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Original question:

TINA/Spice/SN74LV123A: No state change on Pspice

SN74LV123A: Output doesn't change

Achraf HAMMOUD
Prodigy 40 points
Part Number: SN74LV123A


Hello,

I'm trying to simulate a falling edge detection using the SN74LV123A. The input signal (/A) is high @t0. Then it changes to low and remains low for 1us only. When the falling edge is detected the output (/Q) should change to low and should remain low for 230ms approximately. To do so i used Rext = 490k & Cext = 470nF.

The problem is that when I run the simulation the output /Q does not change as you can see below :

Green =  /A

Blue = /Q

Red = Rext_Cext

Thank you in advance for your assistance.

Sincerely,

Achraf

  • 0
    Guru 318700 points

    The real device certainly works.

    Which model are you using?

  • 0
    Prodigy 40 points

    I'm using the version below (SN74LV123ARGY):

    ** Released by: WEBENCH(R) Design Center, Texas Instruments Inc.
    * Part: SN74LV123A
    * Date: 12/2/2014
    * Model Type: All In One
    * Simulator: Pspice
    * Simulator Version: Pspice 16.2.0.p001
    * EVM Order Number: N/A
    * EVM Users Guide: N/A
    * Datasheet: SCLS393P - APRIL 1998 - Revised OCTOBER 2013
    *
    * Model Version: 1.1
    *
    *****************************************************************************
    *
    * Updates:
    *
    * Version 1.0 : Release
    * Version 1.1 : Revise model
    * Version 1.2 : Revise model to support a wider range of pulse duration.

    Sincerely,

    Achraf

  • 0 in reply to Achraf HAMMOUD
    Guru 318700 points

    It's possible that the output needs a load; try adding 1 MΩ to GND.

  • 0 in reply to Clemens Ladisch
    Prodigy 40 points

    No it doesn't work.

    In order to understand what's happening I modified the simulation: in the simulation below the input /A remains low for 100us (instead of 1us before). Result :

    The IC discharges Cext slowly when /A changes to low. And the output /Q changes to low when the voltage of the pin "Rext_Cext" gets below 1,8V.

    Sincerely,

    Achraf

  • 0 in reply to Achraf HAMMOUD
    TI__Mastermind 20700 points

    Hi Achraf,

    Please see the disclaimer included in this PSpice model below:  

    The most important part to read is the Notes section:

    *****************************************************************************

    * Notes:

    * 1. The MODEL IS INTENDED FOR 5V OPERATION AND REFLECTS THE PULSE DURATION

    *    AS SHOWN IN FIGURE 6 OF THE DATA SHEET.

    * 2. THE MODEL MAY NOT MEET THE OTHER DEVICE PARAMETERS.

    * 3. THIS MODEL IS INTENDED FOR USE IN CADENCE V16.2 AND ABOVE WITH

    *    AUTOCONVERGE ENABLED. IF AUTOCONVERGE IS NOT USED, THE SIMULATION

    *    WILL LIKELY HAVE CONVERGENCE ERRORS.

    * 4. THE VALUES OF R_EXT AND C_EXT IN THE SCHEMATIC MUST BE ENTERED AS

    *    PARAMETERS VIA THE PARAMS STATEMENT IN THE .SUBCKT LINE BELOW.

    *    IF THIS IS NOT DONE, THE MODEL WILL GIVE INCORRECT RESULTS.

    *

    *****************************************************************************

    I would not recommend using this model. 

    Regards,

    Sebastian 

    Sebastian