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SN74LVC8T245: Output state when power is off

Lin_Charles
Expert 4260 points
Part Number: SN74LVC8T245


Hi team,

It is nice to know you here. My customer need TI explanation about the below phenomenon,

1.When VCCA power off,

2.DIR connect to the HIGH,

3.The input signal is LOW,

4. OE power from VCCA

we can 1.5ms pulse in the B1 even if the A1 is LOW. Could you please help check what may cause this issue? Thanks.

BR,

Charles Lin

  • 0
    TI__Mastermind 46555 points

    Hey Charles,

    It looks as though the pulse happens when VCCB is 0 V which would disable the I/O due to the Ioff circuitry. At that point I would assume the pulse would be coming from something else especially since it has an amplitude of 5 V and our device isn't connected to a 5 V source.

  • 0 in reply to Dylan Hubbard
    TI__Expert 4260 points

    Hi Dylan,

    I may not describe my issue clearly. The VCCO connects to 5V and it is stable when VCCA power off. This is why B1 has a 5V pulse.

    The question is that when A1 is LOW and VCCA power off, OE power from VCCA and it also power off, we can see a 5V pulse in the B1. Please help find the root cause. Thanks. 

    BR,

    Charles Lin

  • 0 in reply to Lin_Charles
    TI__Mastermind 46555 points

    Hey Charles,

    Okay so the Blue waveform is not connected to our device. When you power down your VCCA supply you put the device in an unknown region of operation (the OE and DIR circuitry are referenced to VCCA) so its possible to see a pulse like this at the output until the I/O get disabled by the Ioff circuitry. We have a new device that is drop in that has features to eliminate issues like this ( See here for product folder). Otherwise, they might need to hold the OE pin High while powering off to keep the I/Os disabled until the Ioff circuitry activates.

    .