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[FAQ] How to calculate max junction temperature (Tj max) and max power dissipation for an analog switch

Other Parts Discussed in Thread: TS5A21366, SN74CB3T16210-Q1, CD4066B

Please can anyone point me in the direction of the Tj max for the TS5A21366 Analogue Switch?

This data is not in the datasheet, and is needed for derating the part in my application.  The only information is the max ambient temperature, and the theta JA, so I can loosely approximate what the junction will rise to, but not the maximum allowed temperature of the junction.




  • Tony,

    Please refer to this application note Semiconductor and IC package thermal metrics




    It is best if you use the RθJA value as a way to compare parts versus using the value as an absolute since the customer’s system PCB design will highly influence this number as stated above. 


    But if you must have a number using this calculation Tjmax= Ambient Tempmax + Pmax(RθJA) here it is below:  


    Signal Switch power dissipation and junction temperature calculator.xlsx 

    The max power for the TS5A21366 can be calculated by


    Pmax = (V+max)(I+max) + 2*[(Ino,Inc,Icommax)^2(Ron)]


    About 112mW = (5.5 V)(10uA) + 2[(200mA)^2(1.4 Ohm)


    Tjmax= Ambient Tempmax + Pmax(RθJA)


    Tjmax = (85°C) + (112mW)(253°C/W)


    Tjmax = 113.3°C


    113.3°C is an approximation of the maximum junction temperature that the device can obtain while operating at its maximum recommended values.  The datasheet abs max table states that max storage temperature for the device is 150°C which is well above 113.3°C.  This will mean that there is little risk of thermally causing damage while running this part to its maximum recommended values. 

    Thank you,


  • This response seems to be imply that -

    (1) the Max Storage Temp and the Max junction Temp are equivalent, in that
    (2) no damage will occur while powered- up to the max storage temperature and that
    (3) the part will also operate- up to the max storage temperature,
    (4) and not experience any reliability issues (such as reduced life).

    Are all of these statements true for the SN74CB3Q32245ZKER & SN74CB3Q3245RGYR? 

  • Matt,

    (1) The Max storage temperature is the maximum temperature the devices may be stored without reliability issues.

    The max junction temperature calculated above is how hot the IC can get if the device is operated a) within the recommended operating conditions and b) in such a way to try and create the most heat inside the device. Since signal switches are low power devices they will not get very hot even if you are operating the device to its maximum limits stated in the datasheet.

    Notice in the equation Tjmax= Ambient Tempmax + Pmax(RθJA) the dominant factor in the Tjmax equation is ambient temperature (85 deg) and the device operation only adds (~30 deg C)

    (2) You cannot reliably power up and operate a signal switch in an ambient environment greater than the limit stated in the datasheet. This limit is typically 85 deg C for most commercially rated devices.

    (3) We cannot guarantee the datasheet performance at ambient temperature greater than the limit stated in the datasheet.

    (4)The abs max ratings and max storage temperature deal with damaging/ reliability of the device while the recommended operating conditions are for operation.

    Some datasheets will state a maximum junction temperature rating based on silicon process limitations but it would almost be impossible for these low power signal switches to reach these temperature when operated within the recommended operating conditions.

    I realized in typing this information is very nuanced between the different terminologies so please don't hesitate to follow up if this didn't add some clarity.

    Thank you,
  • Hi Adam,
    The customer would like to know if there's a way to re-qualify the device for operation in the 105-110degC ambient range.
    Is that possible?
    Alternatively, do we have other parts that can support this temp range?
  • Jason,

    If the datasheet doesn't rate the performance of the device at 105 deg C ambient temperature range TI cannot guarantee the device performance stated in the datasheet

    TI can undertake the effort to characterize the device at higher temperature range but we need to make sure there is the correct business opportunity to offset the cost of properly ensuring the device operation at higher temperature.

    There is a way to search for switches and multiplexers on based on ambient temperature rating.  There are 61 switches that can support > 85 deg C ambient temperature.   What do you think about the CD4066B or SN74CB3T16210-Q1?

    Thank you,