SLUSFM3 September   2024 LMR51425-Q1 , LMR51435-Q1

PRODUCTION DATA  

  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
    6. 6.6 System 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 Peak Current Mode Control
      2. 7.3.2  Adjustable Output Voltage
      3. 7.3.3  Enable
      4. 7.3.4  Switching Frequency
      5. 7.3.5  Power-Good Flag Output
      6. 7.3.6  Minimum ON-Time, Minimum OFF-Time, and Frequency Foldback
      7. 7.3.7  Bootstrap Voltage
      8. 7.3.8  Overcurrent and Short-Circuit Protection
      9. 7.3.9  Soft Start
      10. 7.3.10 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Active Mode
      3. 7.4.3 CCM Mode
      4. 7.4.4 Light-Load Operation (PFM Version)
      5. 7.4.5 Light-Load Operation (FPWM Version)
  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 Output Voltage Set-Point
        3. 8.2.2.3 Switching Frequency
        4. 8.2.2.4 Inductor Selection
        5. 8.2.2.5 Output Capacitor Selection
        6. 8.2.2.6 Input Capacitor Selection
        7. 8.2.2.7 Bootstrap Capacitor
        8. 8.2.2.8 Undervoltage Lockout Setpoint
      3. 8.2.3 Application Curves
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
        1. 8.4.1.1 Compact 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 Development Support
        1. 9.1.1.1 Custom Design With WEBENCH® Tools
    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

7.3.6 Minimum ON-Time, Minimum OFF-Time, and Frequency Foldback

Minimum ON-time (tON_MIN) is the shortest duration of time that the high-side switch can be turned on. tON_MIN is typically 75ns for the LMR514x5-Q1 . Minimum OFF-time (tOFF_MIN) is the shortest duration of time that the high-side switch can be off. tOFF_MIN is typically 135ns. 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 3. D M I N = t O N _ M I N × f S W

The maximum duty cycle without frequency foldback allowed is:

Equation 4. D M A X = 1 - t O F F _ M I N × f S W

Given a required output voltage, the maximum VIN without frequency foldback can be found by:

Equation 5. V I N _ M A X = V O U T f S W × t O N _ M I N

The minimum VIN without frequency foldback can be calculated by:

Equation 6. V I N _ M I N = V O U T 1 - f S W × t O F F _ M I N

In the LMR514x5-Q1, a frequency foldback scheme is employed after the 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. After 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 5.

The frequency foldback scheme also works after 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 6. In such condition, the frequency can be as low as approximately 200kHz. Wide range of frequency foldback allows for the LMR514x5-Q1 output voltage to stay in regulation with a much lower supply voltage VIN, which leads to a lower effective dropout.

With frequency foldback while maintaining a regulated output voltage, VIN_MAX is raised, and VIN_MIN is lowered by decreased fSW.