SNVS029L March   2000  – June 2025 LM2678

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 – 3.3 V
    6. 5.6  Electrical Characteristics – 5 V
    7. 5.7  Electrical Characteristics – 12 V
    8. 5.8  Electrical Characteristics – Adjustable
    9. 5.9  Electrical Characteristics – All Output Voltage Versions
    10. 5.10 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 Switch Output
      2. 6.3.2 Input
      3. 6.3.3 C Boost
      4. 6.3.4 Ground
      5. 6.3.5 Feedback
      6. 6.3.6 ON/OFF
    4. 6.4 Device Functional Modes
      1. 6.4.1 Shutdown Mode
      2. 6.4.2 Active Mode
  8. 7 Application and Implementation
    1. 7.1 Application Information
      1. 7.1.1 Design Considerations
      2. 7.1.2 Inductor
      3. 7.1.3 Output Capacitor
      4. 7.1.4 Input Capacitor
      5. 7.1.5 Catch Diode
      6. 7.1.6 Boost Capacitor
      7. 7.1.7 Additional Application Information
    2. 7.2 Typical Application
      1. 7.2.1 All Output Voltage Versions
        1. 7.2.1.1 Design Requirements
        2. 7.2.1.2 Detailed Design Procedure
          1. 7.2.1.2.1 Custom Design With WEBENCH® Tools
          2. 7.2.1.2.2 Capacitor Selection Guides
        3. 7.2.1.3 Application Curves
      2. 7.2.2 Fixed Output Voltage Design Example
        1. 7.2.2.1 Detailed Design Procedure
          1. 7.2.2.1.1 Capacitor Selection
      3. 7.2.3 Adjustable Output Design Example
        1. 7.2.3.1 Detailed Design Procedure
          1. 7.2.3.1.1 Capacitor Selection
    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 Device Support
      1. 8.1.1 Development Support
        1. 8.1.1.1 Custom Design With WEBENCH® Tools
    2. 8.2 Documentation Support
      1. 8.2.1 Related Documentation
    3. 8.3 Receiving Notification of Documentation Updates
    4. 8.4 Support Resources
    5. 8.5 Trademarks
    6. 8.6 Electrostatic Discharge Caution
    7. 8.7 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information
    1. 10.1 VSON Package Devices

7.1.4 Input Capacitor

Fast changing currents in high current switching regulators place a significant dynamic load on the unregulated power source. An input capacitor helps to provide additional current to the power supply as well as smooth out input voltage variations.

Like the output capacitor, the key specifications for the input capacitor are RMS current rating and working voltage. The RMS current flowing through the input capacitor is equal to one-half of the maximum DC load current so the capacitor must be rated to handle this. Paralleling multiple capacitors proportionally increases the current rating of the total capacitance. The voltage rating must also be selected to be 1.3 times the maximum input voltage. Depending on the unregulated input power source, under light load conditions the maximum input voltage can be significantly higher than normal operation and must be considered when selecting an input capacitor.

The input capacitor must be placed very close to the input pin of the LM2678. Due to relative high current operation with fast transient changes, the series inductance of input connecting wires or PCB traces can create ringing signals at the input terminal which can possibly propagate to the output or other parts of the circuitry. It can be necessary in some designs to add a small valued (0.1 μF to 0.47 μF) ceramic type capacitor in parallel with the input capacitor to prevent or minimize any ringing.