SNVSCT0 July   2025 LMR71915

ADVANCE INFORMATION  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Device Comparison Table
  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
  8. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Control Architecture
      2. 7.3.2  Internal VCC Regulator and Bootstrap Capacitor
      3. 7.3.3  Internal Soft Start
      4. 7.3.4  On-Time Generator
      5. 7.3.5  Current Limit
      6. 7.3.6  N-Channel Buck Switch and Driver
      7. 7.3.7  Synchronous Rectifier
      8. 7.3.8  Enable, Undervoltage Lockout (EN/UVLO)
      9. 7.3.9  Power Good (PGOOD)
      10. 7.3.10 Thermal Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Active Mode
      3. 7.4.3 Sleep Mode
  9. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Fly-Buck™ Converter Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Switching Frequency (RT)
        2. 8.2.2.2 Transformer Selection
        3. 8.2.2.3 Secondary Output Diode
        4. 8.2.2.4 CBST Selection
        5. 8.2.2.5 Minimum Secondary Output Load
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
        1. 8.4.1.1 Compact PCB Layout for EMI Reduction
        2. 8.4.1.2 Feedback Resistors
      2. 8.4.2 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
      1. 9.1.1 Third-Party Products Disclaimer
    2. 9.2 Documentation Support
      1. 9.2.1 Related Documentation
    3. 9.3 Receiving Notification of Documentation Updates
    4. 9.4 Support Resources
    5. 9.5 Trademarks
    6. 9.6 Electrostatic Discharge Caution
    7. 9.7 Glossary
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information
    1. 11.1 Tape and Reel Information

8.2.2.1 Switching Frequency (RT)

The switching frequency of the LMR719xx is set by the on-time programming resistor connected to the RT pin. Use Equation 2to calculate RT based on the desired switching frequency. For this example of 500kHz, 60.4kΩ is used.

Note that at very low duty cycles, the 30ns minimum controllable on time of the high-side MOSFET, tON(min), limits the maximum switching frequency. In CCM, tON(min) limits the voltage conversion step-down ratio for a given switching frequency.

Ultimately, the choice of switching frequency for a given output voltage affects the available input voltage range, design size, and efficiency.