SLVSDQ6A July   2018  – November 2018 TL431LI , TL432LI


  1. Features
  2. Applications
  3. Description
    1.     Device Images
      1.      Simplified Schematic
  4. Revision History
  5. Device Comparison Table
  6. Pin Configuration and Functions
    1.     Pin Functions
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Thermal Information
    4. 7.4 Recommended Operating Conditions
    5. 7.5 Electrical Characteristics
    6. 7.6 Typical Characteristics
  8. Parameter Measurement Information
    1. 8.1 Temperature Coefficient
    2. 8.2 Dynamic Impedance
  9. Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
    4. 9.4 Device Functional Modes
      1. 9.4.1 Open Loop (Comparator)
      2. 9.4.2 Closed Loop
  10. 10Applications and Implementation
    1. 10.1 Application Information
    2. 10.2 Typical Applications
      1. 10.2.1 Comparator With Integrated Reference
        1. Design Requirements
        2. Detailed Design Procedure
          1. Basic Operation
            1. Overdrive
          2. Output Voltage and Logic Input Level
            1. Input Resistance
        3. Application Curve
      2. 10.2.2 Precision Constant Current Sink
        1. Design Requirements
        2. Detailed Design Procedure
          1. Basic Operation
            1. Output Current Range and Accuracy
          2. Power Consumption
      3. 10.2.3 Shunt Regulator/Reference
        1. Design Requirements
        2. Detailed Design Procedure
          1. Programming Output/Cathode Voltage
          2. Total Accuracy
          3. Stability
          4. Start-up Time
        3. Application Curve
      4. 10.2.4 Isolated Flyback with Optocoupler
        1. Design Requirements
          1. Detailed Design Procedure
            1. TL431 Feedback Loop Error Calculation
    3. 10.3 System Examples
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
    2. 12.2 Layout Example
  13. 13Device and Documentation Support
    1. 13.1 Related Links
    2. 13.2 Documentation Support
      1. 13.2.1 Device Nomenclature
      2. 13.2.2 Related Documentation
    3. 13.3 Receiving Notification of Documentation Updates
    4. 13.4 Community Resources
    5. 13.5 Trademarks
    6. 13.6 Electrostatic Discharge Caution
    7. 13.7 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

TL431 Feedback Loop Error Calculation

Figure 26 shows the simplified version of the feedback network. The accuracy of the output voltage is dependent on the regulation voltage accuracy of the TL431LI. A simplified VOUT can be seen in Equation 2 but this equation does not include errors that will deviate the output.

Equation 2. TL431LI TL432LI Accuracy2.gif

The primary sources of error are the Error|Vref and Error|Iref. The Error|Vref primarily consists of the errors that affect the internal bandgap voltage reference of the TL431LI. This consists of errors from the initial accuracy, temperature drift, ratio of change in reference voltage to the change in cathode voltage, and dynamic impedance. The benefit of the TL431LI is its low temperature drift, VI(dev), which allows the Vref to be more accurate across the full temperature range compared to typical TL431LI devices. Equation 3 shows a simplified worst case Vref with initial accuracy and temperature drift.

Equation 3. TL431LI TL432LI Accuracy1.gif

The Error|Iref in Figure 26 is dependent on the Iref and II(dev) along with R1. The TL431LI has improved Iref and II(dev) which allows the values of the resistor R1 to be increased to save power. Typically optocoupler feedback design requires the Iref to be taken into account when doing VOUT calculations but the error comes from the deviation from the maximum to typical value of Iref. In addition to this, the II(dev) is the temperature deviation on the Iref current which will affect the overall reference current into the TL431LI. Equation 4 shows the VOUT of the TL431LI for Figure 26 which includes the improved Iref and II(dev). The VOUT equation assumes that the resistors R1 and R2 have a 0.5% accuracy tolerance.

Equation 4. TL431LI TL432LI Accuracy0.gif

Comparing the calculated VOUT without and without error the expected worst case max error is 2.1% which meets the 3% error target.