SLUSEP7 December   2022 LMR51440 , LMR51450

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Comparison Table
  6. 6 Pin Configuration and Functions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2.     ESD Ratings
    3. 7.2 Recommended Operating Conditions
    4. 7.3 Thermal Information
    5. 7.4 Electrical Characteristics
    6. 7.5 System Characteristics
    7. 7.6 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1  Fixed Frequency Peak Current Mode Control
      2. 8.3.2  Adjustable Output Voltage
      3. 8.3.3  Enable
      4. 8.3.4  Switching Frequency
      5. 8.3.5  Power-Good Flag Output
      6. 8.3.6  Minimum ON-Time, Minimum OFF-Time, and Frequency Foldback
      7. 8.3.7  Bootstrap Voltage
      8. 8.3.8  Overcurrent and Short-Circuit Protection
      9. 8.3.9  Soft Start
      10. 8.3.10 Thermal Shutdown
    4. 8.4 Device Functional Modes
      1. 8.4.1 Shutdown Mode
      2. 8.4.2 Active Mode
      3. 8.4.3 CCM Mode
      4. 8.4.4 Light-Load Operation (PFM Version)
      5. 8.4.5 Light-Load Operation (FPWM Version)
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1 Output Voltage Set-Point
        2. 9.2.2.2 Switching Frequency
        3. 9.2.2.3 Inductor Selection
        4. 9.2.2.4 Output Capacitor Selection
        5. 9.2.2.5 Input Capacitor Selection
        6. 9.2.2.6 Bootstrap Capacitor
        7. 9.2.2.7 Undervoltage Lockout Set-Point
      3. 9.2.3 Application Curves
    3. 9.3 Best Design Practices
    4. 9.4 Power Supply Recommendations
    5. 9.5 Layout
      1. 9.5.1 Layout Guidelines
        1. 9.5.1.1 Compact Layout for EMI Reduction
        2. 9.5.1.2 Feedback Resistors
      2. 9.5.2 Layout Example
  10. 10Device and Documentation Support
    1. 10.1 Device Support
      1. 10.1.1 Development Support
        1. 10.1.1.1 Custom Design With WEBENCH® Tools
    2. 10.2 Documentation Support
      1. 10.2.1 Related Documentation
    3. 10.3 Receiving Notification of Documentation Updates
    4. 10.4 Support Resources
    5. 10.5 Trademarks
    6. 10.6 Electrostatic Discharge Caution
    7. 10.7 Glossary
  11. 11Mechanical, Packaging, and Orderable Information

Package Options

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

9.2.2.7 Undervoltage Lockout Set-Point

The system undervoltage lockout (UVLO) is adjusted using the external voltage divider network of RENT and RENB. The UVLO has two thresholds, one for power up when the input voltage is rising and one for power down or brown outs when the input voltage is falling. Equation 14 can be used to determine the VIN UVLO level.

Equation 14. VIN_RISING=VENH×REBT+RENBRENB

The EN rising threshold (VENH) for LMR514x0 is set to be 1.25 V (typical). Choose a value of 21.5 kΩ for RENB to minimize input current from the supply. If the desired VIN UVLO level is at 6.0 V, then the value of RENT can be calculated using Equation 15:

Equation 15. REBT=(VIN_RISINGVENH-1)×RENB

The above equation yields a value of 81.7 kΩ, a standard value of 82 kΩ is selected. The resulting falling UVLO threshold, equals 4.8 V, can be calculated by Equation 16 where EN hysteresis voltage, VEN_HYS, is 0.25 V (typical).

Equation 16. VIN_FALLING=(VENH-VENH_HYS)×REBT+RENBRENB