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DAC80508: thermal concern about DSBGA package(2.4mm x 2.4mm)

K.Yaita
Guru 29720 points
Part Number: DAC80508


Hi Team,

My customer considers to use DAC80508 DSBGA package(2.4mm x 2.4mm).
He concerns if the device is usable without damaging under following conditions.
-------------------------------------
Ta: 95°C
output current: 20mA(DC) x 8ch 
output voltage: 2.5V x 8ch
VDD: 5.5V
-------------------------------------

May I have your advice?

Best Regards,
Yaita

  • 0
    TI__Genius 14220 points

    Hi Yaita,

    Thank you for your query. The total power dissipation under the given conditions would be (5.5 - 2.5)*8*20mA = 0.48W. Considering a junction temperature of 150oC, the maximum allowed Rth-ja will be (Tj - Ta)/Pd = 114.58 oC/W. Considering a Rth-ja of the DSBGA package of 68 oC/W, the junction temperature will be Tj = (Pd*Rth-ja) + Ta = 127.64 oC.

    Although this is within the max Tj limit of 150 oC, operating the device continuously at this junction temperature will impact the life of the device. So, there are three ways to limit the junction temperature:

    1. Limit the power dissipation by reducing the voltage headroom

    2. Provide hatsinking to reduce Rth-ja

    3. Use the WQFN package

    Regarding (1), assuming the max output voltage is 2.5V, the DAC power supply can be kept at 2.7V. This will reduce Pd to 0.032W. And new Tj will be 97.176 oC. This might be a good Tj for long-term operation. In case the headroom cannot be maintained low by default, an adaptive power supply scheme can be employed so that the output range is not affected. 

    Regarding (2), a heatsink can be chosen to bring down the Rth-ja. This will now be Rth-jc (junction to top case) + Rth-ca (heatsink). However, for such a small package using a heat sink might be tricky.

    Regarding (3), even a barefoot calculation of the Tj with Rth-ja of 33 gives Tj = 110.84 oC at a Pd of 0.48W. Please note that there will be significant thermal dissipation from the bottom part of the DAC. Considering a Rth-jb (junction to board) of 7.3 oC/W and a PCB design to take away the heat, the Tj can be significantly reduced.

    Hope that answers your question.

    Regards,

    Uttam Sahu

    Applications Engineer, Precision DACs

  • 0 in reply to Uttam Sahu
    Guru 29720 points

    Hi Uttam-san,

    Thank you for your kind support!

    >Although this is within the max Tj limit of 150 oC, operating the device continuously at this junction temperature will impact the life of the device.
    Is there any criteria of Tj_max that doesn't damage the device when operating continuously?

    My customer wants to use DSBGA package because his application requires small package.
    In addition, I heard the application requires VDD=5.5V because output swings 0~5V range in this case.

    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Genius 14220 points
    Hi Yaita-san,

    I didn't say that operating the device at 150 oC junction temperature will damage the device. All semiconductor devices have a life and one of the factors that impact the life majorly, is temperature stress. Hence, it is recommended to operate the device lower than the rated temperature spec. Intermittent usage at high temperature should be fine but continous usage will have negative impact.

    Given that the customer is going to use the device at 0-5V output range, could you please let me know the current drawn at different codes? Is the load resistive or a constant current source? If it is resistive, the max power dissipation will happen at mid-code. However, if it is a constant current, the max dissipation will happen at zero code.

    Regards,
    Uttam
  • 0 in reply to Uttam Sahu
    Guru 29720 points

    Hi Uttam-san,

    Thank you for your kind support.

    Given that the customer is going to use the device at 0-5V output range, could you please let me know the current drawn at different codes? Is the load resistive or a constant current source? If it is resistive, the max power dissipation will happen at mid-code. However, if it is a constant current, the max dissipation will happen at zero code.
    -> I heard it is load resistive 350ohm(typ) and my customer think 20mA x 8ch is excess value..
    May I have your comments if operating DAC80508(DSBGA package) continuously under the folowing condition will impact the life of the device?
    -------------------------------------
    Ta: 95°C
    output voltage: 2.5V x 8ch
    load resistive per channel: 350ohm
    VDD: 5.5V
    -------------------------------------

    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Genius 14220 points
    Hi Yaita-san,

    Apologies for responding late. With the new information, we can do the following calaculations:

    1. The total power dissipation for a resistive load will be at mid-code i.e. (5.5V*6mA) + 3*8*(2.5V/350) = 0.033W + 0.171W = 0.2W
    2. With Rth-ja of 68 oC/W and TA of 95 oC, the Tj will be (0.2W*68) + 95 = 108.6 oC

    This junction temperature should be fine to operate at.

    If you are interested in calaculating the actual MTTF and FIT values with a given junction temperature, you can use the derating tool available on the following page:

    www.ti.com/.../estimator.tsp

    Hope that answers your question.

    Regards,
    Uttam
  • 0 in reply to Uttam Sahu
    Guru 29720 points

    Uttam-san,

    I understand.
    I appreciate for your great support!

    Best Regards,
    Yaita