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CD4503B: High to Low transient time of output voltage

Part Number: CD4503B
Other Parts Discussed in Thread: CD4053B,

Hello experts,

My customer is interested in CD4053B. Customer has question about tTHL. Does it vary by sink current? If sink current is vary 5mA to 10mA, tTHL will be varied? If so, what is change, ns or more? Customer mentioned that output is connected photo coupler, and there is pull up resister between CD4053B and photo coupler. Then customer want to know how much effect a tTHL by sink current.

Best Regards,
Fujiwara

  • The timing depends mainly on the capacitive load (the photocoupler's input capacitance) and the drive strength of the device that actually sinks the current.

  • Hello Clemens-san,

    If capacitive load and drive strength of the device (it means device has enough sink current capability) and actual sink current is only varied (ex. 5mA to 10mA), how much tTHL would be varied? It would be few nsec or more?

    Best Regards,
    Fujiwara

  • Yasushi-San,

    The RC time constant decides the rise and fall time. The R is the internal NFET resistance which can be calculated from Vol/ Iol and C is the external capacitance.

    It is similar for the Rise time where the R is decided by the PFET. 

    Someone from Mux signal switches group will get back to you on your query.

  • Hello Fujiwara-san,

    Your question can be answered by basic circuit analysis.

    The CD4503B has standard push-pull outputs, which will act very much like resistors when active. This allows us to simplify the circuit to this:

    My simulation used R2 as a parameter and shows outputs for the values of R2 = {1k, 10k, and 100k} ohms (in that order from the top for each I_sink and V_out).

    As you can see, the timing does not change due to changes in R2. The only change is in the final voltage, which you can see in the red V_out (which has the most sink current).

    Doing the same simulation again and varying the internal resistance instead produces a very different result:

    Here you can see a significant difference in timing for each simulation (R1 = {50, 100, 150} ohms.

    As Clemens said,

    The timing depends mainly on the capacitive load (the photocoupler's input capacitance) and the drive strength of the device that actually sinks the current.


    The drive strength of the device changes with supply voltage as described in the datasheet (by the V_OL / I_OL values given), and the load capacitance should be described by the photocoupler's manufacturer, as well as the board design (traces on a PCB will add capacitance).

  • Fujiwara-san,

    Wanted to clarify which device you are talking about for us to classify this post correctly.

    Explanation given above applies to the driver device ( CD4503B is buffer driver whereas CD4053B is an analog mux which needs an external driver device).

  • Shreyas,

    The question is for CD4503B. Sorry for confusing you. Thank you for answer, I understand.

    Best Regards,
    Fujiwara