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TPS7A85: BIAS pin cap questions

Brian Antheunisse
Expert 5305 points
Part Number: TPS7A85


Hi guys,

My customer is using a ton of the TPS7A85, over twenty on a board, all having a bias supply fed to their bias pins, all pretty tightly packed together. 

They realized that they were using 1uF (with effective .97uF) on accident but have found no issues. Do you guys think this is ok, since the bias pins are all sharing capacitance? What does the failure mode look like if they were to just keep them 1uF?

Thanks,

Brian 

  • 0
    TI__Guru 55940 points
    Hi Brian,

    It is certainly possible depending on your layout that the shared capacitance is enough to satisfy the requirements and depending on your Vin, some of the LDOs may not be using the BIAS rail (If Vin is higher than 2.2 V an external BIAS is not needed). The BIAS rail is used to power a few internal blocks, the most critical of which is the high rail of the internal error amplifier; however, the actual current draw is so low that unless you have a large transient, you are unlikely to see a degradation in performance due to the other capacitors being close by. The most likely failure mode that you would see is if Vbias were to suddenly collapse to trigger UVLO.

    Very Respectfully,
    Ryan