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CD4047B: The voltage of Q and Q bar can't not drop to 0V

Jimmy Lai
Expert 6845 points
Part Number: CD4047B

Dear team,

We appreciated your great support always!

Our customer use our CD4047B to design a inverter as attached,

The voltage of Q and Q bar can't not drop to 0V (2V), please refer below waveform.

May I know why the voltage can't not drop to 0V?    

 

Many Thanks,

Jimmy

  • 0
    TI__Guru* 96681 points

    Hi Jimmy,

    I can't really tell much from your scope shot - I assume you're talking about the blue trace and I also assume it's being measured at "INA". At the time scale you're showing (I think about 1us per div), it appears to level off at ~2V for ~4us, but given more time I would expect it to discharge completely to 0V.

    It looks like you have 2 diodes in series in the discharge path of INA, so I would expect that point to very quickly discharge down to about 2V, and then slowly discharge to 0V over time. 

    Can you measure the output of the CD4047B directly (at R251 or directly at the device pin 10)? Is the output switching from ~0V to ~12V?

  • 0 in reply to Emrys Maier
    TI__Expert 6845 points

    Hi Emrys,

    We appreciated your great support!

    Please see below with more accurate resolution. The oscillator output can't be directly pulled low when the signal is less than 2V.

    Many Thanks,

    Jimmy

  • 0 in reply to Jimmy Lai
    Guru 259060 points

    Apparently, C123 needs some time to discharge.

    (And when Q217 is off, the gate of Q218 is floating. Is this intended?)

    The waveform shows that the high-level output does not reach 12 V. Is the supply voltage lower, or is there a pull-down somewhere?