TL431 Shunt Regulator Voltage Sizer
Choose feedback resistors for a target voltage, include reference-pin current and tolerance, then check whether the supply resistor can maintain cathode current across input voltage and load.
Divider & operating limits
For the conventional TL431 shunt circuit with its anode at ground. Defaults are illustrative assumptions, not a preset for a specific manufacturer, grade, or package.
Programmed voltage & current
A resistor ratio can set a voltage even when the supply cannot sustain it. Review the current budget before treating the setpoint as an operating output.
Feedback network
- Exact RT for the target
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- Entered bottom resistor RB
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- Nominal RB current
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- Nominal current through RT
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- Nominal IREF × RT contribution
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- Error range vs. target
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| Resistor | Maximum DC dissipation | Rating target at 50% use |
|---|---|---|
| RT — top | — | — |
| RB — bottom | — | — |
Rating targets are 2× modeled dissipation, not package recommendations. Check ambient derating, working voltage, pulse rating, and actual component availability.
Supply & cathode-current budget
| Condition | IK budget | Entered limit | Check |
|---|---|---|---|
| Low VIN / maximum load | — | — | — |
| High VIN / no external load | — | — | — |
- Nominal RS lower bound from IK ceiling
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- Nominal RS upper bound from minimum IK
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- Load-current ceiling from minimum IK
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- RS voltage drop, conservative maximum
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- RS dissipation, regulated-state maximum
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- RS power-rating target at 50% use
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- TL431 dissipation, conservative DC bound
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- Maximum total current through RS
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The RS interval checks current only, not thermal or stability limits. The TL431 power bound combines the highest programmed voltage and highest positive IK budget; those extremes need not occur together. It is deliberately conservative.
Save the inputs, resistor values, voltage range, and operating checks as a text file.
The REF current changes the divider
With current flowing into REF, the top resistor carries the bottom-resistor current plus IREF. The reference-current correction therefore adds to the output voltage in the circuit shown.
Programmed output voltage
IRB = VREF / RB
IRT = IRB + IREF
VOUT = VREF × (1 + RT/RB) + IREF × RTThe output is measured from cathode to the grounded anode. RT and RB must not be interchanged.
Top resistor for a target
RT = (VTARGET − VREF) / (VREF/RB + IREF)E24 / E96 selection minimizes nominal voltage error with RB held fixed. It does not optimize worst-case error, power, cost, or resistor availability.
Current left for the TL431
IRS = (VIN − VOUT) / RS
IK = IRS − ILOAD − IRTThe feedback network consumes current too. Do not subtract only the external load or count IREF twice.
Model context: Texas Instruments — Setting the Shunt Voltage on an Adjustable Shunt Regulator. The present tool uses a fixed-reference DC approximation; the application note also discusses effects that require a more detailed model.
A nominal 5 V setpoint
With VREF = 2.495 V, IREF = 2 µA, and RB = 2.49 kΩ, the ideal top resistor is about 2.495 kΩ. The nearest E96 value is 2.49 kΩ.
VOUT = 2.495 × (1 + 2.49/2.49) + 2 µA × 2.49 kΩ
VOUT = 4.99498 VThe nominal error is about −0.10%. That is not the complete tolerance result: independent ±1% reference and resistor bounds, with IREF from 0–4 µA, produce a wider programmed-voltage range. The calculator evaluates those limits separately.
Why not use very large resistors?
Increasing both feedback resistors preserves their ratio but increases the IREF × RT voltage term. For example, 2 µA through a 100 kΩ top resistor contributes 0.20 V; through 2.49 kΩ it contributes 4.98 mV.
Smaller values reduce this sensitivity but draw more divider current and consume more of the input-current budget. Choose the ratio and divider current together rather than treating voltage ratio as the only constraint.
Low line sets current. High line sets heat.
Low input, maximum load
Check the smallest input-to-output voltage difference, highest RS, and largest feedback-network current. Enough current must remain for the TL431 after supplying the external load.
A negative calculated IK budget means the assumed setpoint demands more current than the feed can provide. It does not mean the TL431 sources that negative current. The actual rail will need a different operating-point model.
High input, no load
Removing the external load leaves more feed current for the shunt device. Check cathode current, TL431 dissipation, and RS heating at the high-input corner.
The package’s current rating is not its thermal rating. Evaluate junction temperature using the chosen package, board, ambient, and appropriate thermal information. Fault and startup stresses need separate analysis.
A correct DC voltage can still oscillate
Adding capacitance across cathode and anode changes the regulator’s loop response. A generic decoupling value is not automatically suitable for every TL431 circuit.
Match the actual device
Use stability information for the exact manufacturer, variant, and operating conditions. Similar part names do not establish identical behavior.
Check voltage, current & ESR
Review the load capacitance with its ESR and the expected cathode voltage and current range, including no-load operation.
Verify the complete loop
Check startup and load-step waveforms on the real circuit. An optocoupler feedback loop or pass-transistor regulator requires its own loop and bias analysis.
Reference: Texas Instruments — Understanding Stability Boundary Conditions Charts in TL431, TL432 Data Sheet.
TL431 resistor-sizing questions
Can I use the same values with TLV431 or another “431” device?
Do not assume the reference voltage, current requirements, pinout, or stability behavior is the same. This page is scoped to the conventional approximately 2.5 V TL431 circuit. Use the exact alternative device’s model and datasheet.
Why can a 1% reference produce more than 1% output error?
The feedback resistor ratio and reference-input current contribute additional error. Temperature and operating-point shifts also matter unless they are already covered by your input bounds. Reference accuracy alone is not an output-accuracy specification.
Does the supply resistor set the output voltage?
The feedback network sets the programmed voltage. RS supplies the load, feedback network, and cathode current. Too large an RS can starve the regulator; too small an RS can cause excessive shunt current and heating.
Can I regulate a large load directly with this circuit?
Check the current and heat budget before using a shunt topology for a load rail. The supply must feed the load while preserving cathode current, and the no-load condition can waste substantial power. A series regulator or switching regulator may be more suitable.
Does the voltage range represent every real-world error?
No. It covers the independent VREF, IREF, RT, and RB bounds entered here. This is not a temperature simulation or statistical accuracy estimate. Include justified operating variation in the bounds or use a more detailed device model and validation.
Source the reference & resistor network
Send your exact TL431 ordering code, package, accuracy grade, resistor values, tolerances, and quantities for a sourcing review.