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SN74LVC541A: Excessive propagation delay time (~18ns instead of 5)

William Mackin
Intellectual 460 points
Part Number: SN74LVC541A

One of our programs is using this part at 3.3V and started seeing part failures.  When they replace this component, the board works again.

We sent the failing parts, and new-from-stock parts to our lab for testing.

The lab used 3.6V supply.  All VoH, VoL, and input current tests passed just fine, but the parts fail AC testing.

The lab ran H->L, L->H, Enable, and Disable tests at room temperature with no load on the outputs.

They measured anywhere from 12.6 to 26.6 ns delay time.  The datasheet says the limit is 5-7ns.  Here's the results from one part:

AC Test AC3THL: *Failed @ 18.1ns (Range 60.0 to 4.0 - HiLimit: 5ns)
AC Test AC3TLH: *Failed @ 17.9ns (Range 60.0 to 4.0 - HiLimit: 5ns)
AC Test AC3OEN: *Failed @ 12.6ns (Range 60.0 to 4.0 - HiLimit: 7ns)
AC Test AC3Tdis: *Failed @ 26.4ns (Range 60.0 to 4.0 - HiLimit: 7ns)

We've seen 2-4 ns difference in times on similar tests due to the tester, but that doesn't account for these results.  I'm including a couple of the scope traces.

Ch1 input, Ch2 output, Ch4 is a trigger used to select a random signal from within a repeating test, so we're not picking up the very first result.

Can you offer any suggestions as to why we're seeing such slow propagation times?

TPHL_LH_SN74LVC541APW.pdf

  • 0
    Guru 281000 points

    You did not mention it, but I guess the new parts have a proper propagation delay?

    Can you check whether there is a clamping diode between any input pin and VCC?

  • 0 in reply to Clemens Ladisch
    Intellectual 460 points

    W e have 44 digital signals ( all in +3.3V level ) connecting to the input of LVC541A buffers. All these inputs would be having either of both options below (either 158K pull up or 158K pulldown resistors). There are external clamping diode used in the schematic for any of the signals.

  • 0 in reply to William Mackin
    TI__Guru* 96862 points

    Hi Bill,

    I assume that the inputs are measured at the inputs of the LVC541 directly (red circles).

    The output slew rate seems quite slow for having no load -- can you show what loading is connected / how the output is measured?

  • 0 in reply to William Mackin
    Guru 281000 points

    Sorry, my question was unclear. I want you to check whether the chip itself has an integrated diode between A and VCC. (Because I suspect you do not have an LVC device.) Also, please show the marking.

  • 0 in reply to Clemens Ladisch
    Intellectual 460 points

    The schematic above is where we discovered the issue.  Then we put samples on a tester in our lab in order to get the scope display above.  I asked our tester about the internal diodes.  He said, " I setup the tester and add a test to look for a diode from signal to VCC. Please note that I am looking for a diode that is part of the device, NOT an external diode on the test hardware. Well, there is no diode built into the die."

    I'm also attaching a pic of the part.

  • 0 in reply to William Mackin
    TI__Guru* 96862 points

    Hey Bill,

    I can confirm that the shown device is an LVC541 with an expected lot trace code.

    I'd say for next steps to see if you have any other lot trace codes for the same device available to test -- or just one that is working correctly to compare data against on your tester. This is just for us to verify that it's not something in the test that's resulting in erroneous data. It will also be helpful to see another lot if it shows significantly improved performance under the same test conditions - ie the one lot may have an issue.

    The device should easily be faster than the datasheet limit at 3.6-V supply and 25C (datasheet specs are for down to 3V and across temp). If the test is working as expected and the device is really having a delay of more than 5.1ns, then I would highly recommend starting a return through your supplier so that we can start an internal investigation into the cause and get to the bottom of the issue - it is likely either a damaged or defective part.

  • 0 in reply to Emrys Maier
    Intellectual 460 points

    Lot codes tested:

      CS9D

      CQL1

      ALQG

      AZ56

    I will initiate a return/FAI process.

  • 0 in reply to William Mackin
    Intellectual 460 points

    Sorry; those may be incomplete:

      1CK CS9D

      21K CQL1

      27K ALQG

      1CK AZ56

  • 0 in reply to William Mackin
    TI__Guru* 96862 points

    Hey Bill,

    Thank you for the extra information - please make sure to include as much detail as possible in the return request. The following items will help to expedite the return process:

    * Schematic of your application showing all input/output connections of the device. It also helps to indicate input signal levels, supply value, and output loading if it is not clear from just the schematic.

    * Which datasheet spec is failing, the test methodology, and evidence of the failures (same plots you posted here)

    * Which lots have failed in testing

    * Which lots are working correctly

    * Link to this E2E thread

    I'm going to close out this thread as 'resolved' indicating the resolution as starting a return.

    Just so you know, the people who handle the returns (our regional quality engineers) will eventually get the parts to me, but my team doesn't directly handle returns, so giving details to them like I mentioned above will help to speed things along.