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TPS24750: Inrush current tripping the fast trip threshold

Owen Hooker
Prodigy 40 points
Part Number: TPS24750
Other Parts Discussed in Thread: TPS25982

Hi,

I have design using the TPS24750 as an efuse on a power bus.  The design has peripheral devices that can be plugged in after the TPS24750 has already started. These peripheral devices are not our products and we have no control over the inrush current they may have. The problem we have is that sometimes the efuse will fast trip thinking there is a short when it sees a momentary inrush from a device being connected.  The inrush current is only a couple microseconds but it can still manage to trip the efuse.  We have tried adding more capacitance to the output and this does help but not as much as we would like.  Is there any way to slow down the response of the fast trip? should I just keep adding capacitors to the output until I no longer have a problem?  Any help is greatly appreciated.

Regards,

Owen

  • 0
    TI__Guru** 105361 points
    Hi Owen,

    Ideally, peripheral devices should have inrush current protection.
    For your case, please add RC filter across sense resistor to slow down the fast-trip response. For more details, please refer Section-3 in www.ti.com/.../slva703.pdf

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 40 points

    Hi Rakesh,

    Thank you for the valuable information.  I do plan to implement the RC filter across the sense resistor as you have recommended but I do have a couple of questions.  Firstly I wonder if it would still be beneficial for me to try to add more capacitance to c-out to help mitigate the inrush current. Currently, the design has 100uf on c-out and we can try to match it to the value on the datasheet but it would require a significant board redesign to make room for additional electrolytic capacitors. also, would there be any benefit to using a ceramic capacitor with a lower ESR in addition to the electrolytic at c-out?

    In regards to implementing the RC filter across the sense resistor do you have a recommendation on a starting point for the values of Cfilter and Rfilter?  I have included a pdf of the current schematic without the RC filter for reference. Also, I have included the specification below.

    Vin: 14.8v 

    Sense resistor: 0.002ohm

    c-out: 100uf

    Itrip: 10amp

    Regards,

    Owen

    sandwich_plate.pdf

  • 0 in reply to Owen Hooker
    TI__Guru** 105361 points
    Hi Owen,

    For your question on Cout: Increasing low ESR cap on the output would help. How much cap value is determined by the peak inrush current of the downstream load. Also, the path parasitics (from Vout to external board) will influence the performance.

    Since, you mentioned that the inrush interval is only few microseconds. I suggest to select RC filter with 3~5 us time constant and check.

    Alternatively, please look at TPS25982 our new device to manage system transients. Please refer Figure 19 and 20.

    Best Regards,
    Rakesh