SLUSF22 November   2024 TPS54538

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics
    6. 5.6 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1  Fixed Frequency Peak Current Mode
      2. 6.3.2  Mode Selection
      3. 6.3.3  Voltage Reference
      4. 6.3.4  Output Voltage Setting
      5. 6.3.5  Switching Frequency Selection / Synchronization
      6. 6.3.6  Phase Shift
      7. 6.3.7  Enable and Adjusting Undervoltage Lockout
      8. 6.3.8  External Soft Start and Prebiased Soft Start
      9. 6.3.9  Power Good
      10. 6.3.10 Minimum On Time, Minimum Off Time, and Frequency Foldback
      11. 6.3.11 Frequency Spread Spectrum
      12. 6.3.12 Overvoltage Protection
      13. 6.3.13 Overcurrent and Undervoltage Protection
      14. 6.3.14 Thermal Shutdown
    4. 6.4 Device Functional Modes
      1. 6.4.1 Modes Overview
      2. 6.4.2 Heavy Load Operation
      3. 6.4.3 Pulse Frequency Modulation
      4. 6.4.4 Forced Continuous Conduction Modulation
      5. 6.4.5 Dropout Operation
      6. 6.4.6 Minimum On-Time Operation
      7. 6.4.7 Shutdown Mode
  8. 7 Application and Implementation
    1. 7.1 Application Information
    2. 7.2 Typical Application
      1. 7.2.1 Design Requirements
      2. 7.2.2 Detailed Design Procedure
        1. 7.2.2.1 Custom Design With WEBENCH® Tools
        2. 7.2.2.2 Output Voltage Resistors Selection
        3. 7.2.2.3 Choosing Switching Frequency
        4. 7.2.2.4 Soft-Start Capacitor Selection
        5. 7.2.2.5 Output Inductor Selection
        6. 7.2.2.6 Output Capacitor Selection
        7. 7.2.2.7 Input Capacitor Selection
        8. 7.2.2.8 Feedforward Capacitor CFF Selection
        9. 7.2.2.9 Maximum Ambient Temperature
      3. 7.2.3 Application Curves
    3. 7.3 Best Design Practices
    4. 7.4 Power Supply Recommendations
    5. 7.5 Layout
      1. 7.5.1 Layout Guidelines
      2. 7.5.2 Layout Example
  9. 8 Device and Documentation Support
    1. 8.1 Device Support
      1. 8.1.1 Third-Party Products Disclaimer
      2. 8.1.2 Development Support
        1. 8.1.2.1 Custom Design With WEBENCH® Tools
    2. 8.2 Documentation Support
      1. 8.2.1 Related Documentation
    3. 8.3 Receiving Notification of Documentation Updates
    4. 8.4 Support Resources
    5. 8.5 Trademarks
    6. 8.6 Electrostatic Discharge Caution
    7. 8.7 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

Package Options

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

7.2.2.7 Input Capacitor Selection

The TPS54538 device requires an input decoupling capacitor and, depending on the application, a bulk input capacitor. The typical recommended value for the decoupling capacitor is 10μF, and TI recommends an additional 0.1µF capacitor from the VIN pin to ground to provide high frequency filtering.

The value of a ceramic capacitor varies significantly over temperature and the amount of DC bias applied to the capacitor. TI recommends X5R and X7R ceramic dielectrics because X5R and X7R ceramic dielectrics have a high capacitance-to-volume ratio and are fairly stable over temperature. The capacitor must also be selected with the DC bias taken into account. The effective capacitance value decreases as the DC bias increases.

The capacitor voltage rating needs to be greater than the maximum input voltage. The capacitor must also have a ripple current rating greater than the maximum input current ripple. The input ripple current can be calculated using Equation 23.

Equation 23. I C I N _ R M S = I O U T × V O U T V I N _ M I N × V I N _ M I N - V O U T V I N _ M I N

For this example design, three Murata GRM32ER7YA106KA12K (10μF, 35V, 1210, X7R) capacitors have been selected. The effective capacitance under input voltage of 24V for each one is 6.16μF. The input capacitance value determines the input ripple voltage of the converter. The input voltage ripple can be calculated using Equation 24. Using the design example values, IOUT_MAX = 5A, CIN_EFF = 3 × 6.16 = 18.48μF, and fSW = 500kHz, yields an input voltage ripple of 113mV and a RMS input ripple current of 2.03A.

Equation 24. ∆ V I N = I O U T _ M A X × 0.25 f S W × C I N + ( I O U T _ M A X × R E S R _ M A X )

where

  • RESR_MAX is the maximum series resistance of the input capacitor, which is approximately 1mΩ of three capacitors in parallel.