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
8.2.3.2.6 Primary Output Capacitor

In a conventional buck converter, the output ripple voltage is calculated as shown in Equation 27.

Equation 27. GUID-E58D881E-7A62-48FC-82F7-A42EF85C4078-low.gif

To limit the primary output ripple voltage ΔVOUT1 to approximately 50 mV, an output capacitor COUT1 of 0.45 µF would be required for a conventional buck converter.

Figure 8-6 shows the primary winding current waveform (IL1) of a Fly-Buck converter. The reflected secondary winding current adds to the primary winding current during the buck switch off-time. Because of this increased current, the output voltage ripple is not the same as in conventional buck converter. The output capacitor value calculated in Equation 27 must be used as the starting point. Optimization of output capacitance over the entire line and load range must be done experimentally. If the majority of the load current is drawn from the secondary isolated output, a better approximation of the primary output voltage ripple is given by Equation 28.

Equation 28. GUID-AD42BA55-D8B2-4EDE-988A-257C4360C5E9-low.gif
GUID-95DC2459-9530-477D-B27B-4320C3417B7A-low.gifFigure 8-6 Current Waveforms for COUT1 Ripple Calculation

To limit the maximum primary output voltage ripple due to reflected secondary current to 50 mV, an output capacitor (COUT1) of 4 μF is required. A standard 4.7-µF, 16-V capacitor is selected for this design. If lower output voltage ripple is required, a higher value must be selected for COUT1 and COUT2.