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SN54LS123-SP: Cext and RT

Part Number: SN54LS123-SP


Good day. I hope everything is fine. 

Our customer want to use the SN54LS123-SP and they have a few question about it. Kindly refer to their question below. 

1. Can the Cext be a ceramic capacitor? If yes, will a switching diode be required when using a ceramic capacitor for Cext?
2. Is there any tools available for calculating Cext and RT?

I would appreciate if you could help us with this inquiry. Thank you. 



  • Hi Cedrick,

    Yes, Cext can be any type of capacitor - ceramic works fine.  The diode is required for large capacitances (greater than 1000 pF), but not otherwise.

    The equations are pretty easy for this, but I can help if I need to.  Do you know what output pulse length is desired?

    I would highly recommend building a prototype and testing the RC values selected across the required operating temperature - these devices are notoriously inaccurate, and it's best to find out that your pulse is too short or long in the prototype stage.

  • Hi Emrys, 

    Thank you for your support on this matter. 

    Our customer has a follow up question about this. Kindly refer to his question below. 

    "In the datasheet it states that the LS123 does not require a switching diode. If a diode is truly required, would 1N6638 be okay to use?"

    Thank you and have a great day. 



  • Hi Cedrick,

    Can you tell me where it says that the LS121 doesn't need one in the datasheet? I didn't see that anywhere.

    This image indicates the requirement:

    It's specifically referencing a large external capacitor and using the 'clear' input.

    I can't recommend diodes since we don't make them, but any silicon diode should work.

  • Hi Emrys, 

    Thank you for your response. 

    The statement that the LS121 does not need a switching diode is on page 8 of the datasheet (snapshot below). 

    Furthermore, our customer said that from their understanding, that image you provided previously is for typical application for 122, 123, 130. Page 8 of the datasheet is for LS122 and LS123. For my application, I'll be using the LS123. 

    I would appreciate if you could provide further guidance on this. Thank you. 



  • Hi Cedrick,


    It shouldn't matter anyways - the diode is there to prevent reverse bias of an electrolytic capacitor based on the datasheet, and they said they are using a ceramic capacitor. It's likely that when this document was written (37 years ago), that the only capacitors available that were so large were electrolytic.