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TPS2121EVM-023: Inrush Current Shut Down

Part Number: TPS2121EVM-023
Other Parts Discussed in Thread: TPS2121


Currently I am performing tests on the TPS2121 EVM and have encountered an error with inrush current. When I have a load that is drawing around 2A of current the device cannot switch between sources. At input 1 I have a 5V source supplying 2.25A to a load. Then I connect another source to input 2 at 20V, the TPS2121 tries to swap to over to the second input but in doing so the inrush current on the output spikes to 5.9A. This seems to create an error in the device as it shuts down and tries again after a few ms, but the current keeps rising to 5.9A every time the device tries to power from the second output so it is never able to produce an output. The current limiting resistor was set to 18.7k to allow as much current as possible, but the typical current value for this is 5.2A.

  1. I think the device shuts down because the inrush current exceeds the 5.2A but shouldn't the 5.2A limit be for continuous current rather than inrush current?
  2. If the device is this sensitive to current, is there a proposed workaround for this?

In my setup I have tried this with and without the output capacitors added and they seem to have little to no effect on the inrush current. This error doesn't happen when the 20V input is connected first to input 2 then the 5V input is connected to input one. With the configuration the load will get power from input 2 but when input 2 is removed the device will swap to the 5V on input 1 with no issue. 

Please reach out if any additional information is needed.


  • Hi Jesus, 

    Welcome to E2E and thanks for joining! 

    The TPS2121 protects against overcurrent through its short circuit protection feature described below. 

    During a transient event which draws greater than 2.4*ILM from the device, short-circuit protection will trigger within 1us of the overcurrent event and the device will shut off for about 25ms, after which the device will turn back on. 

    If the short is still present, then the current-limit loop will regulate the output current to ILM. 

    Since you current limiting resistor is set to 18.7kohm, disregarding the variance of the resistor (which should be considered), you can expect a max ILM of 5.8A. If the actual resistance is a bit higher, it is not impossible to see 5.9A for ILM. 

    Could you send a waveform with a small time scale (around 500us/division) to confirm whether you are seeing a larger inrush (around 2.4 * 5.9A) within 1us and a restart current of 5.9A every 25ms? 

  • Hi Jesus, 

    Were you able to see a larger inrush current within 1us and a restart current of 5.9A every 25ms? 

    Please let me know if you have any further questions

    Best Regards, 


  • Hi Jesus,

    Since this thread has gone on two weeks without a reply, I am closing/locking it. If there is another update here, please make a new thread.

    Best Regards,