How to Calculate the Operating Junction Temperature Tj – KBA220732

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    Question: How to calculate the operating junction temperature Tj?


    Answer: The operating junction temperature Tj is the temperature of the integrated circuit under given operating conditions. To calculate the operating junction temperature Tj for respective device, the user needs to know the correct package type (such as LAA064, FAB024, SOC008, etc.) to determine the correct theta JA value, as well as the typical power dissipation in mW. Then the equation below can be used to calculate Tj, as follows:


              Tj = TA + (ƟJA)×(Pd)




              ƟJA = Junction-to-ambient thermal resistance (Theta JA). ƟJA value is in the Cypress Customer Reliability Qualification Summary available in the Cypress website.


              TA = Ambient operating temperature (temperature grade of the actual device)


              Pd = Voltage (V) × Current (I). Typical Voltage and Current values are found in the datasheet


    Sample calculation:


              The thermal resistance value for the S25FS512SAGBHI210 flash device


              24-ball-BGA (FAB-024) package is:

                        ƟJA = 39°C/W


              The operating conditions are defined by:

                        Voltage = 2.0 V

                        Current = 70 mA


              Therefore, the power dissipation is:

                        Pd = (2.0 * 70 mA) = 140 mW


              From the given formula, the Operating Junction Temperature is:

                        Tj = 85°C + (39 °C/W) (140 mW) = 90.46°C