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TPS22968: Load Switch On Off Signal

Albert Lee83
Prodigy 60 points
Part Number: TPS22968
Other Parts Discussed in Thread: TXS0108E,

Hi,

Why is the output still high on Vout when the On/Off signal is low?

Vbias: 5V, Vin1 & 2: 3.3V, On1 & 2: low (0.032V), Vout1 & 2: 3.329V

The On/Off signal is driven low (measured through DMM) through a level shifter TXS0108E.

Thanks,

ALbert

  • 0
    TI__Genius 10855 points
    Hi Albert,

    Thanks for reaching out on E2E.

    Can you elaborate on the system setup? What is the downstream load that you're powering? Can we get oscilloscope shots of the device turning on/off with the ON pin being driven low by the TXS01083? If the ON pin isn't driven between 0V - 0.5V, then the switch may remain on.

    Thanks,
    Arthur Huang
  • 0 in reply to Alpha H
    Prodigy 60 points

    The downstream load is currently not connected to anything, just using a 1M probe to monitor the output.

    Attached is the scope shot with the 1st probe probing the On/Off pin of the chip, and the 2nd probe probing the Vout of the chip.

    The On/Off signal is driven by the FPGA, but in between there is a 1.5V to 3.3V level shifter (TXS0108E).

    Initially when the system is powered up but before FPGA has been programmed (for about 0.8sec), the FPGA will not drive the signals (tri-state), and signals coming out of the level shifter will be held high for around 0.8sec until the FPGA drives the signals low.

    As you can see, after around 0.8sec, the On/Off signal is driven low (below 0.5V); however, the Vout still remains high at 3.3V.

  • 0 in reply to Albert Lee83
    TI__Genius 10855 points

    Hi Albert, 

    Hmm...that is a very strange behavior. A few questions for you:

    1. Are you using the TPS22968 or the TPS22968N? 
    2. What downstream load are you powering, is it purely a capacitive load? 
    3. Is the yellow signal the input to the TXS0108E or the direct input to the load switch?
    4. Have you tried replacing the TPS22968? Does the circuit work properly after replacing the device?

    Thanks,

    Arthur 

  • 0 in reply to Alpha H
    Prodigy 60 points
    Hi,

    1. TPS22968N

    2. At the end, the downstream load will be powering up three Hall Effect sensors for a motor.
    (currently for testing, nothing is hooked up downstream, only a probe or DMM for checkout.)

    3. The yellow signal is the output of the Level Shifter (TXS0108E) (or direct input to the load switch On/Off pin).

    4. I have two boards that have this same issue, so the TPS22968 IC is acting the same in both boards/circuitries.

    Please advise on any other suggestions or comments.
    Highly appreciated, thanks.

    Thanks,
    ALbert
  • 0 in reply to Albert Lee83
    TI__Genius 10855 points

    Hi Albert, 

    Ah, I think you might be using the wrong version of the TPS22968, the one without quick output discharge (TPS22968N). I believe that the device is turning off, but there is no reason for the output to be pulled down to GND, especially if the load is mostly capacitive. 

    To elaborate, the function of the switch is to disconnect the power supply from the load. Once the switch is turned off, the connection between these two is broken and the capacitance is left on its own. Without a load to discharge the capacitance, there is no reason for the capacitance voltage to lower at a fast rate.

    I recommend you try the TPS22968 with the QOD feature. This will pull down the rail with an internal resistor whenever EN is low.

    Thanks,

    Arthur Huang

     

  • 0 in reply to Alpha H
    Prodigy 60 points

    Hi Arthur,

    I added different resistive loads on the Vout; varying from 100K, 10K, 1K.

    Adding a load will ensure the charged capacitance will discharge in a reasonable matter.

    The issue you are talking about might not be the issue I see.

    Again, probe 1 is the On/Off pin of the switch device, probe 2 is the Vout pin of the switch device.

    === 100K Resistive Load ===

    === 10K Resistive Load ===

    === 1K Resistive Load ===


    Please kindly advise what the issue or root cause might be.

    Thanks,

    ALbert

  • 0 in reply to Albert Lee83
    TI__Genius 10855 points

    Hi Albert, 

    Since we can't guarantee the behavior of the output without QOD functionality, I recommend using the TPS22968 version of the device. 

    Can you try replacing the TPS22968N with the TPS22968? 

    Thanks,

    Arthur

  • 0 in reply to Alpha H
    Prodigy 60 points

    Hi,

    After replacing the non quick output discharge "TPS22968N" with the quick output discharge "TPS22968", it worked.

    Thanks,

    ALbert