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TCA9617B: Tj of TCA9617B

Ryan Wang
Expert 7465 points
Part Number: TCA9617B

There is only Ta 85 degree in datasheet. what's Tj of TCA9617B?

  • 0
    TI__Guru 73591 points
    Hey Ryan,


    The max recommended operating Junction temperature is also 85 C.

    -Bobby
  • 0 in reply to BOBBY
    TI__Expert 7465 points

    Bobby,

    I'm a little puzzled.  My understanding is Tj=Ta+Power consumption*thermal resistance=85+5.5V*0.03A*2*90=115 degree

    How can Tj 85 degree match Ta 85 degree?

  • 0 in reply to Ryan Wang
    TI__Expert 7465 points
    Bobby,

    Does below statement from Logic team also fit your device?

    e2e.ti.com/.../3539.04-power-thermals

    For logic devices, TJ(max) is equal to the maximum storage temperature (Tstg(max)) as listed in the Absolute Maximum Ratings table of the datasheet unless otherwise specified.
  • 0 in reply to Ryan Wang
    TI__Guru 73591 points

    Hey Ryan,

    Typically the calculation you made is correct however this device does not dissipate very much power.

    Let me expand on this a bit. The 30mA of current will flow through our pull down FETs which will dissipate heat. Now calculating the power dissipation follows:

    Rdson(max) = VoL(max)/IoL(max)

    0.23V/0.030A=7.67 ohms

    Power = R*I^2

    Therefore power across one FET is as follows:

    P=7.67 ohms * (0.030A)^2

    6.9mW

    We have two FETs which can pull low therefore: multiply the power by 2 = 13.8mW

    Now calculating Tj using theta JB as it seems you have used this value of 90C/W:

    Tj=85C+0.0138*90

    "How can Tj 85 degree match Ta 85 degree?"

    Tj = 86.242 which is ~85C

    Sorry if my original answer seemed confusing. I answered based on my assumption this device would not dissipate much power at all.

    "Does below statement from Logic team also fit your device?"

    I actually do not see the full link you are referring to. If you are looking for absolute max junction temperature then I would think it should match the storage temperature max but this isn't always the case. If the customer is asking for an absolute max junction temperature then I would suggest 125C is the number you provide.

    Thanks,

    -Bobby