TL431

Features

• Operating temperature range: −40°C to +125°C.
• Adjustable output voltage from VREF to 36 V.
• Cathode sink-current capability: 1 mA to 100 mA.
• Low output noise.
• Typical output impedance: 0.2 Ω.
• Low temperature drift, particularly for the TL431B.
• High-precision internal reference voltage.

Reference Voltage Accuracy

The nominal TL431 reference voltage is approximately 2.495 V.

For a nominal reference voltage of approximately 2.495 V, the corresponding limits are approximately:

Typical temperature-drift values specified for TL431B include:

• 6 mV for the C temperature grade;
• 14 mV for the I and Q temperature grades.

Adjustable Output Voltage

The TL431 is a programmable shunt regulator. Its output voltage is set using an external resistor divider.

The adjustable voltage range is:

VREF to 36 V

For a typical configuration:

VOUT = VREF × (1 + R1/R2)

where VREF ≈ 2.495 V.

Cathode Sink Current

The device provides cathode current capability from:

1 mA to 100 mA

This allows the TL431 to operate as a programmable shunt regulator and as a precision feedback element.

Applications

• AC/DC switch-mode power supplies;
• Flyback converters;
• Voltage regulators;
• Feedback control circuits;
• Optocoupler feedback circuits;
• Power adapters and chargers.

Practical Repair Note

In switch-mode power supplies, the TL431 is commonly used together with an optocoupler.

The output voltage is sensed by a resistor divider connected to the REF pin. The TL431 then changes its cathode current, which changes the current through the optocoupler and provides feedback to the primary-side PWM controller.

A faulty TL431, incorrect resistor-divider values, or a defective optocoupler can therefore cause:

• excessive output voltage;
• low output voltage;
• unstable output;
• periodic startup and shutdown.

The TL431 should not be confused with a conventional fixed-output linear regulator. It is a programmable shunt regulator controlled through the REF input.

Tags: TL431, TL431A, TL431B, SOT-23, SOT23, Voltage Reference, Shunt Regulator, 2.5V, 36V, 100mA