SNVSBK8A October   2020  – March 2021 LM5157-Q1 , LM51571-Q1

PRODMIX  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Description (continued)
  6. Device Comparison Table
  7. Pin Configuration and Functions
  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
  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/EN 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  Dual Random Spread Spectrum - DRSS (MODE Pin)
      6. 9.3.6  Clock Synchronization (UVLO/SYNC/EN Pin)
      7. 9.3.7  Current Sense and Slope Compensation
      8. 9.3.8  Current Limit and Minimum On Time
      9. 9.3.9  Feedback and Error Amplifier (FB, COMP Pin)
      10. 9.3.10 Power-Good Indicator (PGOOD Pin)
      11. 9.3.11 Hiccup Mode Overload Protection (MODE Pin)
      12. 9.3.12 Maximum Duty Cycle Limit and Minimum Input Supply Voltage
      13. 9.3.13 Internal MOSFET (SW Pin)
      14. 9.3.14 Overvoltage Protection (OVP)
      15. 9.3.15 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
        1. 9.4.3.1 Spread Spectrum Enabled
        2. 9.4.3.2 Hiccup Mode Protection Enabled
  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 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 Diode Selection
      3. 10.2.3 Application Curve
    3. 10.3 System Examples
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
    2. 12.2 Layout Examples
  13. 13Device and Documentation Support
    1. 13.1 Device Support
      1. 13.1.1 Third-Party Products Disclaimer
      2. 13.1.2 Development Support
        1. 13.1.2.1 Custom Design With WEBENCH® Tools
      3. 13.1.3 Export Control Notice
    2. 13.2 Documentation Support
      1. 13.2.1 Related Documentation
    3. 13.3 Receiving Notification of Documentation Updates
    4. 13.4 Support Resources
    5. 13.5 Trademarks
    6. 13.6 Electrostatic Discharge Caution
    7. 13.7 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

Package Options

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

Current Sense and Slope Compensation

The device senses switch current which flows into the SW pin, and provides a fixed internal slope compensation ramp, helping prevent subharmonic oscillation at high duty cycle. The internal slope compensation ramp is added to the sensed switch current for the PWM operation, but no slope compensation ramp is added to the sensed inductor current for the current limit operation to provide an accurate peak current limit over the input supply voltage (see Figure 9-17).

GUID-EEEE1EE4-CE69-4A88-91CD-1DBC98D43F7B-low.gifFigure 9-17 Current Sensing and Slope Compensation
GUID-3BA19FB2-16DA-4883-A16E-885BA3E9D8CD-low.gifFigure 9-18 Current Sensing and Slope Compensation (a) at PWM Comparator Inputs
GUID-9AD6F538-B5F5-496A-8253-5FCFA4D1E60D-low.gif Figure 9-19 Current Sensing (b) at Current Limit Comparator Inputs

Use Equation 6 to calculate the value of the peak slope voltage (VSLOPE).

Equation 6. GUID-BC7AB38B-B295-4AA1-B3A4-403A1439D62A-low.gif

where

  • fSYNC is fRT if clock synchronization is not used

According to peak current mode control theory, the slope of the compensation ramp must be greater than half of the sensed inductor current falling slope to prevent subharmonic oscillation at high duty cycle. Therefore, the minimum amount of slope compensation in boost topology must satisfy the following inequality:

Equation 7. GUID-3611BE1B-E2D9-4731-A79E-6AA1BBCFB697-low.gif

where

  • VF is a forward voltage drop of D1, the external diode

Typically 82% of the sensed inductor current falling slope is known as an optimal amount of the slope compensation. By increasing the margin to 1.6, the amount of slope compensation becomes close to the optimal amount.

If clock synchronization is not used, the fSW frequency equals the fRT frequency. If clock synchronization is used, the fSW frequency equals the fSYNC frequency.