SPRADB7 September   2023 AM2431 , AM2432 , AM2434 , AM2631 , AM2631-Q1 , AM2632 , AM2632-Q1 , AM2634 , AM2634-Q1 , AM263P4 , AM263P4-Q1 , AM2732 , AM2732-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
    1. 1.1 How to Use This Application Note
    2. 1.2 Glossary
  5. 2Thermal Resistance Overview
    1. 2.1 Junction Vs. Ambient Temperature
    2. 2.2 Package Defined Thermal Resistance Characteristics
    3. 2.3 Board Defined Thermal Resistances
  6. 3Board Design Choices that Affect Thermal Performance
    1. 3.1 Thermal Vias
    2. 3.2 Board Size
    3. 3.3 Air Flow, Heat Sinking, and Enclosures
    4. 3.4 Copper Thickness
    5. 3.5 Relative Position of Heat Emitters
    6. 3.6 Layer Count
    7. 3.7 Breaks in Thermal Pathing
  7. 4Thermal Design Best Practices Review
  8. 5AM263x EVM Thermal Comparison with Data
    1. 5.1 Test Setup and Materials
    2. 5.2 Measurement Logging Software
    3. 5.3 AM263x EVM Comparison
    4. 5.4 Measurement Results
      1. 5.4.1 Lid Temperature Readings
      2. 5.4.2 Power Readings over Temperature
      3. 5.4.3 Calculated Thermal Resistance Values
      4. 5.4.4 Recorded Junction and Ambient Temperatures
      5. 5.4.5 Calculated Junction Temperature at Ambient Temperature Extremes
  9. 6Using the Thermal Model
  10. 7References

2.1 Junction Vs. Ambient Temperature

Before learning about the intricacies of thermal design, it is important to develop an understanding of the different temperatures of interest on a board. While the temperature on the package lid is what stands out on a thermal camera, the temperature beneath the lid is what determines the functionality of the device. Specifically, the area near hotspots on the die or junction temperature (TJ). Junction temperature is influenced by many things but there is a clear relationship between the junction temperature and the temperature of the free air surrounding a system or ambient temperature (TA). This relationship between junction temperature and ambient temperature can be defined by the thermal characteristic: RΘJA or the thermal resistance between junction and ambient temperature.

Equation 1. T J u n c t i o n - T A m b i e n t O p e r a t i n g   P o w e r   ( W a t t s ) = R Θ J A ( ⁰ C / W )

Thermal properties can be represented similarly to a simple electrical circuit. From this perspective, "Ohm's law" still applies to the thermal "circuit". The operating power of the device in Watts is analogous to the current of the circuit and the thermal resistance between the two temperatures is the resistance.

GUID-D933988E-7E5F-4CBF-BAFB-DC1E737BB39D-low.png Figure 2-1 Electrical vs Thermal Nodal Analysis
Note: RΘJA is not a constant value and depends heavily on the PCB design.

The AM263x Sitara™ Microcontrollers Data Sheet lists values for thermal resistance characteristics but these values do not always tell the whole story. The thermal resistance characteristics in the data sheet are based upon a specific test condition based on a Joint Electron Device Engineering Council (JEDEC) defined 2S2P system.

Note: Due to the thermal resistance values in the data sheet being based on the JEDEC defined, 2-signal and 2-power layer (2S2P) system, it is important to not use these values to calculate junction temperature with a known ambient or lid temperature. The AM263x has a thermal manager module with two thermal sensing ADC's near hotspots of the die to record junction temperature.