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CD74HC123: Maximum CX value & powerdown considerations

John Wright
Prodigy 10 points
Part Number: CD74HC123
Other Parts Discussed in Thread: CD74HC4538

I have designed a circuit using the CD74HC4538 using timing values or 15M and 22uF to give a little under 4 minutes duration (not critical, anything between 3.5 and 4 mins is fine) . The datasheet explains that a 51R  needs to be added to limit discharge current during power off.

Unfortunately, my PCB manufacturer doesn't stock the 4538 but does have the 123 so I'm redesigning the circuit around that.

The datasheet makes no mention of a maximum value for CX nor does it contain any guidance 're powerdown.

So:

1 - could I use, say,  100F and 5M? 

2 - do I need to add any power down current limiting protection?

   

 

  • 0
    TI__Guru* 96661 points

    Hello,

    My biggest concern would be with the large resistor causing problems. The device can have up to 1uA of input leakage -- with a 5Mohm resistor, that's 5V of loss, and thus a totally inoperative circuit.

    Typically the leakage will be much lower (just a few nanoamps). Although this configuration may work for you, I cannot guarantee operation with such large values.

    The device has internal ballast resistors to prevent over-current during discharge, so you don't need to add anything external. That being said, I have never tested more than 100uF with a device like this, so using 100F may have unexpected issues.

  • 0 in reply to Emrys Maier
    Prodigy 10 points

    Thanks for prompt response Emrys.

    Comparing the datasheets I see that leakage for the 123 is specified 2 x that of the 4538. However, the 1uA figure is at higher temperatures and this circuit will be used in a domestic situation so, hopefully, the actual leakage will be less than the 0.1uA max quoted.

    I'll build it and report back.   

  • 0 in reply to Emrys Maier
    Prodigy 10 points

    Hi again,

    Built the circuit using 1uF & 4.7M on one side and 100uF and 4.7M on the other. Short timer gives as near to 2 secs as I can measure (on my phone) which is fine. Long timer expired at about 2 mins 50. Added 1M in series (5.7M total) and it now gives about 3:30 - c210secs. A bit less than 0.45 Cx*Rx (c0.37). May be at least in part due to tolerance of electrolytic C.

    Anyway - it's good enough for my purposes.

    Thanks for your help.     

  • 0 in reply to John Wright
    TI__Guru* 96661 points

    Hey John,

    Glad to hear that's working for you. I've seen one or two customers in the past do similar things, but we always have concerns  operation over the full range + possible variation from mass production (lot to lot variation).