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TPS25921A: Css design to prevent inrush current larger than 1.6A

Hung Ching Hsu
Expert 3086 points
Part Number: TPS25921A

Hi Team, 

My customer is using TPS25921A for 5V USB protection. 

Now USB device is connecting to the port and if they turn on power, 2~3A inrush current flows through for 100~200us. 

In this case, FLT pin will activate, is it correct? (Since current limitation is for max 1.6A)

If so, customer is thinking use SS pin to prevent inrush current being larger than 1.6A, but customer can't understand how to choose Css. 

Could you help advise how can customer choose Css to suppress inrush current to under 1.6A?

Thanks.

Regards,

Jo

  • 0
    TI__Guru* 79181 points

    Hi Jo,

    FLT will go low if device hits thermal shutdown. If device is able to startup in 1.6A current limit then FLT will not go low.

    But it is always good to use SS pin to control inrush current.

    For calculating SS cap customer can refer design calculator on product folder and datasheet application section(10.2.1.2.4 Setting Output Voltage Ramp time (tSS)).

    Customer need to know load cap. How much is load cap ? 

    Regards

    Kunal Goel

  • 0 in reply to Kunal Goel
    TI__Expert 3086 points

    Hi Kunal, 

    Thank you. May I confirm with you for following? 

    1. Both "9.3.3 Hot Plug-in and In-Rush Current Control" and "10.2.1.2.4 Setting Output Voltage Ramp time (tSS) " in datasheet explains the inrush current calculation, 

    Customer wants to know what is the difference between these two, when should customer use which one, could you help? 

    Datasheet says following, but I couldn't get it. 

    "For systems where load is present during start-up, the current never exceeds the overcurrent limit set by R(ILIM) resistor for the application. For defining appropriate charging time/rate under different load conditions, refer to the Setting Output Voltage Ramp time (tSS) section."

    2. Customer is consider use TPS25921A for USB device protection, datasheet said this device can be used for "USB Switch", can this device also used for protection for USB devices(Such as USB memory, mouse, adaptor...)? 

    3. Is there any suggestion value for Css connected to TPS25921A for USB device application? Customer wants to know it as reference.  

    Thanks.

    Regards,

    Jo

  • 0 in reply to Hung Ching Hsu
    TI__Guru* 79181 points

    Hi Jo,

    You can follow 9.3.3 for understanding. Main thing is to make sure that Css you choose make inrush current low enough so you have successful startup.

    I assume that in USB application , COUT is around 10uF. I think customer can keep Css minimum 1nF.

    Yes this device can be used for USB device protection like memory, mouse, adaptor.

    Regards

    Kunal Goel

  • 0 in reply to Kunal Goel
    TI__Expert 3086 points

    Hi Kunal, 

    Thank you for suggesting Css for USB application and I understood this device can also use for USB device protection. 

    For inrush current calculation, I read datasheet but not quite understood the difference between 9.3.3 and 10.2.1.2.4, why both is existing and in what situation should customer use which one.

    It seems 10.2.1.2.4 is focus on limiting inrush current so junction temp will not exceed abs. max temp rating when Cout exists. And 9.3.3 is talking about the situation that Cout is not existing. Is my understanding correct? 

    Thanks.

    Regards,

    Jo

  • +1 in reply to Hung Ching Hsu
    TI__Guru* 79181 points

    Hi Jo,

    It seems 10.2.1.2.4 is focus on limiting inrush current so junction temp will not exceed abs. max temp rating when Cout exists. And 9.3.3 is talking about the situation that Cout is not existing. Is my understanding correct? 

    9.3.3 is just telling what formula to use to calculate slew rate for a particular Css cap. 10.2.1.2.4 is next step , considering real application. Here based on COUT in application we calculate inrush current and see power dissipation and check if we will not have thermal shutdown. 

    Regards

    Kunal Goel