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SN74LVC1G123: output malfunction

CK Ho
Expert 1780 points
Part Number: SN74LVC1G123


Dear sir,

Our customer is using SN74LVC1G123, and below is the schematic:


Ideally, the output Q (pin5) should change form Low to High and produce a pulse when the input A (pin1) in the transition from High to Low, right??

But belows are we measired waveforms:

Sometimes the transition from Low to High of the input A would cause the output Q also change form Low to High.

Do you have any idea about it?? Or any comment??

Does the Cext (pin6) can be connected to GND??

Look forward your feedback. Thank you very very much.

  • 0
    TI__Genius 16735 points

    Hello,

    This is an unpredictable behavior. When a trigger event occurs, an internal fet will discharge the capacitor. I am not sure how your external ground will be causing a problem but try removing it as the first step and retest. Use an ohmmeter to check the resistance between the Cext and GND. If the resistance reads less than 1 Ω, it is safe to connect both to ground.

    Here is an appnote on designing with monostable multivibrators. Based on the information you gave, I don't see anything else specifically wrong. Is your VCC pin stable? Are you triggering long enough after start up for the Cext to reach stability? 

    I am going to continue looking at this, see if I can come up with something else.

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    Expert 1780 points

    Dear Karan,

    Thanks very much for your feedback.

    Now we have removed the external ground, but it looks also abnormal.

    The VCC is stable. Please see the waveform below (pin1, pin7, pin5 respectively). Do you know why Q output a pulse at A's rising edge??

  • 0 in reply to CK Ho
    TI__Genius 16735 points

    Hi,

    Is the blue signal the A pin? If so can you zoom into it so I can see at what voltage on A is the output triggering.
    The device has schmitt trigger inputs so it should handle slow inputs but if possible you should speed the signals up.

    Can you also provide scope shot of the CLR and B pins so I know what they are doing when this is being triggered?

    Thanks!
    -Karan

  • 0 in reply to Karan Kotadia
    Expert 1780 points

    Dear Karan,

    We found that the root cause maybe is the signal bounce of input A (the blue signal).

    Thanks very much for your support.

    Appreciate!!

  • 0 in reply to CK Ho
    TI__Genius 16735 points

    Hello,

    I am glad you were able to find the problem!

    Thanks!

    -Karan