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SN74AHCT367 Output serial resistor in case of coupled interference

Zhou Fang67
Expert 4250 points
Other Parts Discussed in Thread: SN74AHCT367

Hi Expert,

SN75AHC367 is used to as a 3.3V => 5V level shift to control some logic/controller IC on board(Enable and input from other ICs), several output are connected to a connector.

There is a possibilty that SN75AHC367 output 1Y1, 2Y2 and 2Y1 are damaged(shorted to GND) due to unknow reason. We want to add serial resistor at the output to protect it again interference.

Is there a recommend value for serial resistor? What is the maximum resistor we can add, is there a calculation for this resistor?

And the resisor should be as close as possible to device pin?

Thanks a lot.

Zhou

  • 0
    TI__Mastermind 28495 points
    Hey Zhou,

    I moved this thread into a forum better equipped to help you

    Thanks,
    Daniel
  • 0
    TI__Expert 6335 points

    Zhou,

    The applications engineer for these products has been notified of your request. They will get back to you.

    Best Regards,
    Nirav

  • 0
    TI__Guru 62050 points

    Hi Zhou ,

    I have some clarifications to ask you.

    The subject reads SN74AHCT367 , you have SN75AHC367 in the message (part which doesn't exist).

    could you please clarify which one you are referring to ? SN74AHCT is different from the SN74AHC part.

    You might be violating the recommended specs with SN74AHC device using it for translation of 3.3V to 5V .

  • 0 in reply to ShreyasRao
    TI__Expert 4250 points
    Hi Shreyas,

    Sorry to typo error.

    It is SN74AHCT367.
    We want to know if a resistor is added at the output, is there a suggested value to prevent interfereance meanwhile keep signal integrity?
    Is there a calculation for this resistor value in theory?

    Thanks a lot.

    Zhou
  • 0 in reply to Zhou Fang67
    TI__Guru 62050 points
    Hi Zhou ,

    Instead of adding a resistor at the output , do we know why the device is being damaged in the first place ?
    are there any voltage spikes on the inputs or currents higher than normal being sourced in/out when the damage happens ? For the resistors recommendation at the output , there is no calculation as such but it depends on how much the current limit needed to prevent damage to the device and power source. A 1k at the output will limit the current to 5.5mA if shorted to gnd which should be fine to prevent source damage .
  • 0 in reply to ShreyasRao
    TI__Expert 4250 points
    Hi Shreyas,

    The damage pins are 1Y1, 2Y2 and 2Y1. The key difficulty is that we are not able address the root cause of damage. Because these damaged devices are come from the end customer PP product on field with 4/25 rate of failture. They found the failure came from SN74AHCT367.

    On the previous design, 10 Ohm resistors were added on 1Y1, 2Y2 etc... for protection. A key note was that 10 Ohm resistors were not closed to device pins, they were put on the middle of PCB trace between device pin and a connector. (The traces are around 50mm-150mm long).

    We plan to add another 10 Ohm resistor next to device pins for better protection, in total, 20 Ohm across 1Y1, 2Y2 etc...

    However, I worry about the signal integrity, 50 Ohm or 20 Ohm on 1Y1, 2Y2 etc.. do not decrease slew rate a lot, right?
    I know it is hard to estimate the precise specifications, in gerneral purpose, 20 Ohm or 50 Ohm are commonly used?

    Thanks.

    Zhou