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LMV762 A Output Failures

Denis
Intellectual 505 points
Other Parts Discussed in Thread: LMV762, LMP2012, LMV393, TLV7256, TLV3501, LMC7211, LPV7215, LMV7239, TLV7211

I'm having a very peculiar problem on a design that is using the LMV762MMX. In manufacturing I am seeing about a 20-30% failure on the A component output (pin 1) of the LMV762. The B component output (pin 7) is working properly.

Here's a bit of background, I have redundant circuits that monitor temperature and the pair of comparators in one package monitor the open and short condition of the RTD. When a failure occurs both comparator packages for monitoring the open condition (A) fail to logic low on both of the redundant circuits (the short monitor sections (B) continue to work properly. The inputs to each of the comparators is buffered by LMP2012 op-amps.

The LMV762 are powered at 5V and feed into a 3.3V CPLD which is 5V tolerant.

Is there a layout difference in the A comparator and the B comparator die/packaging that may cause one to fail over another?

Would power up sequencing be a possible cause?

Thank you for your time in reviewing the problem.

~Denis

  • 0
    TI__Intellectual 2630 points

    Denis,

    With dual comparators there may be minor offset differences between comparator channels but that would not explain the behaviour you are seeing. Three things:

    - Though difficult on a production board have you tried cutting traces, rewiring, and seeing if the failure follows the channel?

    - Have you tried replacing the LMV762 with another dual comparator in the same package (i.e. LMV393IDD, TLV7256IDD)?

    - Can you share a schematic for review?

  • 0 in reply to Jason Seitz
    Intellectual 505 points

    Hi Jason,

    I agree, I wouldn't expect the offset differences to be a problem, especially when the outputs are connected to different inputs on the CPLD.

         - I have cut the traces and found that the output of the bad comparator will not change states (damaged output). Upon replacing the faulty comparator, the circuit works fine.

         - I have replaced the LMV762 with a LMV393 and changed the input of the CPLD to have a pull-up. The circuit works with this change. My concern has to do with product in the field and what the exposure is because of this failure. Understanding the cause of failure will help in a risk analysis. Best I can tell at this time, all parts are from the same manufacturing lot/date code.

    Here's a portion of the schematic:

    U31A is the portion that fails, if any failure occurs. Both outputs connect to the same style port on the CPLD.

    ~Denis

  • 0 in reply to Denis
    TI__Intellectual 2630 points

    Denis, Thanks for the extra detail and schematic. After looking at your schematic the suspect U31A comparator has a reference voltage (-IN) set to 4.25V. This voltage exceeds the LMV762 maximum Input Common Mode Voltage Range (CMVR) specification of 3.8V at V+ = 5V listed in the datasheet. Remember that for a comparator the common mode for each input must be considered separately.

    The LMV393 which you swapped to will have less of a fallout with a CMVR spec up to 4.2V. But just to be safe you'd probably also want to try the TLV7256 with CMVR 0 to V+.

  • 0 in reply to Jason Seitz
    Intellectual 505 points

    Jason,

    After reviewing the rest of the design, I'm concerned that I'm exceeding the CMVR of the LMV761MFX (the control section of the design). Do you have similar replacement for this part that has a CMVR from 0 to V+?

  • 0 in reply to Denis
    TI__Intellectual 2630 points

    Denis,

    As I mentioned in my previous post the TLV7256 has a CMVR 0 to V+.

  • 0 in reply to Jason Seitz
    Intellectual 505 points

    Jason,

    The TLV7256 is a dual comparator. I was wondering if there is single comparator in the SOT23-6 pin package.

  • 0 in reply to Denis
    TI__Intellectual 2630 points

    Denis,

    There is the TLV3501 in 6 lead SOT-23. Vin max 0.2V from V+.

    Did you need it in 6 lead SOT-23 for the shutdown function? If you don't need shutdown and 5 lead SOT-23 works we also have from highest Vin max to lowest:

    LPV7215, LMV7239, LMC7211, TLV7211