SNVSBA0D February   2020  – August 2021 LM61480-Q1 , LM61495-Q1 , LM62460-Q1


  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 Characteristics
    7. 7.7 Switching Characteristics
    8. 7.8 System Characteristics
    9. 7.9 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1  Output Voltage Selection
      2. 8.3.2  Enable EN Pin and Use as VIN UVLO
      3. 8.3.3  SYNC/MODE Uses for Synchronization
      4. 8.3.4  Clock Locking
      5. 8.3.5  Adjustable Switching Frequency
      6. 8.3.6  RESET Output Operation
      7. 8.3.7  Internal LDO, VCC UVLO, and BIAS Input
      8. 8.3.8  Bootstrap Voltage and VCBOOT-UVLO (CBOOT Pin)
      9. 8.3.9  Adjustable SW Node Slew Rate
      10. 8.3.10 Spread Spectrum
      11. 8.3.11 Soft Start and Recovery From Dropout
      12. 8.3.12 Overcurrent and Short Circuit Protection
      13. 8.3.13 Hiccup
      14. 8.3.14 Thermal Shutdown
    4. 8.4 Device Functional Modes
      1. 8.4.1 Shutdown Mode
      2. 8.4.2 Standby Mode
      3. 8.4.3 Active Mode
        1. Peak Current Mode Operation
        2. Auto Mode Operation
          1. Diode Emulation
        3. FPWM Mode Operation
        4. Minimum On-time (High Input Voltage) Operation
        5. Dropout
        6. Recovery from Dropout
        7. Other Fault Modes
  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.  Choosing the Switching Frequency
        2.  Setting the Output Voltage
        3.  Inductor Selection
        4.  Output Capacitor Selection
        5.  Input Capacitor Selection
        6.  BOOT Capacitor
        7.  BOOT Resistor
        8.  VCC
        9.  CFF and RFF Selection
        10. RSPSP Selection
        11. RT Selection
        12. RMODE Selection
        13. External UVLO
        14. Maximum Ambient Temperature
      3. 9.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
      1. 11.1.1 Ground and Thermal Considerations
    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 Receiving Notification of Documentation Updates
    3. 12.3 Support Resources
    4. 12.4 Trademarks
    5. 12.5 Glossary
    6. 12.6 Electrostatic Discharge Caution
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

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

Soft Start and Recovery From Dropout

When designing with the LM6x4xx-Q1, slowed rise in output voltage due to recovery from dropout and soft start must be considered separate phenomena. Soft start is triggered by any of the following conditions:

  • EN is used to turn on the device.
  • Recovery from a hiccup waiting period; see Section 8.3.13.
  • Recovery from shutdown due to overtemperature protection
  • Power is applied to the VIN of the IC or the VCC UVLO is released.

Once soft start is triggered, the IC takes the following actions:

  • The reference used by the IC to regulate output voltage is slowly ramped from zero. The net result is that output voltage, if previously 0 V, takes tSS to reach 90% of its desired value.
  • Operating mode is set to auto, activating diode emulation. This allows start-up without pulling output low if there is a voltage already present on the output.
  • Hiccup is disabled for the duration of soft start; see Section 8.3.13.

All of these actions together provide start-up with limited inrush currents. They also allow the use of output capacitors and loading conditions that cause current to border on current limit during start-up without triggering hiccup. In addition, if output voltage is already present, output is not pulled down. See Figure 8-13.

The left curves show soft start from 0 V. The right curves show soft starting behavior from a pre-biased or non-zero voltage. In either case, the output voltage reaches within 10% of the desired setpoint tSS time after soft start is initiated. During soft start, FPWM and hiccup are disabled. Both hiccup and FPWM are enabled once output reaches regulation or tSS2, whichever happens first.
Figure 8-13 Soft-Start Operation

Any time output voltage is more than a few percent low for any reason, output voltage ramps up slowly. This condition, called recovery from dropout, differs from soft start in three important ways:

  • Hiccup is allowed only if output voltage is less than 0.4 times its set point. Note that during dropout regulation itself, hiccup is inhibited. See Section 8.3.13.
  • FPWM mode is allowed during recovery from dropout. If output voltage were to suddenly be pulled up by an external supply, the LM6x4xx-Q1 can pull down on the output. Note that all the protections that are present during normal operation are in place, protecting the device if output is shorted to a high voltage or ground.
  • The reference voltage is set to approximately 1% above that needed to achieve the current output voltage. It is not started from zero.

Despite the name, recovery from dropout is active whenever output voltage is more than a few percent lower than the setpoint for long enough that:

  • Duty factor is controlled by minimum on-time or
  • When the part is operating in current limit.
This primarily occurs under the following conditions:

  • Dropout: When there is insufficient input voltage for the desired output voltage to be generated. See Section
  • Overcurrent that is not severe enough to trigger hiccup or if the duration is too short to trigger hiccup. See Section 8.3.13.
Whether output voltage falls due to high load or low input voltage, once the condition that causes output to fall below its setpoint is removed, output climbs at the same speed as during start-up. Even though hiccup does not trigger due to dropout, it can, in principal, be triggered during recovery if output voltage is below 0.4 times output the setpoint for more than 128 clock cycles during recovery.
Figure 8-14 Recovery From Dropout