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TPS2121: Fast Switchover when one supply is removed

Stephen Esquibel
Prodigy 30 points
Part Number: TPS2121
Other Parts Discussed in Thread: TPS2120

I have a TPS2121 configured like Figure 31 of the datasheet. The resistors are adjusted for 5V supplies. Fast switchover works as long as the supplies stay above 1.5V. When a supply is completely removed fast switchover does not work. Is it possible to get fast switchover to work when the supply is completely removed (in1 -> 0V)?

  • 0
    TI__Genius 13760 points

    Hi Stephen,

    Welcome to E2E!

    In short the TPS2121 can perform a fast switchover when a power source gets completely removed. To accomplish a fast switchover the voltages on both PR1 and CP2 must be over VREF = 1.06V. If PR1 is greater than CP2 then VIN1 will be the output. If PR1 is less than CP2 then VIN2 will be the output. For Figure 31, with PR1 connected to VIN1 through the voltage divider, when the power source to VIN1 gets completely disconnected, the TPS2121 will first recognize if the voltage on PR1 and CP2 was greater than VREF. It will then recognize that the voltage on PR1 went lower than CP2 and perform the fast switchover before it recognizes that PR1 was removed.

    If PR1 was higher than VREF, and CP2 before IN1 was disconnected, then the device should've performed the fast switchover. In order to assist you in trouble shooting the problem further I would need some additional information. Would you be able to share:

    • Schematic of your configuration
    • 2 scope shots at the time of the switchover
      • First scope shot: VIN1, PR1, CP2, VOUT
      • Second scope shot: VIN1, VIN2, VOUT, ST(if configured)

    Best regards,

    Andy Robles

  • 0 in reply to Andy Robles
    Prodigy 30 points

    Here is the schematic. I am considering adding a 3.3V zener from Vs(+5V) to PR1. That should keep PR1 above the 1.06V Vref while not affecting the input transition, which occurs near 3.3V. Will that work?

  • 0 in reply to Stephen Esquibel
    TI__Genius 13760 points

    Hi Stephen,

    With 5V at VIN1 the voltage on the PR1 pin is about 3.4V. Once VIN1 gets removed, the device should be performing a fast switchover once the voltage on PR1 drops lower than the 3.3 reference voltage connected to CP2 which is triggered once VIN1 drops to 4.85V. Since the device is not behaving as expected we will have to find the root cause before being able to suggest a solution. Wold you be able to share scope shots of the switchover showing the voltages on VIN1, PR1, CP2, and VOUT?

    Best regards,

    Andy Robles

  • 0 in reply to Andy Robles
    Prodigy 30 points

    Hello,

    I will work on the requested captures.

    In the meantime, is there a recommended circuit which will fast switch from In1 to In2 and from In2 to In1 even when supplies are being completely removed?

    Thank you,

    Steve

  • 0 in reply to Stephen Esquibel
    TI__Genius 13760 points

    Hi Steve,

    The current configuration you have in your system with the TPS2120 is a recommended configuration for fast switchover regardless of the supplies dropping their voltage or being completely removed. The scope shots will help me trouble shot the system since this configuration should achieve the desired functionality.

    Best regards,

    Andy Robles