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TPS2121: Power Selection

Alan Hoehn
Intellectual 470 points
Part Number: TPS2121

I am designing a product that will have two sources of power.  They are POE regulated down to 5V and USB C.  I want to be able to seamlessly switch between the two sources with a very low voltage drop.  Also, have one source be selected if they both are connected..  Is the TPS2121 a good candidate to accomplish this?  The current draw would be 2A.

  • 0
    TI__Genius 13760 points

    Hi Alan,

    Welcome to E2E!

    Judging by the design requirements the TPS2121 should be the right fit for your system.

    In order to achieve the desired functionality, the TPS2121 will have to be in the XCOMP/XREF mode of operation. This mode allows for a fast seamless switchover between inputs, typically 5us, in order to provide uninterrupted power at the output. More details on the XREF and XCOMP modes of operation can be found in pages 20 and 25 respectively in the datasheet.

    I would suggest the XCOMP configuration to give one of your power supplies priority when present. In order to ensure functionality in the XCOMP mode, a voltage greater than Vref = 1.06V has to be applied to the PR1 and PC2 pins of the TPS2121. This voltage will be applied through a voltage divider from IN1 and IN2 respectively. With the proper voltage divider configurations priority can be given to any of the input sources. When the voltage at PR1 is greater than CP2 the TPS2121 will output IN1. In the other hand, when PR1 is less than CP2 the TPS2121 will output IN2.

    The 2A of current that will be drawn from the device will not be a problem for the TPS2121 can handle up to 4.5A of continuous current.

    Let me know if you have any questions!

    Best regards,

    Andy Robles

  • 0 in reply to Andy Robles
    Prodigy 60 points

    HI，Im use of TPS2121RUXR in 12V（In1）&8.4V （In2），the datasheet said:seamless switchover between inputs, typically 5us,but I test when 8.4V to 12V its use 25ms,result the system power out restart. the load is 0.2A.

  • 0 in reply to bo yang
    TI__Genius 13760 points

    Hi Alan,

    In order to help you with your problem I have a couple questions about your set up:

    1. Are you using the TPS2121EVM in your test?

    2. Are you using long cables to input the power sources to the TPS2121?

    3. What is your input capacitance on each input?

    4. What is your output capacitance?

    5. Are you using the XCOMP/XREF to achieve the fast seamless switchover?

    6. Would you be able to share a schematic of your set-up? If using the EVM would be able to show the diagram with which jumpers are in place?

    7. Would you be able to share scope shots of this event with VIN1, VIN2, VOUT, and ST pin signals?

    Best regards,

    Andy Robles

  • 0 in reply to Andy Robles
    Prodigy 60 points

    Im test on my board  ：picture 1 has no loads；2 has 0.3A。3 is my SCH

  • 0 in reply to bo yang
    TI__Genius 13760 points

    Hi,

    The device seems like its functioning as expected without the load. This means that the load current is affecting the device and causing it to shutdown.

    The device is more than likely shutting down due to its relative thermal shutdown protective feature. Relative thermal shutdown shuts down the device when the temperature of the FET of the active power supply rises 60 degrees Celsius above the rest of the die. In order to mitigate this rise in temperature we need to address the inrush current.

    Could you test the set up and decrease the output capacitance and share your results?

    Would you be able to measure the current on VIN1=12V to see if we can capture the inrush current going into the system?

    Best regards,

    Andy Robles