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TPS7A7001: Inrush Current Effect on TPS7A7001

Rifa S
Prodigy 20 points
Part Number: TPS7A7001

Hello

We are using TPS7A7001 in an application where Inrush Current of 4.2 A for 50 us is observed. Will this have an effect in the LDO over a long period of time.

I would also like to know the peak current the LDO can withstand in this time duration (50 us).

Thank you

Rifa

  • +1
    TI__Mastermind 37005 points

    Hi Rifa,

    4.2 A is nowhere near enough to fuse the internal bond wires, which is over 6A, so that's not an issue.  However, the current limit minimum is 2.1A.  While we don't have the current limit bandwidth characterized for this LDO, in general current limit protection circuitry takes 20-50us to engage in modern LDO's.  There is a nonzero risk that your design will experience current limiting during turn on which will delay the startup time and possibly cause a nonmonotonic turn on, if 4.2A is pulled from Cout while the LDO can only deliver 2.1A. 

    The LDO will potentially get slightly hotter during turn on but thermal time constants are measured in milliseconds.  Your system is only experiencing this level of current during turn on, which is infrequent, and only for 50us.  The impact to the LDO reliability will be negligible. 

    Thanks,

    Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 20 points

    Hello Stephen

    Thanks for the answer.

    Can you kindly state the minimum, maximum and typical value of current above which current limiting starts and it will shift to constant current loop? As well the minimum duration to trigger this constant current loop. 

    Thank You

    Rifa S

  • +1 in reply to Rifa S
    TI__Mastermind 37005 points

    Hi Rifa,

    We don't have this characterized for this older device other than what is already listed in the datasheet.  I would use the rules of thumb of 20-50us for the time it takes to enter current limit.  The minimum value for current limit in the datasheet is 2.1A.

    Thanks,

    Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 20 points

    Hello Stephen

    This is the inrush current we are observing. As given the inrush current of 4.42 A after a duration of 50 us is getting current limited for 130 us. This happens during power on when the output capacitor is completely discharged.

    Can you tell about the effect of this in the LDO.

    Thank You

    Rifa S

  • +1 in reply to Rifa S
    TI__Mastermind 37005 points

    Hi Rifa,

    From this oscilloscope plot, I think the LDO itself will be fine.  The inrush current does not exceed the bond wire fusing current with lots of margin to spare, the duration of the inrush is small (well under 1ms) so the junction temperature increase will be limited. The only item to investigate is your output voltage which is a system concern (not something that will hurt the LDO): I would make sure that Vout turns on monotonically, especially around the time that current limit is engaged.

    Thanks,

    Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 20 points

    Hi Stephen,

    Can you also clarify the Current Limit for the following test point; Vin = 5.5V; Vout = 4V as the current limit data mentioned in the datasheet is only when Vin = 1.425V and Vout = 0.9*Nominal Vout.

    Thank You

    Rifa S

  • +1 in reply to Rifa S
    TI__Mastermind 37005 points

    Hi Rifa,

    The current limit loop is its own feedback loop monitoring the load, and I would not expect significant changes in performance based on Vin or Vout. I looked in the original design files from around 15 years ago, and test data collected at that time, and I don't see a significant difference based on Vin or Vout.  You should use the datasheet guidance here even with a different Vin and Vout.

    Thanks,

    Stephen