SNVS217G May   2004  – September 2015 LM2731

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 Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
      1. 7.1.1 Theory of Operation
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Pin Operation
      2. 7.4.2 Thermal Shutdown
      3. 7.4.3 Current Limit
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1  Selecting the External Capacitors
        2. 8.2.2.2  Selecting the Output Capacitor
        3. 8.2.2.3  Selecting the Input Capacitor
        4. 8.2.2.4  Feedforward Compensation
        5. 8.2.2.5  Selecting Diodes
        6. 8.2.2.6  Setting the Output Voltage
        7. 8.2.2.7  Switching Frequency
        8. 8.2.2.8  Duty Cycle
        9. 8.2.2.9  Inductance Value
        10. 8.2.2.10 Maximum Switch Current
        11. 8.2.2.11 Calculating Load Current
        12. 8.2.2.12 Design Parameters VSW and ISW
        13. 8.2.2.13 Inductor Suppliers
      3. 8.2.3 Application Curves
    3. 8.3 System Examples
  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 Community Resources
    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 to get stable operation and low noise. All components must be as close as possible to the LM2731 device. TI recommends that a 4-layer PCB be used so that internal ground planes are available.

As an example, a recommended layout of components is shown in Figure 41.

Some additional guidelines to be observed:

  • Keep the path between L1, D1, and C2 extremely short. Parasitic trace inductance in series with D1 and C2 will increase noise and ringing.
  • The feedback components R1, R2 and CF must be kept close to the FB pin of U1 to prevent noise injection on the FB pin trace.
  • If internal ground planes are available (recommended), use vias to connect directly to ground at pin 2 of U1, as well as the negative sides of capacitors C1 and C2.

10.2 Layout Example

LM2731 20059116.png Figure 41. 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 LM2731 FET switch. The switch power dissipation from ON-state conduction is calculated by:

Equation 11. P(SW) = DC × IIND(AVE)2 × RDS(ON)

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