TL431 Calculator

Input Voltage (V):

Resistor R1 (kΩ):

Resistor R2 (kΩ):

Series Resistor Rseries (kΩ):

Calculate Output Voltage

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TL431 Calculator – Formula and Usage Instructions

Purpose:

This tool helps you calculate the output voltage, current, and power dissipation of the TL431 adjustable voltage shunt regulator IC based on your input voltage and external resistors. The calculation also accounts for the series resistor R_series that connects between the input voltage and the cathode of the TL431.

How to Use:

1. Input Voltage (V): Enter the input voltage V_in in volts (V). This voltage is applied to the anode of the TL431.
2. Resistor R1 (kΩ): Enter the value of resistor R₁ in kilo Ohms (kΩ). This resistor is connected between the input voltage and the reference voltage (V_ref) pin of the TL431.
3. Resistor R2 (kΩ): Enter the value of resistor R₂ in kilo Ohms (kΩ). This resistor is connected between the reference voltage (V_ref) pin of the TL431 and the cathode pin.
4. Series Resistor R_series (kΩ): Enter the value of the series resistor R_series in kilo Ohms (kΩ). This resistor is placed between the input voltage and the cathode of the TL431 to limit the current through the TL431. It is crucial to select a proper value to avoid exceeding the TL431’s current limit.

How It Works:

The TL431 is a shunt regulator, and the output voltage is determined by the ratio of the resistors R₁ and R₂. The TL431 regulates the voltage at its reference pin to approximately 2.495V.

Formulas Used in the Calculation:

• Voltage at the Junction of R₁ and R₂: V_junction = V_in × (R₂ / (R₁ + R₂))
• Calculated Output Voltage V_out:
  • If V_junction ≥ V_ref (TL431 shunt regulator behavior): V_out = V_ref × (1 + (R₁ / R₂))
  • If V_junction < V_ref (Including series resistor R_series): V_out = V_in × (R₂ / (R₁ + R₂ + R_series))
• Maximum Output Current: I_out = (V_in – V_out) / R_series
• Power Dissipated in R_series: P_Rseries = I_out² × R_series

TL431 Current Limit:

The maximum output current for the TL431 is 100 mA (0.1 A). If the calculated output current exceeds this limit, the tool will show an error and provide a suggestion to adjust the R_series value accordingly. Please check the maximum limit for your chosen variant of TL431 in respective datasheet.

Recommendation for R_series:

To ensure the TL431 operates within its limits, it is important to choose a series resistor R_series that limits the current to below the maximum allowable current (100 mA). You can adjust R_series to prevent excessive current flow and avoid damaging the TL431.