SLVS529D April   2004  – January 2015 TL1431-EP

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Simplified Schematic
  5. 5 Revision History
  6. 6 Pin Configuration and Functions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Dissipation Rating Table
    6. 7.6 Electrical Characteristics
    7. 7.7 Typical Characteristics
  8. 8 Parameter Measurement Information
  9. 9 Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
    4. 9.4 Device Functional Modes
  10. 10Application and Implementation
    1. 10.1 Application Information
    2. 10.2 Typical Application
      1. 10.2.1 Design Requirements
      2. 10.2.2 Detailed Design Procedure
      3. 10.2.3 Application Curves
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
    2. 12.2 Layout Example
  13. 13Device and Documentation Support
    1. 13.1 Trademarks
    2. 13.2 Electrostatic Discharge Caution
    3. 13.3 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

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10 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

10.1 Application Information

The ability to set the shunt voltage, VKA, to any voltage between VREF and the maximum rated voltage for the shunt regulator provides a lot of flexibility. It takes two resistors to set the shunt voltage. In an ideal common anode shunt regulator, the shunt voltage would be VREF × (R1/R2 + 1).

Real world shunt regulators have limited gain, non-zero reference input current, and suffer from cathode voltage modulation. This application report derives comprehensive formulas that accurately represent the relationship between the shunt voltage and feedback resistors. It also shows a practical example.

Shunt Regulator Limitations

Real world shunt regulators have three parameters that should be considered.

  • Dynamic impedance, ZKA
  • Reference pin current, IREF
  • Ratio of change in reference voltage to the change in cathode voltage, ΔVREF/ΔVKA.

The first parameter will cause a VREF shift for all VKA values and the last two only apply when VKA, is set greater than VREF.

ZKA offsets the VREF in direct proportion to the cathode current. The data sheet generally specifies VREF at a specific current. At any other current ZKA impacts VREF.

IREF causes an inequality in the feedback resistor currents which changes the effective DC feedback ratio. This factor is often included in data sheet formulas.

ΔVREF/ΔVKA specifies how much the VREF voltage changes when the cathode voltage changes. This is a frequently ignored factor although the effect can be significant.

10.2 Typical Application

shunt_reg_lvs529.gif
R should provide cathode current ≥1 mA to the TL1431-EP at minimum V(BATT).
Figure 21. Shunt Regulator

10.2.1 Design Requirements

To calculate the values for resistors R1 and R2, the values of the following parameters must be known: the feedback current, (IFB), cathode current, (IKA), and desired shunt voltage, (VKA).

example_lva445.gif

The Electrical Characteristics table specifies when VKA = VREF and IKA is 10 mA the nominal VREF, (labeled VNOM) is 2.5 V. The reference voltage varies with cathode voltage at two different rates. The reference voltage is –1.1 mV/V from VREF to 10 V then –1.5 mV/V above 10 V. The reference pin current is 4 µA.

The ZKA parameter offsets VREF by (IKA – INOM) × ZKA .

In addition, the ΔVREF / ΔVKA parameter offsets VREF by either –1.1 mV × (VKA – 2.5 V) if VKA ≤ 10 V or –8.25 mV –1.5 mV/V × (VKA – 10 V) if VKA>10 V. The –8.25-mV constant is the VREF offset as VKA changes from
VNOM to 10 V, (10 V – 2.5 V) × –1.1 mV/V.

Therefore:

If VKA ≤ 10 V then;

Equation 1. VREF = VNOM + (IKA – INOM) x ZKA + (VKA – VNOM) × –1.1 mV/V

If VKA > 10 then;

Equation 2. VREF = VNOM + (IKA – INOM) × ZKA + (VKA – 10 V) x –1.5 mV/V –8.25 mV

Now that the value of VREF is calculated, use Equation 1 and Equation 2 to calculate the value of R1 and R2.

Equation 3. R1 = (VKA – VREF) / IFB
Equation 4. R2 = VREF / (IFB – IREF)

NOTE

R2 has less current than R1.

10.2.2 Detailed Design Procedure

The goal of the design is: the TL1431 cathode set to 12 V, the cathode current at 2 mA, and a feedback current of 0.2 mA.

Using the formula derived in the general example for VKA>10 V.

Equation 5. VREF = VNOM + (IKA – INOM) × ZKA + (VKA – 10 V) × -1.1 mV – 8.25 mV
Equation 6. VREF = 2.500 V + (2 mA – 10 mA) x 2 Ω+ (12 V – 10 V) × –1.1 mV – 8.25 mV

Using Equation 5 and Equation 6, the value of VREF = 2.473 V

Equation 7. R1 = (VKA – VREF) / IFB
Equation 8. R1 = (12 V – 2.473 V) / 0.2 mA

Using Equation 7 and Equation 8, the value of R1 = 46.285 kΩ

Equation 9. R2 = VREF / (IFB – IREF)
Equation 10. R2 = 2.473 V / (0.2 mA – 4 µA)

Using Equation 9 and Equation 10, the value of R2 = 12.617 kΩ

The closest standard 1% resistor values are R1 = 46.4 kΩ and R2 = 12.7 kΩ. Other resistor combinations may provide a shunt voltage that is centered better. A formula to test for R1 values that may be closer to standard values using standard R2 resistors is R1= (VKA – VREF) / (VREF / R2 + IREF).

10.2.3 Application Curves

D012_SLVS529.gifFigure 22. Stability Bounderies for load capacitance on Shunt Regulator
D011_SLVS529.gifFigure 23. Pulse Response at Startup of Shunt Regulator
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