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Original question:

SN74AHCT244: SN74AHCT244 datasheet question about temperature

SN74AHCT244: SN74AHCT244 Junction Temperature

Kevin Smalley
Prodigy 20 points
Part Number: SN74AHCT244
Other Parts Discussed in Thread: CD74HCT244

Regarding SN74AHCT244PW, I'm looking for a value for Tj-max.

Per the datasheet, the part has a listed max operating temperature of +85C (packaging information table) and internal resistances (Table 7.4), but no way to validate that the junction temperature is not being exceeded for a given max power dissipation per my application.

If I dissipate less than the max allowed power of the component, can I have a higher case temperature?

  • 0
    TI__Guru* 96591 points

    Hi Kevin and welcome to the forums!

    Just as a mention -- it's pretty rare to exceed the max junction temperature on a logic device.

    There's an FAQ located here that explains Tj(max) for standard logic devices. For the quick answer - 150C.

    The max case temperature is not 85C -- 85C is the max ambient operating temperature. So, the answer is yes - you can have a higher case temp than the max ambient air temp.

    Can you tell me what you are doing with the SN74AHCT244 that's causing so much power draw?  I don't think I've ever seen an application where the device had more than a 1 or 2 degree increase in temperature.

  • 0 in reply to Emrys Maier
    Prodigy 20 points

    Emrys,

    Thanks for the response.

    I'm doing the thermal analysis and working on getting an estimated power for this part. In the mean time, I'm gathering all of the necessary information to calculate the junction temperature and see how much margin I have for a temperature rise through the board.

    For my application there are a couple of reasons for my question:
    1) Our initial estimates are assuming a worst-case coldplate (card edge) temperature of +85C for our application. Hence using the Max Ambient temperature +85C will not show that this part will work.
    2) There are many other dissipating components that will drive up the board temperatures in certain areas. Since I'm starting at +85C, my case temp can only be higher.
    3) Finally, I de-rate the component junction temperature and then compare my calculated junction temperature (Tcase + P*Theta-jc) to this de-rated value to show compliance.

    So based on the link provided, which says Tj-max is +150C for logic devices, is it safe to say that my ambient operating temperature can exceed +85C for this component, provided that I do not exceed the listed max voltage and current values, and my Tj is below the max rated +150C for a given power dissipation?

    Kevin

  • 0 in reply to Kevin Smalley
    TI__Guru* 96591 points
    Hey Kevin,
    The Tj(max) value is given as an _additional_ constraint beyond the ambient max temp. Unfortunately, we cannot guarantee operation outside of any of the datasheet specs.

    Have you considered switching to the CD74HCT244? It's rated to 125C and should bypass the issue entirely.