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LM555: LM555 delay setting

Randy Chen
Intellectual 1800 points
Part Number: LM555

Tool/software:

Hi, 

Customer has an application question about using LM555CMX/NOPB to implement the virtual reseat. Below is my schematic design.

As I know, LM555 would output a high pulse when trigger voltage less than 1/3 Vcc untill the external capacitor charged to 2/3 Vcc.

Is it possible to cause the system power on with a 11s delay because the BMC initial not ready and trigger level is low?

Best regards,

Randy

  • 0
    TI__Intellectual 1500 points

    Hi Randy,

    Yes, in your design the LM555 would output a high pulse for 11s when the trigger level falls below 1/3 Vcc. If the trigger pin is pulled to less than 1/3 Vcc (4V in this case), the output will go high for 11s while the capacitor charges to 8V. For more information on designing 555 timer circuits in monostable mode, see this FAQ post: [FAQ] How do I design monostable timer circuits using LMC555, TLC555, LM555, NA555, NE555, SA555, or SE555? - Clock & timing forum - Clock & timing - TI E2E support forums

    To answer your question, it depends on the voltage of the BMC_SELF_PWR_CYCLE line, since that's what triggers the LM555. From my understanding of your design, if the voltage of BMC_SELF_PWR_CYCLE is below 4V, then the output of the LM555 will go high, pulling the P12V_OFF_N line to ground for 11 seconds.

    Best regards,

    Alex Curtis

  • 0 in reply to Alex Curtis
    TI__Intellectual 1800 points

    Hi Alex, 

    One more application question from customer as below.

    If the trigger pin is pull-up to P12V_AUX and not controlled by BMC. Does LM555 still generate a high pulse from output pin until the external capacitor be charged to 2/3 Vcc when the system just power on?

    Best regards,

    Randy

  • 0 in reply to Randy Chen
    TI__Intellectual 1500 points

    Hi Randy,

    It looks like P12_AUX is not present in your schematic, but the LM555 should not generate a high pulse if the trigger pin is at 12V. The external timing capacitor only charges to 2/3 Vcc when the device is triggered and is normally in a discharged state. As long as the trigger pin is above 1/3 Vcc when the system starts up, the output will stay low, and the capacitor will remain discharged. 

    One way to further reduce the possibility of accidentally triggering the LM555 during power up would be to have the RESET pin at ground initially, then bring it up to Vcc when the rest of the system is stable. RESET has highest pin priority, so if the RESET pin is low, the output will stay low regardless of the trigger or threshold voltage, so you can use it to suppress unwanted outputs. 

    Let me know if you have any other questions,

    Alex Curtis