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

6.3.10 Minimum On Time, Minimum Off Time, and Frequency Foldback

Minimum on time (tON_MIN) is the smallest duration of time that the high-side switch can be on. tON_MIN is typically 70ns in the TPS54538. Minimum off time (tOFF_MIN) is the smallest duration that the high-side switch can be off. tOFF_MIN is typically 114ns. In CCM operation, tON_MIN, and tOFF_MIN, limit the voltage conversion range without switching frequency foldback.

The minimum duty cycle without frequency foldback allowed is:

Equation 8. D M I N = t O N _ M I N × f S W

The maximum duty cycle without frequency foldback allowed is:

Equation 9. D M A X = 1 - t O N _ M I N × f S W

Given a required output voltage, the maximum VIN without frequency foldback is:

Equation 10. V I N _ M A X = V O U T t O N _ M I N × f S W

The minimum VIN without frequency foldback is:

Equation 11. V I N _ M I N = V O U T 1 - t O N _ M I N × f S W

In TPS54538, a frequency foldback scheme is employed once tON_MIN or tOFF_MIN is triggered, which can extend the maximum duty cycle or lower the minimum duty cycle.

The on time decreases while VIN voltage increases. Once the on time decreases to tON_MIN, the switching frequency starts to decrease while VIN continues to go up, which lowers the duty cycle further to keep VOUT in regulation according to Equation 8.

The frequency foldback scheme also works once larger duty cycle is needed under low VIN condition. The frequency decreases after the device hits the tOFF_MIN, which extends the maximum duty cycle according to Equation 9. A wide range of frequency foldback allows the TPS54538 output voltage to stay in regulation with a much lower supply voltage VIN, which allows a lower effective dropout. With frequency foldback, VIN_MAX is raised, and VIN_MIN is lowered by decreased fSW.