SNVS487E December   2006  – January 2015 LM27313 , LM27313-Q1

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: LM27313
    3. 6.3 ESD Ratings: LM27313-Q1
    4. 6.4 Recommended Operating Conditions
    5. 6.5 Thermal Information
    6. 6.6 Electrical 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
    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 VIN=5.0 V, VOUT=12.0 V, Iload=250 mA
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1  Selecting the External Capacitors
          2. 8.2.1.2.2  Selecting the Output Capacitor
          3. 8.2.1.2.3  Selecting the Input Capacitor
          4. 8.2.1.2.4  Feed-Forward Compensation
          5. 8.2.1.2.5  Selecting Diodes
          6. 8.2.1.2.6  Setting the Output Voltage
          7. 8.2.1.2.7  Duty Cycle
          8. 8.2.1.2.8  Inductance Value
          9. 8.2.1.2.9  Maximum Switch Current
          10. 8.2.1.2.10 Calculating Load Current
          11. 8.2.1.2.11 Design Parameters VSW and ISW
          12. 8.2.1.2.12 Minimum Inductance
          13. 8.2.1.2.13 Inductor Suppliers
          14. 8.2.1.2.14 Shutdown Pin Operation
        3. 8.2.1.3 Application Curves
      2. 8.2.2 Application Circuit VIN=5.0V, VOUT=20.0V, Iload=150mA
        1. 8.2.2.1 Design Requirements
        2. 8.2.2.2 Detailed Design Procedure
        3. 8.2.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
    3. 10.3 Thermal Considerations
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
    2. 11.2 Related Links
    3. 11.3 Trademarks
    4. 11.4 Electrostatic Discharge Caution
    5. 11.5 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

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10 Layout

10.1 Layout Guidelines

High-frequency switching regulators require very careful layout of components in order to get stable operation and low noise. All components must be as close as possible to the LM27313 device. It is recommended that a 4-layer PCB be used so that internal ground planes are available.

Some additional guidelines to be observed:

  1. Keep the path between L1, D1, and C2 extremely short. Parasitic trace inductance in series with D1 and C2 will increase noise and ringing.
  2. The feedback components R1, R2 and CF must be kept close to the FB pin of the LM27313 to prevent noise injection on the high impedance FB pin.
  3. If internal ground planes are available (recommended) use vias to connect directly to the LM27313 ground at device pin 2, as well as the negative sides of capacitors C1 and C2.

10.2 Layout Example

20216822.pngFigure 22. Recommended PCB Component Layout

10.3 Thermal Considerations

At higher duty cycles, the increased ON time of the FET means the maximum output current will be determined by power dissipation within the LM27313 FET switch. The switch power dissipation from ON-state conduction is calculated by:

Equation 18. PSW = DC x IIND(AVG)2 x RDS(ON)

There will be some switching losses as well, so some derating needs to be applied when calculating IC power dissipation.