SNVSB75E december   2018  – august 2023 LM5155 , LM51551

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Revision History
  6. 5 Description (continued)
  7. 6 Device Comparison Table
  8. 7 Pin Configuration and Functions
  9. 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 Typical Characteristics
  10. 9 Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1  Line Undervoltage Lockout (UVLO/SYNC Pin)
      2. 9.3.2  High Voltage VCC Regulator (BIAS, VCC Pin)
      3. 9.3.3  Soft Start (SS Pin)
      4. 9.3.4  Switching Frequency (RT Pin)
      5. 9.3.5  Clock Synchronization (UVLO/SYNC Pin)
      6. 9.3.6  Current Sense and Slope Compensation (CS Pin)
      7. 9.3.7  Current Limit and Minimum On-time (CS Pin)
      8. 9.3.8  Feedback and Error Amplifier (FB, COMP Pin)
      9. 9.3.9  Power-Good Indicator (PGOOD Pin)
      10. 9.3.10 Hiccup Mode Overload Protection (LM51551 Only)
      11. 9.3.11 Maximum Duty Cycle Limit and Minimum Input Supply Voltage
      12. 9.3.12 MOSFET Driver (GATE Pin)
      13. 9.3.13 Overvoltage Protection (OVP)
      14. 9.3.14 Thermal Shutdown (TSD)
    4. 9.4 Device Functional Modes
      1. 9.4.1 Shutdown Mode
      2. 9.4.2 Standby Mode
      3. 9.4.3 Run Mode
  11. 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 Custom Design With WEBENCH® Tools
        2. 10.2.2.2 Recommended Components
        3. 10.2.2.3 Inductor Selection (LM)
        4. 10.2.2.4 Output Capacitor (COUT)
        5. 10.2.2.5 Input Capacitor
        6. 10.2.2.6 MOSFET Selection
        7. 10.2.2.7 Diode Selection
        8. 10.2.2.8 Efficiency Estimation
      3. 10.2.3 Application Curve
    3. 10.3 System Examples
    4. 10.4 Power Supply Recommendations
    5. 10.5 Layout
      1. 10.5.1 Layout Guidelines
      2. 10.5.2 Layout Examples
  12. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
      2. 11.1.2 Development Support
        1. 11.1.2.1 Custom Design With WEBENCH® Tools
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Receiving Notification of Documentation Updates
    4. 11.4 Support Resources
    5. 11.5 Trademarks
    6. 11.6 Electrostatic Discharge Caution
    7. 11.7 Glossary
  13. 12Mechanical, Packaging, and Orderable Information

9.3.3 Soft Start (SS Pin)

The soft-start feature helps the converter gradually reach the steady state operating point, thus reducing start-up stresses and surges. The device regulates the FB pin to the SS pin voltage or the internal reference, whichever is lower.

At start-up, the internal 10-μA soft-start current source (ISS) turns on 50 µs after the VCC voltage exceeds the 2.85-VCC UV threshold, or if the VCC voltage is greater than 4.5 V, whichever comes first. The soft-start current gradually increases the voltage on an external soft-start capacitor connected to the SS pin. This results in a gradual rise of the output voltage. The SS pin is pulled down to ground by an internal switch when the VCC is less than VCC UVLO threshold, the UVLO is less than the UVLO threshold, during hiccup mode off-time or thermal shutdown.

In boost topology, soft-start time (tSS) varies with the input supply voltage. The soft-start time in boost topology is calculated as shown in Equation 3.

Equation 3. GUID-20230720-SS0I-DDD1-XQHP-XVTMGV1SDVCM-low.gif

In SEPIC topology, the soft-start time (tSS) is calculated as follows.

Equation 4. GUID-20230720-SS0I-9CRH-WZTK-H05GVBM1QGDV-low.gif

TI recommends choosing a soft-start time long enough so that the converter can start up without going into an overcurrent state. See Section 9.3.10 for more detailed information.

Figure 9-8 shows an implementation of primary side soft start in flyback topology.

GUID-B9757D7C-A06F-4152-B230-DD40AC9C4785-low.gifFigure 9-8 Primary-Side Soft-Start in Flyback

Figure 9-9 shows an implementation of secondary side soft start in flyback topology.

GUID-F183B914-9CB9-4925-9F55-7713976BEDA1-low.gifFigure 9-9 Secondary-Side Soft Start in Flyback