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TPS22990: TPS22990 Vin dip caused by inrush current during startup

Reagan Li
Prodigy 545 points
Part Number: TPS22990

Hi team,

Customer follows the datasheet to put Cin to Cout of 10:1 shown below.

However, there are about 600uF(SP_MLCC) at P1V05 side and about 200uF MLCC at the CPU side of P1V05_VCCIO. 

Customer has concern about the Vin dip caused by inrush current during startup, so he set the Ct to 10nF.

Could you help to confirm if it is fine?

If not , could you give some advice to improve?

Thanks!

Best regards

Reagan

  • 0
    TI__Mastermind 32650 points

    Hi Reagan,

    I see that the bias voltage here is 3.3V and not 5V so the below calculation is not exact but it is a good representation of the situation. With a lower bias voltage, the rise time will be slightly slower and this will decrease the inrush current.

    Assuming that the BIAS is 5V, the inrush current will be I=C*dv/dt

    I = 600uF * 1.05/146.9us  -> (from Table 1 in the datasheet)

    I = 4.288A

    This amount of inrush is acceptable for the device but could pull the supply down. With a bias voltage of 3.3V, I would still expect the inrush current to be over 3A which still can pull the supply down if it is a low power supply.

    The mitigation methodology for this is to test the system and then add more CT capacitance to increase the rise time further thereby dropping the inrush current a lot more. 

  • 0 in reply to Shreyas Dmello
    TI__Prodigy 545 points

    Hi Shreyas,

    Thanks for your reply!

    The Vin power supply is P1V05, and the OCP for this rail is around 30A which comes from SN1701022.
    For this Ct MLCC, is there any limitation value, like a recommended range? I'm not sure if 10000pF is the maximum value or just an example. For this application, 0.5ms can also be acceptable.

    Best regards

    Reagan

  • 0 in reply to Reagan Li
    TI__Mastermind 32650 points

    Hi Reagan,

    We don't have a max CT value for this pin. We do not have data for CT caps larger than 10nF readily available.

    Using large CT caps will result in slower and slower rise times which will clamp the inrush to acceptable levels with the trade of of longer delay and rise times.

    If the supply OCP is 30A, I believe the supply is quite strong. I think there may be some slight dip (which is natural) but a 4A inrush is acceptable for this device as it is rated for 10A continuous. If the load can handle this inrush, then no change may be needed to the system.