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TO-92 / SOT-23 · TL431
ActiveVoltage References
TL431
Texas Instruments · TO-92 / SOT-23 · Voltage References
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Overview
The TL431 is a three-terminal adjustable precision shunt regulator that behaves like a temperature-compensated Zener diode with programmable breakdown voltage. Internally it contains a 2.495 V bandgap reference, an op-amp, and an NPN pass transistor, all in a TO-92, SOT-23 or SOP-8 package. Two external resistors set the output voltage anywhere from 2.5 V to 36 V, making it the most flexible voltage reference on the market at under $0.10.
The device operates by sinking current from cathode to anode — like a Zener, but with much sharper turn-on (0.2 Ω dynamic impedance) and far better temperature stability (30 ppm/°C typical). Cathode current can range from 1 mA to 100 mA, and the reference accuracy is ±1% (TL431A), ±2% (standard TL431) or ±0.5% (TL431B).
Its dominant application is the feedback loop of switch-mode power supplies: connect the cathode to the secondary-side output through a resistor divider, and the reference pin to the divider midpoint. When output voltage rises, TL431 sinks more current through an optocoupler LED, which pulls down the primary-side PWM controller's feedback pin, closing the loop. This topology appears in virtually every phone charger, LED driver and PC power supply built since the 1990s.
Other uses include precision current sources (with a sense resistor), voltage supervisors, adjustable Zener replacements, and analog-to-digital front-end references when a 2.5 V or 3.3 V reference is needed.
Available from TI, onsemi, STMicroelectronics, Diodes Inc and many Asian vendors — all pin-compatible. Shenzhen stock of TL431IDBZR (SOT-23) MOQ 3,000 pcs on reel; TL431AIDBZR (1% grade) same package.
Key Features
- Programmable 2.5 V to 36 V output with two resistors
- ±0.5% / ±1% / ±2% accuracy grades
- 30 ppm/°C temperature drift — 10× better than a Zener
- Sinks up to 100 mA cathode current
Typical Applications
- Feedback reference in switch-mode power supplies (flyback, forward, LLC)
- Adjustable precision Zener replacement
- Precision current source (with sense resistor)
- Voltage supervisor and reset generator
- Battery charger termination detection
Specifications
- Reference Voltage
- 2.495 V
- Adjustable Range
- 2.5 to 36 V
- Cathode Current
- 1 to 100 mA
- Tolerance
- 0.5 / 1 / 2 %
- Output Impedance
- 0.2 Ω
- Operating Temperature
- -40 to 85 °C
- Mounting Type
- Through Hole / Surface Mount
- RoHS
- Compliant
Procurement Tips
For SMPS feedback, always specify the ±1% (TL431A) or ±0.5% (TL431B) grade — the ±2% standard grade translates directly to ±2% output voltage error, which may violate downstream regulator dropout margins.
Frequently Asked Questions
How do I calculate the output voltage?
Vout = 2.495 × (1 + R1/R2), where R1 goes from cathode to reference pin, R2 from reference pin to anode (ground). For 3.3 V output: R1 = 3.3 kΩ, R2 = 10 kΩ. For 5 V: R1 = 10 kΩ, R2 = 10 kΩ. Use 0.1% thin-film resistors for accuracy.
What is the minimum cathode current?
About 1 mA for the TL431 to enter regulation. Below that the output is undefined. Ensure your bias resistor provides at least 1 mA at minimum load. Some low-current variants (TLV431) work down to 80 µA.
Can I use it as a 2.5 V reference?
Yes — tie reference pin directly to cathode, and the device regulates at 2.495 V. Add a series resistor to limit cathode current to your target value (1-100 mA).
TL431 vs LM4040 vs REF3025 — which reference should I use?
TL431 is adjustable and cheapest but noisiest (~50 µVpp). LM4040 is a fixed-voltage shunt reference with 0.1% accuracy and low noise. REF3025 is a series (not shunt) reference with 25 ppm/°C drift. Use TL431 for SMPS feedback, LM4040 for general reference, REF3025 for precision ADC front-ends.
Why does my SMPS oscillate with a TL431?
The TL431 has an internal pole around 100 kHz that can interact badly with the optocoupler CTR and the SMPS control loop. Add a compensation network: 10 nF from reference pin to cathode, plus a series RC on the optocoupler collector. Consult TI application note SLVA482 for detailed compensation design.
MOQ and lead time?
MOQ 3,000 pcs on reel for TL431IDBZR (SOT-23), or 100 pcs loose. TI authorized 8-16 weeks; Shenzhen stock 1-2 days.
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