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DS8921: What is the nominal ICC and power derating?

Ed Taylor
Prodigy 30 points
Part Number: DS8921

The datasheet lists max ICC (35 mA) but not a nominal.

If the devices runs at max ICC, the power dissipation is 175 mW (35 mA * 5 V).  At 85C, the part is rated for 172 mW (730 mW - [(9.3 mW/C)*(85C-25C)]) however the DS8931ATM is rated up to 85C.  If the 9.3 mW/C derating is correct and the device is running at max ICC, how can the part operate at 85C?  Is the derating correct?

Thank you.

  • 0
    TI__Guru 62920 points

    Hi Ed,

    It looks like the derating factors have been swapped between the two package types.  (You can tell since the derating equations should show that the device can not dissipate any power when at an ambient temperature of 150 C since this corresponds to the maximum junction temperature.  This would only work out if you swapped either the derating factors or the maximum power dissipation specifications.  We know that the DIP variant should have better thermal performance overall, though, so the power dissipation ratings must be correct.)

    That being the case, you can compute the maximum power dissipation at TA = 85 C as:

    730 mW - (85 C - 25 C)*(5.8 mW/C) = 382 mW.

    I would also like to point out that computing the device power dissipation simply based on ICC * VCC would over-estimate the dissipation.  This would be the total power delivered by the supply, but a portion of it would be dissipated in the transceiver and a portion of it would be dissipated in the load resistance placed on the output driver.  (For example, if the device were outputting a 2-V differential signal across a 100-Ohm load, the power dissipation in the load would be 40 mW.)

    Regards,
    Max