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LMC555: Pulse Extender Change recent change in behaviour.

Owain Boorman
Prodigy 10 points

Part Number: LMC555

Tool/software:

We have used a LMC555CMM Timer as a pulse extender for a power button using the circuit below. The system can either be powered from an external DC Supply or from a removable battery. +V.PERM is the power supply applied be it either external DC or the Battery, +3V3.PIC is turned on by the pic microcontroller. 

For over 6 years the circuit above has worked as expected. In the last few months, with newer batches of components,  the behaviour has changed and an intermittent fault has appeared. When the removable battery is connected the LMC555 acts as if the power button has been triggered. 

Has something internally changed that could have caused this change in behaviour? 

  • 0
    TI__Expert 5340 points

    Hello Owain,

    We're looking into this further, and I'll get back to you tomorrow with an update. 

    In the meantime, could you please provide me with additional information about the behavior the part is exhibiting, such a scope capture? 

    Best Regards,

    Alex Curtis

  • 0
    Prodigy 10 points

    Hello Alex,

    When power is applied to the circuit the output of the LMC555 goes high as if it was triggered.

    VIN - Supply voltage being applied

    OUT - Output of the LMC555

    This behaviour seemed to be dependent on the batch/lot code of the devices - PCBs started working when the LMC555 chip was swapped for a different, potentially older batch code. 

    Batch codes that worked:

    08VE

    0991

    09X2

    9BCH

    967V

    Batch codes that didn't work:

    152R

    32H9

    It would be good to know what has changed and to know if there is a fix.

    Thanks,

    Owain

  • 0 in reply to Owain Boorman
    TI__Expert 5340 points

    Hi Owain,

    Thank you for the scope plots, and I'll see what information I can find on those batch codes. In the meantime:

    • Can you confirm that you've replicated this issue across multiple devices within the same batch? 
    • Given that the device is triggering unexpectedly, could you also provide a capture of the trigger pin during the same period?
    • Is the VIN trace in your scope plot the 3V3PIC line? Or the V.PERM line? 

    Best Regards,

    Alex Curtis

  • 0 in reply to Alex Curtis
    Prodigy 10 points

    Hi Alex,

    Yes, we have replicated the issue across multiple devices within the same batch, we have tested over 60 boards which have exhibited this issue. We have come across another Batch Code that exhibits the issue: 32JC.

    Please find updated plots below:

    Monitoring pins 2,3,6,7,8:

    Monitoring pins 2,3,6,7,8, zoomed in:

    Monitoring pins 2,5,6,7,8:

    Monitoring pins 2,5,6,7,8, zoomed in:

    VIN was the +3.3V.PIC line, plots have been renamed.

    Thanks,

    Owain

  • 0 in reply to Owain Boorman
    TI__Expert 5340 points

    Hi Owain,

    Have you done an A-B-A swap to verify that the timer is causing the issue?

    Best,

    Alex Curtis

  • 0 in reply to Alex Curtis
    Prodigy 10 points

    Hi Alex,

    Yes we have, we swapped out the LMC555CMM on the same PCB and it looks to be the timer causing the issue.

    Test Number Marking Test Result
    1 152R FAIL
    2 9BCH PASS
    3 152R FAIL
  • 0 in reply to Owain Boorman
    TI__Mastermind 30985 points

    Hi Owain, 

    There is no power on reset on the timer devices so the output will not be predictable during power up. 

    Best Regards, 

    Chris 

  • 0 in reply to Chris Featherstone
    Prodigy 10 points

    Hi Chris,

    Does this mean that the datasheet has always been incorrect, or just the recent die changes have made it unreliable?

    The pin function table states that RESET_n should be connected to V+ to avoid false triggering, is this no longer the case?

    Should the datasheet be changed to state that power on reset circuitry is needed?

    We have procured 1000pcs of National Semiconductor LMC555CMM from May 2010 and they seem to work as our original design, and datasheet suggests.

    Best regards,

    Owain

  • 0 in reply to Owain Boorman
    TI__Mastermind 30985 points

    Hi Owain, 

    The section you highlighted is correct. The reset pin is not related to power on reset. Power on reset circuits are used in designs to provide a known state on power up. The timers do not have power on reset circuits. The reset pin has never provided this feature and is only intended to operate with the expectation explained in the product datasheet when the device is fully powered on. Because there is no power on reset the output will be unknown when the device is powered on. 

    Best Regards, 

    Chris Featherstone