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TXS0108E: Thermal via

Michiaki Tanii
Expert 4340 points
Part Number: TXS0108E

Hello team,

 

Why does TXS0108 recommend to place via ?

Is it for improvement thermal performance ?

 

They will design via with phy0.4.

Is if ok for the phy size ?

 

Thank you and best regards,

Michiaki

  • 0
    Guru 242810 points

    Where does TI recommend vias? The RGY package drawing says that "vias are optional".

  • 0 in reply to Clemens Ladisch
    TI__Expert 4340 points

    Hello team,

     

    Could you share the data of Junction-to-ambient thermal resistance for the RGY pkg with no vias ?

     

    Thank you and best regards,

    Michiaki

  • 0 in reply to Michiaki Tanii
    TI__Guru* 96591 points

    Hello Tanii-san,

    The junction-to-ambient thermal resistance, RθJA, is determined assuming only basic solder connections to a standard board and no additional heatsink.

    If you are attempting to calculate thermal dissipation through the bottom of the device with added vias, use RθJC(bot) in combination with your heatsink's thermal properties.

  • 0 in reply to Emrys Maier
    TI__Expert 4340 points

    Hello Emrys,

     

    Customer would like to use vias with phy 0.4.

    Is it ok for the above ?

     

    Thank you and best regards,

    Michiaki

  • 0 in reply to Michiaki Tanii
    TI__Guru* 96591 points

    Hello Tanii-san,

    There is no right or wrong way to add vias to the thermal pad - so long as the pad is either floating or connected to ground. It's up to the customer to decide on their needs for thermal performance.

  • 0 in reply to Emrys Maier
    TI__Expert 4340 points

    Hello Emrys,

     

    Please share the RthetaJA and RthetaJC of RGY pkg without thermal pad ?

    I understand what you mentioned but I cannot estimate the thermal performance in the worst case.

     

    Thank you and best regards,

    Michiaki

  • 0 in reply to Michiaki Tanii
    TI__Guru* 96591 points

    Hello Tanii-san,

    Can you tell me why this is so critical? It is highly unusual for a voltage translator to require such lengths to calculate thermal dissipation. The device typically dissipates very little power.

  • 0 in reply to Emrys Maier
    TI__Expert 4340 points

    Hello Emrys,

     

    I know what you want to say but how can customer judge without information thermal resistance ?

    There would be no concern even if customer doesn’t use thermal pad but they need evidence.

     

    Your understanding would be highly appreciated.

    Michiaki

  • 0 in reply to Michiaki Tanii
    TI__Guru* 96591 points

    Hi Tanii-san,

    How can I help if the application information is not shared with me?

    I have requested a thermal model without the thermal pad, but I'm not sure what you expect to learn from this. Why would the customer design with no connection for the thermal pad?

    If there is enough information to produce a model from the request, it will be return in approximately two weeks.

  • 0 in reply to Emrys Maier
    TI__Expert 4340 points

    Hello Emrys,

     

    Customer just would like to know if they can use phy 0.4 or not.

    And I asked you the thermal resistance without thermal pad as worst case since you mentioned it depends on customer’s design requirements.

     

    Thank you and best regards,

    Michiaki

  • 0 in reply to Michiaki Tanii
    Guru 242810 points

    There is not enough power dissipation to matter. If the TXS needed thermal performance, it would not be made in other package with much worse thermal impedance.

    Vias are not needed. If you add them, their diameter does not matter. 0.4 is perfectly fine.

  • 0 in reply to Michiaki Tanii
    TI__Guru* 96591 points

    Hi Tanii-san,

    I very clearly stated that they can place whatever vias they want:

    Emrys Maier said:

    There is no right or wrong way to add vias to the thermal pad

    Vias of any size can be added - my apologies if my wording was confusing.

    Asking this question is similar to asking "can I put snow tires on my car" -- the answer is obviously yes, you can do whatever you want... but I assume you are not asking if you _can_ do it, but if _I would recommend_ that you do it... and in order to know that, I would need to know the weather in your area... do you get a lot of snow? Are you living in Hokkaido or Okinawa? Are the roads well cared for?

    Similarly, I cannot help you to select a thermal mass requirement for your heat sink if you don't give me any details regarding what power you are trying to dissipate or the environmental factors for the system.

    Michiaki Tanii said:

    ...you mentioned it depends on customer’s design requirements.

    I was referring to their power dissipation design requirements.

    Power dissipation design requires details that you have not provided. How much power are they going to be putting through the TXS0108E? I can calculate from supply voltage, operating frequency, and load conditions if they are given.

    Once we know the power that must be dissipated, then we can start to determine cooling requirements. What is the ambient temperature? What are the board characteristics (materials, stack-up)? If vias are planned to be used, will they be connected to a plane on the opposite side of the board? What size of plane? What copper thickness?

  • 0 in reply to Michiaki Tanii
    TI__Prodigy 525 points

    Hello Emrys,

    Can I contact this information for design?

    I think this information can be used because it is the same pkg.

    Best regards, BANE

  • 0 in reply to Haruo Akaba
    TI__Guru* 96591 points

    Hello,

    The thermal modelling process takes into account many factors - not just the package.

    For example, the mold compound used may be different, resulting in a different thermal property.

    If you can share details regarding the application, I may be able to provide additional advice.

  • 0 in reply to Emrys Maier
    TI__Expert 4340 points

    Hello Emrys,

     

    Can customer estimate the power dissipation by themselves or not ?

    Looking at the datasheet, ICCA and ICCB is definitely low.

    I think it means that power consumption of TXS0108E would be quite low.

     

    And if we can provide the number of thermal resistance in case no vias of RGY pkg is lower than NME pkg, customer can judge easily they can use 0.4 phy diameter vias for RGY pkg .

     

    Your understanding would be highly appreciated.

    Michiaki

  • 0 in reply to Michiaki Tanii
    TI__Guru* 96591 points

    Hi Tanii-san,

    Perhaps some information is not being conveyed well. I will try to be very clear in this response.

    The datasheet includes complete thermal properties for the device assuming normal connections (no additional vias, heat sinks, or other means of thermal dissipation):

    R_θJA (junction to ambient thermal resistance) of 131.4 (NME) is larger than 34.7 (RGY). This indicates that for 1 W of power consumption, the RGY package will increase in temperature by 34.7C, and the NME package will increase by 131.4C.

    131.4C is larger than 34.7C. Therefore the RGY package is better at dissipating thermal energy than the NME package.

    Similarly, ψ_JB (junction to board characterization parameter) indicates that the RGY package has a better junction to board dissipation characteristic than the NME package. You can see this in all of the thermal values listed.

    Adding vias to the thermal pad will increase the thermal mass and improve the RGY package thermal dissipation by a small amount. Adding vias and a large ground plane on the opposite side of the board will significantly improve the thermal dissipation. As I have previously stated, this device does not use very much power, and this is most likely not required. I would need details of the application to help make the determination.

    The TXS0108E uses very little power in general, however this is dependent on the device's operating frequency and loading. The static power consumption is only dependent on VCCA, VCCB, ICCA, and ICCB, however dynamic power consumption depends on operating frequency and capacitive load.

    For more details, please see this application report on calculating CMOS power consumption: 

    CMOS Power Consumption and Cpd Calculation

    This device also includes pull-up resistors, so be sure to include the power dissipation of those resistors in your calculations - P = V^2/R

  • 0 in reply to Emrys Maier
    TI__Guru* 96591 points

    Hello,

    We got back the results for the thermals if the thermal pad is not soldered to the PCB:

    Result- Theta JA-High K (standard datasheet value):

    84.8*

    Result-Theta JC, top (standard datasheet value):

    40.5*

    Result-Theta JB (standard datasheet value):

    51.0*

    Result- Psi JT (standard datasheet value):

    2.3*

    Result- Psi JB (standard datasheet value):

    50.3*

    Result-Theta JC, bottom (standard datasheet value):

    7.3*