SNVA951 November   2020 LM61460-Q1 , LM63615-Q1 , LM63625-Q1 , LM63635-Q1 , LMR33620-Q1 , LMR33630-Q1

 

  1.   Trademarks
  2. 1 Introduction
  3. 2 The Goal of Thermal Management
  4. 3 Junction Temperature Calculation
    1. 3.1 Regulator Junction Temperature (TJ)
    2. 3.2 Ambient Temperature (TA)
    3. 3.3 Power Dissipation (PD)
    4. 3.4 Thermal Resistance (θJA)
      1. 3.4.1 Thermal Metrics
  5. 4 Package Type
  6. 5 PCB Copper Heat Sink
  7. 6 PCB Layout Tips
  8. 7 Estimating and Measuring θJA
    1. 7.1 Simple Guideline
    2. 7.2 Data Sheet Curves
    3. 7.3 Simplified Heat Flow Spreadsheet
    4. 7.4 Online Database
    5. 7.5 Thermal Simulators
  9. 8 Measuring Thermal Performance
    1. 8.1 Thermal Camera
    2. 8.2 Thermocouple
    3. 8.3 Internal Diode
  10. 9 Thermal Design Example
  11. 10Conclusion
  12. 11References

3.4 Thermal Resistance (θJA)

This metric is the total thermal resistance from the junction of the device to the ambient air. It has dimensions of °C/Watt, and you can think of it as electrical resistance. In this case the power dissipation acts like the current and the temperature drop as the voltage. Multiplying the power dissipation by θJA gives the temperature change from ambient to the junction. That is basically what Equation 1 demonstrates. The lower the thermal resistance, the lower will be your junction temperature for a given power dissipation and ambient temperature. The real goal of thermal management is to get the lowest possible θJA for a given set of application restraints.

The metric, θJA is the most important parameter in Equation 1 and the most difficult to calculate, estimate, or measure.

The remainder of this discussion looks at many of the factors that affect thermal resistance to estimate this important parameter.