SLVS839H July   2008  – October 2023 TPS54331

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Revision History
  6. 5 Pin Configuration and Functions
  7. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Switching Characteristics
    7. 6.7 Typical Characteristics
  8. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Fixed-Frequency PWM Control
      2. 7.3.2  Voltage Reference (VREF)
      3. 7.3.3  Bootstrap Voltage (BOOT)
      4. 7.3.4  Enable and Adjustable Input Undervoltage Lockout (VIN UVLO)
      5. 7.3.5  Programmable Slow Start Using SS Pin
      6. 7.3.6  Error Amplifier
      7. 7.3.7  Slope Compensation
      8. 7.3.8  Current-Mode Compensation Design
      9. 7.3.9  Overcurrent Protection and Frequency Shift
      10. 7.3.10 Overvoltage Transient Protection
      11. 7.3.11 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Eco-mode
      2. 7.4.2 Operation With VIN < 3.5 V
      3. 7.4.3 Operation With EN Control
  9. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1  Custom Design with WEBENCH® Tools
        2. 8.2.2.2  Switching Frequency
        3. 8.2.2.3  Output Voltage Set-Point
        4. 8.2.2.4  Input Capacitors
        5. 8.2.2.5  Output Filter Components
          1. 8.2.2.5.1 Inductor Selection
        6. 8.2.2.6  Capacitor Selection
        7. 8.2.2.7  Compensation Components
        8. 8.2.2.8  Bootstrap Capacitor
        9. 8.2.2.9  Catch Diode
        10. 8.2.2.10 Output Voltage Limitations
        11. 8.2.2.11 Power Dissipation Estimate
      3. 8.2.3 Application Curves
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
      2. 8.4.2 Layout Example
      3. 8.4.3 Electromagnetic Interference (EMI) Considerations
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
      1. 9.1.1 Development Support
        1. 9.1.1.1 Custom Design with WEBENCH® Tools
    2. 9.2 Support Resources
    3. 9.3 Receiving Notification of Documentation Updates
    4. 9.4 Trademarks
    5. 9.5 Electrostatic Discharge Caution
    6. 9.6 Glossary
  11. 10Mechanical, Packaging, and Orderable Information
8.2.2.5.1 Inductor Selection

To calculate the minimum value of the output inductor, use Equation 8.

Equation 8. L M I N = V O U T M A X × V I N M A X - V O U T V I N M A X × K I N D × I O U T × F S W

where

  • KIND is a coefficient that represents the amount of inductor ripple current relative to the maximum output current.

In general, this value is at the discretion of the designer; however, the following guidelines may be used. For designs using low-ESR output capacitors, such as ceramics, a value as high as KIND = 0.3 can be used. When using higher ESR output capacitors, KIND = 0.2 yields better results.

For this design example, use KIND = 0.3 and the minimum inductor value is calculated as 5.7 μH. For this design, a large value was selected: 6.8 μH.

For the output filter inductor, do not exceed the RMS current and saturation current ratings. Use Equation 9 to calculate the inductor ripple current (ILPP).

Equation 9. I L P P = V O U T × V I N M A X - V O U T V I N M A X × L O U T × F S W × 0.8

Use Equation 10 to calculate the RMS inductor current.

Equation 10. I L R M S = I O U T M A X 2 + 1 12 × I L P P 2

Use Equation 11 to calculate the peak inductor current.

Equation 11. ILPK=IOUTMAX+ILPP2

For this design, the RMS inductor current is 3.01 A and the peak inductor current is 3.47 A. The selected inductor is a Sumida CDRH103-6R8, 6.8 μH. This inductor has a saturation current rating of 3.84 A and an RMS current rating of 3.6 A, which meets these requirements. Smaller or larger inductor values can be used depending on the amount of ripple current the designer wants to allow, so long as the other design requirements are met. Larger value inductors have lower AC current and result in lower output voltage ripple, while smaller inductor values increase AC current and output voltage ripple. In general, inductor values for use with the TPS54331 device are in the range of 6.8 μH to 47 μH.