SNVS710I March   2011  – December 2015 LMZ13608

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Application Schematic
      2.      Efficiency 3.3-V Output at 25°C
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
    1.     Pin 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
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Output Overvoltage Protection
      2. 7.3.2 Current Limit
      3. 7.3.3 Thermal Protection
      4. 7.3.4 Prebiased Start-Up
    4. 7.4 Device Functional Modes
      1. 7.4.1 Discontinuous Conduction and Continuous Conduction Modes
  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 Design Steps
        2. 8.2.2.2 Enable Divider, RENT, RENB and RENH Selection
        3. 8.2.2.3 Output Voltage Selection
        4. 8.2.2.4 Soft-Start Capacitor Selection
        5. 8.2.2.5 Tracking Supply Divider Option
        6. 8.2.2.6 COUT Selection
        7. 8.2.2.7 CIN Selection
        8. 8.2.2.8 Discontinuous Conduction and Continuous Conduction Modes Selection
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Examples
    3. 10.3 Power Dissipation and Thermal Considerations
    4. 10.4 Power Module SMT Guidelines
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
      2. 11.1.2 Development Support
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Community Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information
    1. 12.1 Tape and Reel Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

10.3 Power Dissipation and Thermal Considerations

When calculating module dissipation use the maximum input voltage and the average output current for the application. Many common operating conditions are provided in the characteristic curves such that less common applications can be derived through interpolation. In all designs, the junction temperature must be kept below the rated maximum of 125°C.

For the design case of VIN = 12 V, VOUT = 3.3 V, IOUT = 8 A, and TA-MAX = 50°C, the module must see a thermal resistance from case to ambient (θCA) of less than:

Equation 15. LMZ13608 30151497.gif

Given the typical thermal resistance from junction to case (θJC) to be 1.0°C/W. Use the 85°C power dissipation curves in the Typical Characteristics section to estimate the PIC-LOSS for the application being designed. In this application it is 3.9 W.

Equation 16. LMZ13608 30151498.gif

To reach θCA = 18.23, the PCB is required to dissipate heat effectively. With no airflow and no external heat-sink, a good estimate of the required board area covered by 2-oz. copper on both the top and bottom metal layers is:

Equation 17. LMZ13608 30151499.gif

As a result, approximately 27.42 square cm of 2-oz. copper on top and bottom layers is the minimum required area for the example PCB design. This is 5.23 × 5.23 cm (2.06 × 2.06 in) square. The PCB copper heat sink must be connected to the exposed pad. For best performance, use approximately 100, 8 mil thermal vias spaced 59 mil (1.5 mm) apart connect the top copper to the bottom copper.

Another way to estimate the temperature rise of a design is using θJA. An estimate of θJA for varying heat sinking copper areas and airflows can be found in the typical applications curves. If our design required the same operating conditions as before but had 225 LFPM of airflow. We locate the required θJA of

Equation 18. LMZ13608 301514a0.gif

On the θJA vs copper heatsinking curve, the copper area required for this application is now only 1 square inches. The airflow reduced the required heat sinking area by a factor of four.

To reduce the heat sinking copper area further, this package is compatible with D3-PAK surface mount heat sinks.

For an example of a high thermal performance PCB layout for SIMPLE SWITCHER power modules, refer to AN-2093 (SNVA460), AN-2084 (SNVA456), AN-2125 (SNVA473), AN-2020 (SNVA419) and AN-2026 (SNVA424).