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When SN74LVC1G07 power off, the output have ground bound

Thomas Chang15933
Expert 4770 points
Other Parts Discussed in Thread: SN74LVC1G07, SN74LVC1G17

Dear Team:

 Our customer used SN74LVC1G07, but got seems gound bound at output, when Power off.

As below is their circuit and waveform. Second 2 picture is Zoom in picture.

  • 0
    Guru 268470 points

    What do the four inputs measure? Which power was switched off? Is there really no decoupling capacitor?

  • 0 in reply to Clemens Ladisch
    TI__Expert 4770 points
    Dear Ladish:
    Four input measure: Scope 4 color signal waveform (P3V3STBY, P3V3, FM_M2_SSD_PEDET, FM_SSATA_PCIE_M2_SEL). Both power turn off happened the issue.
  • 0 in reply to Thomas Chang15933
    Guru 268470 points

    Sorry, I overlooked these labels.

    The Ioff feature of this device works when the power is switched off. However, the power supply for this device is P3V3_STBY, which stays up.

    Switching off P3V3 affects only the input signal, and with the slow decay shown above, the input signal violates the input transition fall time (Δt/Δv in table 6.3 of the SN74LVC1G07 datasheet). Oscillations are expected to happen when you do that; see Implications of Slow or Floating CMOS Inputs (SCBA004).

    You should connect the buffer to the power supply that is always switched off first. If that is not possible, use a buffer with a Schmitt-trigger input (e.g., SN74LVC1G17, with a push/pull output).