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SN65MLVD204B: Hot-plug requirements and protection against damage in unpowered devices

Terry Simpson
Prodigy 10 points
Part Number: SN65MLVD204B

Hi,

We have a design in which there are two SN65MLVD204B's on two distant modules, Module A and Module B, that communicate over a differential cable. The cable can be disconnected which disconnects the communication link and removes power and ground reference from Module B.

Module A is powered up at all times and may drive the SN65MLVD204B's DE (Driver Enable) pin at any time, even if Module B is disconnected.
Module B can be connected at any time, including when Module A is driving the differential pair or is tri-stating the differential pair.
When Module B is connected to the cable, the SN65MLVD204B VCC power rail will take ~100ms to power up. During that time SN65MLVD204B on Module B will have VCC=0V and the differential pair is either being actively driven by the SN65MLVD204B on Module A or is tri-stated and possibly floating to up to 2.4VDC

Would TI recommend adding protection against over-voltage conditions, such as diodes from the differential pair (A,B signals) to VCC?
Would ensuring that the Module A and Module B DE (Drive Enable) signals are de-asserted help minimize risk?

These questions arise because we damaged one of the SN65MLVD204B devices on a system and the damage likely occurred during the process when Module A and Module B were connected.


Thanks!
Terry

  • 0
    Guru 259000 points

    The absolute maximum ratings allows voltages up to 4 V at the pins, regardless of VCC.

    Disconnecting and connecting can generate all kinds of overvoltage (ESD, arcing, etc.). Do the lines have any protection?

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Hi Clemens,

    Based on your absolute max rating statement, it sounds like you're not concerned about Module A driving the diff pair lines into Module B while Module B is unpowered.

    No, the lines do not include any ESD or other overvoltage protection. The SN65MLVD204B includes ESD protection up to 8kV for HBM, so we're relying on that mechanism.

    Are you recommending that we add ESD protection external to the transceiver?

    Thanks again,
    Terry

  • 0 in reply to Terry Simpson
    Guru 259000 points

    I do not definitely know the cause of the damage. But when you have a connector that can be touched by a human, then it is likely that a random failure is caused by ESD.

  • 0
    TI__Genius 11601 points

    Hi Terry,

    As for driving the differential pair lines into module B, the datasheet provides the maximum current values for the device on page 8 under “Receiver or transceiver power-off input current”. The internal ESD protection will protect against small amounts of ESD energy per the ratings on page 6. It will have some surge protection, but if the voltage transients in the system exceed these values, then an external protection diode may be needed. Disabling the DE pin will change the supply current, per table 6.5.

    Regards, Amy

  • +1 in reply to Amy
    TI__Genius 11601 points

    Hi Terry,

    If possible, you could try to check the voltage transient. The internal ESD protection is going to be most useful for ESD events. Figure 8-2 shows how an ESD event will be protected from a high-voltage over a short timeframe. 

    Let me know if you have any other questions.

    Regards, Amy