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

7.2.2.5 Output Inductor Selection

The most critical parameters for the inductor are the inductance, saturation current, and the RMS current. The inductance is based on the desired peak-to-peak ripple current, ΔiL, which can be calculated by Equation 16.

Equation 16. ∆ I L = V O U T V I N _ M A X × V I N _ M A X - V O U T L × f S W

Usually, define K coefficient represents the amount of inductor ripple current relative to the maximum output current of the device, a reasonable value of K is 20% to 60%. Experience shows that the best value of K is 30% to approximately 40%. Because the ripple current increases with the input voltage, the maximum input voltage is always used to calculate the minimum inductance L. Use Equation 17 to calculate the minimum value of the output inductor.

Equation 17. L = ( V I N - V O U T ) K × f S W × I O U T _ M A X × V O U T V I N

where

  • K is the ripple ratio of the inductor current (ΔIL / IOUT_MAX).

In general, choosing lower inductance in switching power supplies is preferable because this choice usually corresponds to faster transient response, smaller DCR, and reduced size for more compact designs. Too low of an inductance can generate too large of an inductor current ripple such that overcurrent protection at the full load can be falsely triggered. The device also generates more inductor core loss since the current ripple is larger. Larger inductor current ripple also implies larger output voltage ripple with the same output capacitors.

After inductance L is determined, the maximum inductor peak current and RMS current can be calculated by Equation 18 and Equation 19.
Equation 18. I L _ P E A K = I O U T + ∆ I L 2
Equation 19. I L _ R M S = I O U T 2 + ∆ I L 2 12

Ideally, the saturation current rating of the inductor is at least as large as the high-side switch current limit, IHS_LIMIT (see Section 5.5). This size makes sure that the inductor does not saturate even during a short circuit on the output. When the inductor core material saturates, the inductance falls to a very low value, causing the inductor current to rise very rapidly. Although the valley current limit, ILS_LIMIT, is designed to reduce the risk of current runaway, a saturated inductor can cause the current to rise to high values very rapidly, this can lead to component damage, so do not allow the inductor to saturate. In any case, the inductor saturation current must not be less than the maximum peak inductor current at full load.

For this design example, choose the following values:

  • K = 0.3
  • VIN_MAX = 28V
  • fSW = 500kHz
  • IOUT_MAX = 5A

The inductor value is calculated to be 5.3μH. Choose the nearest standard value of 5.6μH. The maximum IHS_LIMIT is 7A, the calculated peak current is 5.7A, and the calculated RMS current is 5.02A. The chosen inductor is a Würth Elektronik, 74439346056 , 5.6μH, which has a saturation current rating of 12.1A and a RMS current rating of 6.9A.

The maximum inductance is limited by the minimum current ripple required for the peak current mode control to perform correctly. To avoid subharmonic oscillation, the minimum inductor ripple current must be no less than approximately 10% of the device maximum rated current (5A) under nominal conditions.