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TPS2121EVM-023: TPS2121 – Inrush current question

Part Number: TPS2121EVM-023

We use TPS2121EVM to test the inrush current. We find the peak current will limit at 1.135A, not our OCP setting 4.5A. Below shows the test waveform with test condition. Please have your advice. Thanks

BR, Gary

  • Hi Gary,

    During turn on, the limiting nature of the device is set with the help of the controlled slew rate of the SS pin. Could you please share your schematic? Also when testing we do not recommend using an electronic load in a current mode. Please use the e-load and set the resistance to the required load. 

    Finally, the device could be turning off due to relative thermal shutdown cause by the large output capacitance. Is it possible to retest this with larger SS caps?

  • Hi Dmello,

    We use the TI TPS2121EVM-023 to do test. You can refer to the user's guide.

    All of the test waveform are using the e-load with CR mode to the required load.

    We want to know is why the peak current limit at 1.135A, not our OCP setting 4.5A?

    BR, Gary

  • Please have your recommend. Thanks

    BR, Gary

  • Hi Gary,

    As stated before, during turn on it is the soft start capacitor that helps in limiting current and not the current limit you set. This is because of the linear soft start turn on feature of the device which helps fill large capacitors.

    Looking at your waveforms, your t_rise is around 30ms. Using the inrush current formula you get:

    I= C * dv/dt   

    I = 3300uF * (12/30ms)

    I = 3.3 * (12/30)

    I = 1.32 A

    This is just an approximate but it seems to me that the soft start cap is working as intended. If you want higher inrush currents, then you need to reduce the SS cap.