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
8.2.1.2.3 Frequency Selection

At the minimum input voltage, the maximum switching frequency of LM25018 is restricted by the forced minimum off-time (TOFF(MIN)) as shown in Equation 10.

Equation 10. GUID-8746EB04-DC60-4161-AD58-AA25F70342A7-low.gif

Similarly, at maximum input voltage, the maximum switching frequency of LM25018 is restricted by the minimum TON as shown in Equation 11.

Equation 11. GUID-8405D8E1-ACFC-4758-A90E-147676015A54-low.gif

Resistor RON sets the nominal switching frequency based on Equation 12.

Equation 12. GUID-3F433F27-6379-45D8-91DD-41B206DFFC8B-low.gif

where

  • K = 9 × 10–11

Operation at high switching frequency results in lower efficiency while providing the smallest solution. For this example, 440 kHz was selected as the target switching frequency. The calculated value of RON = 253 kΩ. The standard value for RON = R3 is 237 kΩ is selected.