SNVSAW8E March   2020  – April 2022 LM62440-Q1

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Description (continued)
  6. 6 Device Comparison Table
  7. 7 Pin Configuration and Functions
  8. 8 Specifications
    1. 8.1 Absolute Maximum Ratings
    2. 8.2 ESD Ratings
    3. 8.3 Recommended Operating Conditions
    4. 8.4 Thermal Information
    5. 8.5 Electrical Characteristics
    6. 8.6 Timing Characteristics
    7. 8.7 Systems Characteristics
    8. 8.8 Typical Characteristics
  9. 9 Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1  EN Uses for Enable and VIN UVLO
      2. 9.3.2  MODE/SYNC Pin Operation
        1. 9.3.2.1 Level-Dependent MODE/SYNC Pin Control
        2. 9.3.2.2 Pulse-Dependent MODE/SYNC Pin Control
        3. 9.3.2.3 Clock Locking
      3. 9.3.3  PGOOD Output Operation
      4. 9.3.4  Internal LDO, VCC UVLO, and BIAS Input
      5. 9.3.5  Bootstrap Voltage and VCBOOT-UVLO (CBOOT Pin)
      6. 9.3.6  Adjustable SW Node Slew Rate
      7. 9.3.7  Spread Spectrum
      8. 9.3.8  Soft Start and Recovery From Dropout
      9. 9.3.9  Output Voltage Setting
      10. 9.3.10 Overcurrent and Short Circuit Protection
      11. 9.3.11 Thermal Shutdown
      12. 9.3.12 Input Supply Current
    4. 9.4 Device Functional Modes
      1. 9.4.1 Shutdown Mode
      2. 9.4.2 Standby Mode
      3. 9.4.3 Active Mode
        1. 9.4.3.1 CCM Mode
        2. 9.4.3.2 Auto Mode – Light-Load Operation
          1. 9.4.3.2.1 Diode Emulation
          2. 9.4.3.2.2 Frequency Reduction
        3. 9.4.3.3 FPWM Mode – Light-Load Operation
        4. 9.4.3.4 Minimum On-Time (High Input Voltage) Operation
        5. 9.4.3.5 Dropout
  10. 10Application and Implementation
    1. 10.1 Application Information
    2. 10.2 Typical Application
      1. 10.2.1 Design Requirements
      2. 10.2.2 Detailed Design Procedure
        1. 10.2.2.1  Choosing the Switching Frequency
        2. 10.2.2.2  Setting the Output Voltage
        3. 10.2.2.3  Inductor Selection
        4. 10.2.2.4  Output Capacitor Selection
        5. 10.2.2.5  Input Capacitor Selection
        6. 10.2.2.6  BOOT Capacitor
        7. 10.2.2.7  BOOT Resistor
        8. 10.2.2.8  VCC
        9. 10.2.2.9  BIAS
        10. 10.2.2.10 CFF and RFF Selection
        11. 10.2.2.11 External UVLO
      3. 10.2.3 Application Curves
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
      1. 12.1.1 Ground and Thermal Considerations
    2. 12.2 Layout Example
  13. 13Device and Documentation Support
    1. 13.1 Documentation Support
      1. 13.1.1 Related Documentation
    2. 13.2 Receiving Notification of Documentation Updates
    3. 13.3 Support Resources
    4. 13.4 Trademarks
    5. 13.5 Electrostatic Discharge Caution
    6. 13.6 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

Package Options

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

9.4.3.5 Dropout

Dropout operation is defined as any input-to-output voltage ratio that requires frequency to drop to achieve the required duty cycle. At a given clock frequency, duty cycle is limited by minimum off time. Once this limit is reached, if clock frequency were maintained, output voltage would fall. Instead of allowing the output voltage to drop, the LM62440-Q1 extends on time past the end of the clock cycle until the required peak inductor current is achieved. The clock is allowed to start a new cycle once peak inductor current is achieved or once a pre-determined maximum on time, tON_MAX, of approximately 9 µs passes. As a result, once the needed duty cycle cannot be achieved at the selected clock frequency due to the existence of a minimum off time, frequency drops to maintain regulation. If input voltage is low enough so that output voltage cannot be regulated even with an on time of tON_MAX, output voltage drops to slightly below the input voltage, VDROP1. For additional information on recovery from dropout, reference Figure 9-16.

GUID-F3A9F759-8412-4970-9A99-941E421940DC-low.gif
Output voltage and frequency versus input voltage: If there is little difference between input voltage and output voltage setting, the IC reduces frequency to maintain regulation. If input voltage is too low to provide the desired output voltage at approximately 110 kHz, input voltage tracks output voltage.
Figure 9-26 Frequency and Output Voltage in Dropout
GUID-71649F81-92CD-496C-BEFE-5DA0BAFE524C-low.gif
Switching waveforms while in dropout. Inductor current takes longer than a normal clock to reach the desired peak value. As a result, frequency drops. This frequency drop is limited by tON_MAX.
Figure 9-27 Dropout Waveforms