SNVS509F April   2007  – November 2023 LM25116

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics
    6. 5.6 Switching Characteristics
    7. 5.7 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 High-Voltage Start-Up Regulator
      2. 6.3.2 Enable
      3. 6.3.3 UVLO
      4. 6.3.4 Oscillator and Sync Capability
      5. 6.3.5 Error Amplifier and PWM Comparator
      6. 6.3.6 Ramp Generator
      7. 6.3.7 Current Limit
      8. 6.3.8 HO Output
      9. 6.3.9 Thermal Protection
    4. 6.4 Device Functional Modes
      1. 6.4.1 Soft Start and Diode Emulation
  8. 7 Application and Implementation
    1. 7.1 Application Information
    2. 7.2 Typical Application
      1. 7.2.1 Design Requirements
      2. 7.2.2 Detailed Design Procedure
        1. 7.2.2.1  Timing Resistor
        2. 7.2.2.2  Output Inductor
        3. 7.2.2.3  Current Sense Resistor
        4. 7.2.2.4  Ramp Capacitor
        5. 7.2.2.5  Output Capacitors
        6. 7.2.2.6  Input Capacitors
        7. 7.2.2.7  VCC Capacitor
        8. 7.2.2.8  Bootstrap Capacitor
        9. 7.2.2.9  Soft Start Capacitor
        10. 7.2.2.10 Output Voltage Divider
        11. 7.2.2.11 UVLO Divider
        12. 7.2.2.12 MOSFETs
        13. 7.2.2.13 MOSFET Snubber
        14. 7.2.2.14 Error Amplifier Compensation
        15. 7.2.2.15 Comprehensive Equations
          1. 7.2.2.15.1 Current Sense Resistor and Ramp Capacitor
          2. 7.2.2.15.2 Modulator Transfer Function
          3. 7.2.2.15.3 Error Amplifier Transfer Function
      3. 7.2.3 Application Curves
    3. 7.3 Power Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
      2. 7.4.2 Layout Example
  9. 8 Device and Documentation Support
    1. 8.1 Receiving Notification of Documentation Updates
    2. 8.2 Support Resources
    3. 8.3 Trademarks
    4. 8.4 Electrostatic Discharge Caution
    5. 8.5 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

Package Options

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

7.2.2.2 Output Inductor

The inductor value is determined based on the operating frequency, load current, ripple current and the input and output voltages.

GUID-05B6A0AB-6006-4429-B787-D31FE3157BDB-low.gifFigure 7-2 Inductor Current

Knowing the switching frequency (fSW), maximum ripple current (IPP), maximum input voltage (VIN(MAX)) and the nominal output voltage (VOUT), the inductor value is calculated with Equation 8.

Equation 8. GUID-67AB4FA3-A9B9-41E9-B472-EA904A7EAADA-low.gif

The maximum ripple current occurs at the maximum input voltage. Typically, IPP is 20% to 40% of the full load current. When running diode emulation mode, the maximum ripple current must be less than twice the minimum load current. For full synchronous operation, higher ripple current is acceptable. Higher ripple current allows for a smaller inductor size, but places more of a burden on the output capacitor to smooth the ripple current for low output ripple voltage. For this example, 40% ripple current is chosen for a smaller sized inductor in Equation 9.

Equation 9. GUID-BD7BC7C2-3AB3-490F-A4BE-054CCEB4F41B-low.gif

The nearest standard value of 6 µH is used. The inductor must be rated for the peak current to prevent saturation. During normal operation, the peak current occurs at maximum load current plus maximum ripple. During overload conditions with properly scaled component values, the peak current is limited to VCS(TH) / RS (See Section 7.2.2.3). At the maximum input voltage with a shorted output, the valley current must fall below VCS(TH) / RS before the high-side MOSFET is allowed to turn on. The peak current in steady state increases to VIN(MAX) x tON(min) / L above this level. The chosen inductor must be evaluated for this condition, especially at elevated temperature where the saturation current rating may drop significantly.