SNVSCQ8B November   2025  – July 2026 LM654A5-Q1 , LM654B0-Q1

PRODMIX  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Device Comparison Table
  6. 5 Pin Configuration and Functions
  7. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Typical Characteristics
  8. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Descriptions
      1. 7.3.1  Output Voltage Selection
      2. 7.3.2  EN Pin and Use as VIN UVLO
      3. 7.3.3  Device Configuration
      4. 7.3.4  Mode Selection
        1. 7.3.4.1 MODE/SYNC Pin Uses for Synchronization
        2. 7.3.4.2 Clock Locking
      5. 7.3.5  Adjustable Switching Frequency and Phase Shift
      6. 7.3.6  Dual Random Spread Spectrum (DRSS)
      7. 7.3.7  Internal LDO, VCC UVLO, and BIAS Input
      8. 7.3.8  Bootstrap Voltage (BST Pin)
      9. 7.3.9  Soft Start and Recovery From Dropout
      10. 7.3.10 Safety Features
        1. 7.3.10.1 Power-Good Monitor
        2. 7.3.10.2 Overcurrent and Short-Circuit Protection
        3. 7.3.10.3 Hiccup
        4. 7.3.10.4 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Active Mode
        1. 7.4.2.1 Peak Current Mode Operation
        2. 7.4.2.2 Auto Mode Operation
          1. 7.4.2.2.1 Diode Emulation
        3. 7.4.2.3 FPWM Mode Operation
  9. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Example Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Choosing the Switching Frequency
        2. 8.2.2.2 Inductor Selection
        3. 8.2.2.3 Output Capacitors
        4. 8.2.2.4 Input Capacitor Selection
        5. 8.2.2.5 Setting the Output Voltage
        6. 8.2.2.6 Compensation Components
        7. 8.2.2.7 Feed-forward Capacitor (CFF)
        8. 8.2.2.8 Maximum Ambient Temperature
      3. 8.2.3 Application Curves
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
      2. 8.4.2 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
      1. 9.1.1 Third-Party Products Disclaimer
    2. 9.2 Documentation Support
      1. 9.2.1 Related Documentation
    3. 9.3 Receiving Notification of Documentation Updates
    4. 9.4 Support Resources
    5. 9.5 Trademarks
    6. 9.6 Electrostatic Discharge Caution
    7. 9.7 Glossary
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information

7.3.9 Soft Start and Recovery From Dropout

The soft-start features brings the output voltage up from zero volts to the programmed output voltage in a controlled manner helping to prevent inrush current and overshoots in the output voltage of the regulator. 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 7.3.10.3.
  • Recovery from shutdown due to over-temperature protection.
  • Power is applied to the VIN of the IC or the VCC UVLO is released.

After soft start is initiated, the IC takes the following actions:

  • The reference used by the IC to regulate the output voltage is slowly ramped up from zero. The net result is that output voltage, if previously 0V, takes tSS to reach 90% of the regulation value.
  • Operating mode is set to AUTO, activating diode emulation. This action allows start-up without pulling the output voltage low if there is a voltage already present on the output.
  • Hiccup is disabled for the duration of soft start; see Section 7.3.10.3.

All of these actions together provide start-up with limited inrush currents. These actions also allow the use of output capacitors and loading conditions that can cause current limit during start-up without triggering hiccup. In addition, if the output voltage is already present, the output voltage does not discharge.

With the SS pin open, the default soft-start time, to 90% of the regulation point, is about 2.6ms (typical). This time can be increased by placing a high quality ceramic capacitor on the SS pin to ground. Use Equation 5 to select an appropriate capacitor value.

Equation 5. C s s μ F = 0.031 × T s s m s

If the output voltage goes below the regulation point by several percent for any reason, the output voltage ramps back up slowly after the condition has been removed. This action is the recovery from dropout condition which differs from soft start in three important ways:

  • Hiccup is allowed only if output voltage is less than 0.4 times the regulation point. Note that during dropout regulation, hiccup is inhibited. See Section 7.3.10.3.
  • FPWM mode is allowed during recovery from dropout. If the output voltage suddenly pulled up by an external supply, the LM654xx-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 the regulation point; the reference voltage is not started from zero.

Despite the name, recovery from dropout is active whenever the output voltage is several percent below the regulation point for the following conditions:

  • Duty cycle is controlled by minimum on-time
  • When the device is operating in current limit.

This action primarily occurs under the following conditions:

  • Dropout: when there is insufficient input voltage for the desired output voltage to be regulated.
  • Current limit that is not severe enough to trigger hiccup or if the duration is too short to trigger hiccup. See Section 7.3.10.3.
LM654A5-Q1 LM654B0-Q1 Recovery From Dropout
Whether the output voltage falls due to high load or low input voltage, after the condition that causes the output of regulation event is removed, the output climbs at the same speed as during start-up. Even though hiccup does not trigger due to dropout, hiccup can, in principle, be triggered during recovery if output voltage is below 0.4 times output the set-point for more than 128 clock cycles during recovery.
Figure 7-7 Recovery From Dropout