voltage reference. Output voltage can be programmed from Vref (2.495V) up to 36V using two external resistors. Key operating specifications include a reference input voltage of 2.495V (2.440V to 2.550V for standard grade), low reference input current of 2µA typical (4µA max), minimum regulation current of 0.4mA typical (1.0mA max), continuous cathode current up to 100mA, and typical dynamic output impedance of 0.2Ω. The TL431 is widely utilized in precision voltage references, linear shunt regulators, overvoltage/undervoltage supervisors, and optocoupler feedback loops for isolated switch-mode power supplies.

Package Pinout & Alternate Functions

Interactive package pinout and pin mapping for TO-92-3.

Interactive pinout diagramTop

TL431LP: TO-92 package illustrationInteractive package pinout and pin mapping for TO-92.1: REF2: ANODE3: CATHODE

Package illustration, not a dimensioned mechanical drawing.

3 / 3 pins
#Pin nameType5V TolerantAlternate functions / details
REFanalog inYes
Reference voltage input terminal (threshold Vref = 2.495V nominal relative to Anode; reference input current Iref ~2 µA typical)
ANODEground—
Anode terminal (connected to circuit common/ground return; internal reference common)
CATHODEpassive—
Shunt cathode terminal (current input/sink terminal; regulates voltage from Vref up to 36V with 1mA to 100mA operating current)

Key Specifications

Parametric attributes

ParameterMinTypicalMaxUnitTest condition
Reference Voltage(Vref)2.442.4952.55VVKA = Vref, IKA = 10 mA, TA = 25°C
Reference Voltage Drift Over Temperature(delta_Vref_temp)—1434mVVKA = Vref, IKA = 10 mA, TA = 0°C to 70°C
Ratio of Change in Vref to Change in Cathode Voltage(delta_Vref_delta_VKA)—-1.4-2.7mV/VIKA = 10 mA, delta_VKA = 10 V to Vref, TA = 25°C
Reference Input Current(Iref)—2.04µAIKA = 10 mA, R1 = 10 kΩ, R2 = open, TA = 25°C
Reference Input Current Drift Over Temperature(delta_Iref_temp)—0.41.2µAIKA = 10 mA, R1 = 10 kΩ, R2 = open, TA = 0°C to 70°C
Minimum Cathode Current for Regulation(Imin)—0.41mAVKA = Vref, TA = 25°C
Off-State Cathode Current(Ioff)—0.11µAVKA = 36 V, Vref = 0 V, TA = 25°C
Dynamic Impedance(ZKA)—0.20.5ΩVKA = Vref, IKA = 1 mA to 100 mA, f <= 1 kHz, TA = 25°C

Absolute Maximum Ratings

Max supply voltage (VDD/VCC)37 V
Max output / pin current150 mA
Storage temperature-65 – 150 °C
Thermal resistance θJA (TO-92)140 °C/W
Thermal resistance θJC (TO-92)50 °C/W

Hardware Engineering Gotchas

Real-world hardware caveats, PCB layout constraints and silicon errata to know before manufacturing.

Critical

Capacitive Load Instability (Boundary of Active Stability)

The TL431 can oscillate when driving capacitive loads directly across its cathode and anode terminals without appropriate series damping. The datasheet stability boundary curves (Figure 9-14 in SLVS543S) show that load capacitances in the range of approximately 10 nF to 3 µF (depending on cathode current IKA and regulated voltage VKA) create a conditional stability loop, leading to parasitic oscillation and output ripple.

Mitigation

Avoid placing pure capacitive loads between 10 nF and 3 µF directly between cathode and anode. Either keep bypass capacitance small (CL < 10 nF for high-frequency RF bypassing), select a sufficiently large bulk capacitor (CL > 10 µF) with adequate equivalent series resistance (ESR), or insert a small series damping resistor (typically 1 Ω to 10 Ω) in series with the bypass capacitor to keep operating points outside the instability envelope.

Critical

Minimum Cathode Current Requirement (1.0 mA for Full Regulation)

Unlike simple passive Zener diodes which degrade gradually at low currents, the TL431 incorporates internal active circuitry and an operational amplifier that require a minimum cathode current of 0.4 mA typical and up to 1.0 mA maximum across temperature to maintain regulation. If the net cathode current drops below this threshold, the internal amplifier loses loop gain and the cathode voltage jumps up toward the unregulated supply rail.

Mitigation

Size the series feed resistor RSUP so that under worst-case minimum supply voltage VSUP(min) and maximum external load current ILOAD(max), the cathode current IKA remains strictly above 1.0 mA (designing for 2 mA to 5 mA quiescent current is recommended for robust regulation margin).

Warning

Reference Input Current Error with High-Value Feedback Resistors

The TL431 output voltage equation is VO = Vref * (1 + R1 / R2) + Iref * R1. The reference terminal draws a finite bias current Iref (2 µA typical, 4 µA max at 25°C, with up to 1.2 µA drift over temperature). If high resistor values (e.g. R1 > 100 kΩ) are selected to reduce power consumption, the Iref * R1 term introduces substantial offset error and temperature drift into the regulated output.

Mitigation

Design the resistor divider current Idiv = Vref / R2 to be significantly larger than Iref (at least 100 times, preferably Idiv >= 200 µA to 1 mA). Selecting R2 <= 10 kΩ ensures that reference input current variation contributes less than 0.1% to overall output voltage drift.

Reference Circuits

TL431 Precision Shunt Voltage Reference

Topology: Programmable shunt voltage regulator with series bias resistor and resistive divider.

Inspect schematicOpen in Web CAD Editor

Official Documentation & Cited Datasheets

Primary semiconductor vendor datasheets, errata, and application notes.

DATASHEETTexas Instruments

TL431x, TL432x Precision Programmable Reference

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