SLVSGN5 October   2022 LM51231-Q1

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
  6. 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
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Device Enable/Disable (EN, VH Pin)
      2. 7.3.2  High Voltage VCC Regulator (BIAS, VCC Pin)
      3. 7.3.3  Light Load Switching Mode Selection (MODE Pin)
      4. 7.3.4  VOUT Range Selection (RANGE Pin)
      5. 7.3.5  Line Undervoltage Lockout (UVLO Pin)
      6. 7.3.6  Fast Restart using VCC HOLD (VH Pin)
      7. 7.3.7  Adjustable Output Regulation Target (VOUT, TRK, VREF Pin)
      8. 7.3.8  Overvoltage Protection (VOUT Pin)
      9. 7.3.9  Power Good Indicator (PGOOD Pin)
      10. 7.3.10 Dynamically Programmable Switching Frequency (RT)
      11. 7.3.11 External Clock Synchronization (SYNC Pin)
      12. 7.3.12 Programmable Spread Spectrum (DITHER Pin)
      13. 7.3.13 Programmable Soft-start (SS Pin)
      14. 7.3.14 Wide Bandwidth Transconductance Error Amplifier and PWM (TRK, COMP Pin)
      15. 7.3.15 Current Sensing and Slope Compensation (CSP, CSN Pin)
      16. 7.3.16 Constant Peak Current Limit (CSP, CSN Pin)
      17. 7.3.17 Maximum Duty Cycle and Minimum Controllable On-time Limits
      18. 7.3.18 MOSFET Drivers, Integrated Boot Diode, and Hiccup Mode Fault Protection (LO, HO, HB Pin)
      19. 7.3.19 Thermal Shutdown Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 Device Status
        1. 7.4.1.1 Shutdown Mode
        2. 7.4.1.2 Configuration Mode
        3. 7.4.1.3 Active Mode
        4. 7.4.1.4 Bypass Mode
          1. 7.4.1.4.1 Bypass DE mode
          2. 7.4.1.4.2 Bypass FPWM
      2. 7.4.2 Light Load Switching Mode
        1. 7.4.2.1 Forced PWM (FPWM) Mode
        2. 7.4.2.2 Diode Emulation (DE) Mode
        3. 7.4.2.3 Forced Diode Emulation Operation in FPWM Mode
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
      3. 8.2.3 Application Ideas
      4. 8.2.4 Application Curves
    3. 8.3 System Example
    4. 8.4 Power Supply Recommendations
    5. 8.5 Layout
      1. 8.5.1 Layout Guidelines
      2. 8.5.2 Layout Example
  9. 9 Device and Documentation Support
    1. 9.1 Receiving Notification of Documentation Updates
    2. 9.2 Support Resources
    3. 9.3 Trademarks
    4. 9.4 Electrostatic Discharge Caution
    5. 9.5 Glossary
  10. 10Mechanical, Packaging, and Orderable Information

Package Options

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

7.3.13 Programmable Soft-start (SS Pin)

The soft-start feature helps the converter gradually reach the steady state operating point. To reduce start-up stresses and surges, the device regulates the error amplifier reference to the SS pin voltage or the TRK pin voltage (VTRK), whichever is lower.

The internal 20-μA soft-start (ISS) current turns on 120 μs after the VCC pin crosses VVCC-UVLO. ISS gradually increases the voltage on an external soft-start capacitor (CSS). This results in a gradual rise of the output voltage.

In FPWM mode, the device forces diode emulation while the SS pin voltage is less than 1.5 V. When the SS pin voltage is greater than 1.5 V, the device changes the high-side negative current limit threshold from VZCD-DE to VI-HS-NEG.

GUID-083878E3-CD3C-4C07-A6AC-A7EFD9FB5865-low.gifFigure 7-12 Soft Start and Smooth Transition to FPWM

In boost topology, the soft-start time (tSS) varies with the input supply voltage because the boost output voltage is equal to the boost input voltage at the beginning of the soft-start switching. tSS in boost topology is calculated in Equation 10.

Equation 10. tSS=VTRK×CSS20μA×1-VSUPPLYVLOAD

In general, it is recommended to choose a soft-start time long enough so that the converter can start up without going into an overcurrent state. If the device is used for a pre-boost in automotive application, it is recommended to use 100-pF CSS to reach steady state as soon as possible.

The device also features an internal SS-to-FB clamp (VSS-FB), which clamps SS 55 mV above FB and is activated if 256 consecutive switching cycles occur with current limit. The SS-to-FB clamp is deactivated if 32 consecutive switching cycles occur without exceeding the current limit threshold. This clamp helps to minimize surges after output shorts or over load situations. The device can enter deep sleep mode when SS is greater than 1.5 V. It is not recommended to pulldown SS to stop switching.