SNVS953G December   2012  – May 2021 LM25018

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 Characteristics
    5. 6.5 Electrical Characteristics
    6. 6.6 Switching Characteristics
    7. 6.7 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  Control Overview
      2. 7.3.2  VCC Regulator
      3. 7.3.3  Regulation Comparator
      4. 7.3.4  Overvoltage Comparator
      5. 7.3.5  On-Time Generator
      6. 7.3.6  Current Limit
      7. 7.3.7  N-Channel Buck Switch and Driver
      8. 7.3.8  Synchronous Rectifier
      9. 7.3.9  Undervoltage Detector
      10. 7.3.10 Thermal Protection
      11. 7.3.11 Ripple Configuration
      12. 7.3.12 Soft Start
    4. 7.4 Device Functional Modes
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 Application Circuit: 12.5-V to 48-V Input and 10-V, 325-mA Output Buck Converter
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1 Custom Design With WEBENCH® Tools
          2. 8.2.1.2.2 RFB1, RFB2
          3. 8.2.1.2.3 Frequency Selection
          4. 8.2.1.2.4 Inductor Selection
          5. 8.2.1.2.5 Output Capacitor
          6. 8.2.1.2.6 Type III Ripple Circuit
          7. 8.2.1.2.7 VCC and Bootstrap Capacitor
          8. 8.2.1.2.8 Input Capacitor
          9. 8.2.1.2.9 UVLO Resistors
      2. 8.2.2 Application Curves
      3. 8.2.3 Typical Isolated DC-DC Converter Using LM25018
        1. 8.2.3.1 Design Requirements
        2. 8.2.3.2 Detailed Design Procedure
          1. 8.2.3.2.1  Transformer Turns Ratio
          2. 8.2.3.2.2  Total IOUT
          3. 8.2.3.2.3  RFB1, RFB2
          4. 8.2.3.2.4  Frequency Selection
          5. 8.2.3.2.5  Transformer Selection
          6. 8.2.3.2.6  Primary Output Capacitor
          7. 8.2.3.2.7  Secondary Output Capacitor
          8. 8.2.3.2.8  Type III Feedback Ripple Circuit
          9. 8.2.3.2.9  Secondary Diode
          10. 8.2.3.2.10 VCC and Bootstrap Capacitor
          11. 8.2.3.2.11 Input Capacitor
          12. 8.2.3.2.12 UVLO Resistors
          13. 8.2.3.2.13 VCC Diode
        3. 8.2.3.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 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
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information
8.2.3.2.7 Secondary Output Capacitor

A simplified waveform for secondary output current (IOUT2) is shown in Figure 8-7.

GUID-89BBD4BF-8411-4043-99A9-7FB155F6DCF4-low.gifFigure 8-7 Secondary Current Waveforms for COUT2 Ripple Calculation

The secondary output current (IOUT2) is sourced by COUT2 during on-time of the buck switch, TON. Ignoring the current transition times in the secondary winding, the secondary output capacitor ripple voltage can be calculated using Equation 29.

Equation 29. GUID-A0E01E3B-5C8B-41DF-87E1-63E99EA75D73-low.gif

For a 1:1 transformer turns ratio, the primary and secondary voltage ripple equations are identical. A COUT2 value of 2.2 μF is selected for this design example.

If lower output voltage ripple is required, a higher value must be selected for COUT1 and COUT2.