Use this TL431 Voltage Reference / Feedback Calculator to design and analyze resistor-divider circuits using the TL431 adjustable shunt voltage reference. Enter the desired output voltage and one resistor value to calculate the other resistor, or enter both resistor values to determine the resulting output voltage. The calculator also shows the feedback divider current and resistor power dissipation, helping you select practical resistor values for voltage regulation and SMPS feedback circuits.The calculator uses the TL431 reference voltage as the basis for the calculation and supports standard resistor values for practical circuit design. For SMPS applications, the calculated resistor values are intended as a starting point; compensation, optocoupler characteristics, loop stability, minimum operating current, and other circuit requirements must also be considered.
Results
- TL431 output voltage
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- Output voltage (basic equation)
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- Calculated R1
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- Selected standard R1
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- Calculated R2
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- Selected standard R2
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- R1
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- R2
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- Actual Vout with standard value
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- Difference from target
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- Divider current (through R2)
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- Current through R1 (incl. Iref)
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- Voltage across R1
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- Voltage across R2
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- R1 power
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- R2 power
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- Suggested minimum R1 rating
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- Suggested minimum R2 rating
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How it works
The TL431 is an adjustable shunt regulator. It compares the voltage on its REF pin with an internal reference (about 2.495 V) and sinks as much cathode current as needed to hold REF at that voltage. A resistor divider from the output to REF therefore sets the regulated voltage.
R1 (upper) goes from VOUT to REF; R2 (lower) goes from REF to ground. Because REF is held at Vref, the current through R2 is Vref / R2, and the same current (plus the tiny REF input current) flows through R1.
TL431 Formulas
Basic divider equations:
Vout = Vref × (1 + R1 / R2)
R1 = R2 × (Vout / Vref − 1)
R2 = R1 / (Vout / Vref − 1)
Divider current, and R2 from a desired divider current:
Idiv = Vref / R2
R2 = Vref / Idiv
Resistor power:
P = I² × R
PR2 = Vref² / R2
PR1 = Idiv² × R1
Advanced REF-current approximation (approximate model only):
Vout ≈ Vref + R1 × (Vref / R2 + Iref)
Example: Vref = 2.495 V, R1 = 3.83 kΩ, R2 = 1.00 kΩ gives Vout = 2.495 × (1 + 3.83) ≈ 12.05 V and a divider current of 2.495 mA.
TL431 in SMPS Feedback
In isolated switch-mode power supplies the TL431 is commonly used as an adjustable shunt reference on the secondary side, usually driving the LED of an optocoupler. The output voltage is sensed by the divider VOUT → R1 → REF → R2 → GND.
When the REF voltage approaches the TL431 reference voltage, the TL431 increases its cathode current. That current flows through the optocoupler LED, and the optocoupler transistor tells the primary-side controller to reduce the power delivered, closing the feedback loop.
Voltage divider calculation – what this calculator does: it sets the DC output voltage by choosing R1 and R2 so that REF equals Vref at the target output.
Complete SMPS compensation / feedback design – not covered here: the compensation network around the TL431, the optocoupler LED bias and CTR, the TL431 minimum cathode current, loop gain and phase margin, and the controller-specific feedback components. These must be designed from the controller datasheet and verified on real hardware.
This calculator determines the TL431 voltage-divider values. It does not calculate the complete SMPS compensation network, optocoupler CTR requirements, loop stability, transient response, or controller-specific feedback components.
Notes
- The TL431 needs a minimum cathode current (up to about 1 mA) to regulate properly. This is separate from the divider current and must be supplied by the cathode resistor or optocoupler circuit.
- A standard TL431 is rated for a cathode voltage of up to 36 V and a cathode current of up to 100 mA.
- Keep the divider current well above the REF input current (at least 100 times) so the REF current has little effect on the output voltage.
- The reference voltage has a tolerance (about 2 % for the standard part, 1 % for TL431A and 0.5 % for TL431B). Use 1 % resistors for accurate output voltages.
- Other devices use different reference voltages (for example 1.24 V for the TLV431). Enter the value from your device datasheet.
