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TPS799: Big inrush current

Part Number: TPS799

Hi,

My customer is using TPS79901.
He has issue big inrush current at start-up.
Peak current value is about 2.57A.
He has questions below.

Q1: What is "Inrush Current Protection with EN Toggle" in datasheet ?
      Could you let me know more detail of this function, please?

Q2: How long time does TPS799 need for detection of output current limit?

Q3: At start-up, Inrush Current situation is for about 16us(over 400mA, max value is 2.57A)
       Does this device has damage?

Best reagrds,
Shimizu

  • Hi Shimizu,

    When TPS799 is in an application that allows for the LDO to be enabled separately from Vin (EN is toggled after Vin is established rather than tied to Vin), the current limit is able to engage quicker. This allows for the current limit to limit the inrush current to the output current limit which is 400 mA typ.

    2.57 A is a rather high peak inrush current. How much output capacitance do you have in your application? You could reduce the inrush current by reducing the output capacitance. This could be accomplished by simply moving some of the output capacitance to the input side of the LDO. This allows for the same system level filtering while reducing the amount of inrush current to charge the output capacitors at startup.

    Do you have a scope shot that you could share with Vin, Vout, Ven, and Iout?

    Very Respectfully,
    Ryan
  • Hi Ryan,

    Thank you for your support.
    Output cap is about 32uF.(10uF + 22uF + 0.1uF*5).
    Waveform is below.
    CH1:Vin(5V)
    CH2:EN
    CH3:Vout(3.3V)
    CH4:Iout

    I re-question : How long time does TPS799 need for detection of output current limit?


    Best regards,
    Shimizu

  • Hi Shimizu,

    Thank you for the additional information and scope shot. Actually your scope shot does a good job demonstrating the time it takes for the current limit to engage. As you can see in your scope shot there is ~50 us from the time Vout begins to rise to the peak of the inrush current. The inrush current begins to decrease due to current limit engaging.

    Very Respectfully,
    Ryan
  • Hi Ryan,

    Thank you for your reply.

    I add questions.

    Could you answer me below, please?

    Q1: How much current value should we reduce inrush current by?

    Q2: What kind of calculation did you show Q1's answer by?

    Best regards,

    Shimizu

  • Hi Shimizu,

    Inrush current is normal and will always occur to some effect on TPS799 due to the quickstart circuit. How much you can reduce it will be based on application specific requirements such as how much input capacitance is required by your load.  TPS799 only requires a 2.2 uF capacitor.  Reducing the output current to the minimum output capacitance required for your application will have the maximum impact on reducing the inrush current.

    Inrush current can be calculated by the following equation (taken from the Application Report at the link below)

    One consideration that does not come up in this calculation is how frequently inrush occurs.  How often will the LDO startup (experience inrush current) in your application?  If the LDO is constantly being subjected to inrush, inrush current should be reduced further.

    Please note that TPS799 was designed for applications that require a quick startup time.  As a result the testing and characterization of this device was done with 10 uF or smaller on the output.   

    Very Respectfully,

    Ryan