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DS8921: Part damages when interface powered up before Vcc

Dmitry Babayev
Prodigy 20 points
Part Number: DS8921

I have a second occurrence where when the interface is powered up before Vcc of the device, the device is damaged.

The design is one board with DS8921 Xcvr and the controller would be either NI PC card or in the latest case another DS8921 Xcvr.

I have point to point controller to single slave using as simple differential signal drive (no high speed comm).

How do I prevent and why isn't it designed to withstand these cases?

  • 0
    TI__Mastermind 25705 points

    Dmitry,

    Do you know what part is damaged, R pins or D pins? What's the behavior of the failure parts? How is the interface powered up? What's the status of the DI or RO pins during the power up? How far is it between the two boards? Is there any ground potential difference? Generally the device gets damaged due to the external condition exceeding the IC's range, like the abs max or ESD rating.

    Regards,

    Hao

  • 0 in reply to Hao L
    Prodigy 20 points

    After a visual inspection I realized that part marking is wrong!

    I am having it replaced.

    However, this is still a good case to verify if powering up the interface first will damage the part.

    I am not sure if the D was failing on the previous project.

    The one thing I remember was that when power off the Vcc and R voltages were over 2 volts and that powered up the whole board to over 2 volts.

    I will do a study next week on power to receiver first and get voltages.

    Then I will potentially damage it by powering the interface first and get voltages.

    If it doesn't damage then the previous project (where NI PC card was master) is still a question.

    Dmitry

  • 0 in reply to Dmitry Babayev
    TI__Mastermind 25705 points

    Dmitry,

    Please let me know if you have any new findings. Generally a RS-485 transceiver isn't powered through bus pins. You can refer to the equivalent IO schematic of Figure 18 on page 16 of the datasheet of a similar device.

    https://www.ti.com/lit/ds/symlink/sn65hvd75.pdf

    Regards,

    Hao