SNVSC40E August   2021  – September 2023 LMR38020

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Revision History
  6. 5 Device Comparison Table
  7. 6 Pin Configuration and Functions
  8. 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 System Characteristics
    7. 7.7 Typical Characteristics
  9. 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 and Synchronization (RT/SYNC)
      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 Auto Mode
      2. 8.4.2 Forced PWM Operation
      3. 8.4.3 Dropout
      4. 8.4.4 Minimum Switch On Time
  10. 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 Custom Design With WEBENCH® Tools
        2. 9.2.2.2 Choosing the Switching Frequency
        3. 9.2.2.3 FB for Adjustable Output
        4. 9.2.2.4 Inductor Selection
        5. 9.2.2.5 Output Capacitor Selection
        6. 9.2.2.6 Input Capacitor Selection
        7. 9.2.2.7 CBOOT
        8. 9.2.2.8 External UVLO
        9. 9.2.2.9 Maximum Ambient Temperature
      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 Ground and Thermal Considerations
      2. 9.5.2 Layout Example
  11. 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. 10.1.2 Third-Party Products Disclaimer
    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
  12. 11Mechanical, Packaging, and Orderable Information

Package Options

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

8.3.6 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 75 ns in the LMR38020. Minimum off time (TOFF_MIN) is the smallest duration that the high-side switch can be off. TOFF_MIN is typically 190 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 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. DMAX =1-( TOFF-MIN × fSW )

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

Equation 5. VIN_MAX = VOUTTON-MIN× fSW 

The minimum VIN without frequency foldback can be calculated by:

Equation 6. VIN_MIN = VOUT1-(TOFF-MIN × fSW) 

In the LMR38020, a frequency foldback scheme is employed after 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 3.

The frequency foldback scheme also works after larger duty cycle is needed under low VIN condition. The frequency decreases after the device hits its TOFF_MIN, which extends the maximum duty cycle according to Equation 4. In such condition, the frequency can be as low as approximately 133 kHz minimum. Wide range of frequency foldback allows the LMR38020 output voltage stay in regulation with a much lower supply voltage VIN, which leads to a lower effective dropout.

The fixed frequency operation at FPWM mode with switching frequency greater than 1.2 MHz, is maintained with minimum load current about 100 mA for wider input voltage range. And for very light load and switching frequency over 1.2 MHz, frequency drop can be expected during mintoff frequency foldback.

With frequency foldback, VIN_MAX is raised, and VIN_MIN is lowered by decreased fSW.

GUID-20211109-SS0I-53HB-CQRD-QSJ3GLCJTG35-low.svg
FSW = 1.1 MHz VOUT = 5 V IOUT = 2 A
Figure 8-8 Typical Frequency Foldback at TON_MIN
GUID-20211109-SS0I-DFZD-JMFZ-PBFNQR0N7DNN-low.svg
FSW = 1.1 MHz VOUT = 5 V IOUT = 2 A
Figure 8-9 Typical Frequency Foldback at TOFF_MIN