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TINA/Spice/SN74LV123A: a problem generating the short delay after power up using the RC on CLRNOT

Viktorija Cecil
Mastermind 18491 points
Part Number: SN74LV123A
Other Parts Discussed in Thread: TINA-TI,

Tool/software: TINA-TI or Spice Models

Team, 

Question from my customer:

I want to use the SN74LV123A to produce the ‘PWRKEY’ signal below. VBAT is 3.3V and powers SN74LV123A. I want to use a RC =(time constant) on the SN74LV123A ‘CLRNOT’ input to provide the >30ms delay as stated in the NOTE. Then I wanted to use the rising edge on ‘CLRNOT’ to generate the => 500ms ‘PWRKEY’

There seems to be a problem generating the short delay after power up using the RC on CLRNOT. Would you please look at the attached TINA file?74LV123 PWRKEY.TSC

Thanks!

Viktorija

  • 0
    TI__Genius 16735 points

    Hi Viktorija,

    Please allow me some time to go through the TINA file. I will keep you updated on my progress.

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    TI__Genius 16735 points

    Hello Viktorija,

    The way you currently have your circuit, what you are trying to do will not happen if you are powering the device with the battery thats powering up. For the LV123A to operate you need to have Cext get charged to Vcc prior to attempting to trigger the device. Since you would need to wait for Cext to get fully charged, your trigger after the 30ms from RC delay might get missed. You might need external circuitry to quickly charge up Cext. Our models for the monostable multivibrator are not very accurate. You would need to actually test this for it to work.

    I did get your TINA model to work like you need it to. The RC values you were using were incorrect. The only reason the model is working is because Vcc is ramping immediately. If your battery is not powered on then your trigger after 30ms will be missed by the multivibrator. If I am not understanding correctly please clarify.

    74LV123 PWRKEY (1).TSC

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    TI__Mastermind 18491 points

    Thanks, Karan. 

    Customer will plan to add supervisory chip in place of RC. 

    Thanks again. 

    Viktorija