SLUSEG8B October   2021  – February 2022 LMR54406 , LMR54410

PRODUCTION DATA  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Device Comparison Table
  6. Pin Configuration and Functions
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Timing Requirements
    7. 7.7 System Characteristics
    8. 7.8 Typical Characteristics
  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 Minimum ON Time, Minimum OFF Time, and Frequency Foldback
      5. 8.3.5 Bootstrap Voltage
      6. 8.3.6 Overcurrent and Short Circuit Protection
      7. 8.3.7 Soft Start
      8. 8.3.8 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. 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
        8. 9.2.2.8 Replacing Non Sync Converter
      3. 9.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
      1. 11.1.1 Compact Layout for EMI Reduction
      2. 11.1.2 Feedback Resistors
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Third-Party Products Disclaimer
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
    3. 12.3 Receiving Notification of Documentation Updates
    4. 12.4 Support Resources
    5. 12.5 Trademarks
    6. 12.6 Electrostatic Discharge Caution
    7. 12.7 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

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

Minimum ON Time, Minimum OFF Time, and Frequency Foldback

The minimum ON time (TON_MIN) is the shortest duration of time that the high-side switch can be turned on. TON_MIN is typically 60 ns for the LMR544xx. The minimum OFF time (TOFF_MIN) is the shortest duration of time that the high-side switch can be off. TOFF_MIN is typically 110 ns. 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 2. DMIN = TON_MIN × fSW

The maximum duty cycle without frequency foldback allowed is:

Equation 3. DMAX = 1 - TOFF_MIN × fSW

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

Equation 4. GUID-27A905E1-9C25-4F2C-8FFC-42E678EECEE4-low.gif

The minimum VIN without frequency foldback can be calculated by:

Equation 5. GUID-B48EFFFC-5612-4A38-BBD3-E05F8B97F93B-low.gif

In the LMR544xx, a frequency foldback scheme is employed once 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. 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 2.

The frequency foldback scheme also works once larger duty cycle is needed under a low VIN condition. The frequency decreases once the device hits its TOFF_MIN, which extends the maximum duty cycle according to Equation 3. In such condition, the frequency can be as low as approximately 133 kHz. A wide range of frequency foldback allows for the LMR544xx 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.

VOUT = 0.8VfSW = 1100 kHz
Figure 8-4 Frequency Foldback at TON_MIN
VOUT = 5 VfSW = 1100 kHz
Figure 8-5 Frequency Foldback at TOFF_MIN