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LMC555: Want to output Low after power-up on LMC555

Motoyasu Hattori
Expert 8695 points
Part Number: LMC555

Hi,

Thank you for your support.

When powering up, especially after a long discharge, the output pin goes high without a Trigger to Q1, on the following circuit. What is root cause?

 

In order to avoid this, we are thinking of stopping connecting the RESET pin directly to Vcc, inserting the following RC circuit into the RESET pin, and applying RESET when the power is turned on. Please advise us the recommended value of C and R. Power up time of Vcc is about 20ms.

 

Please advise us..

Thanks and best regards,
M.HATTORI.

  • 0
    TI__Guru*** 140166 points

    Hattori-san,

    Make the RC product 20ms or more. 100ms is a reasonable target.

    The goal is to have a valid VDD before RESET is 0.4V to ensure starting low.  Before RESET is 1.1V the timer will be ready for operation. 

    Green is VDD, Red is RESET voltage. R=200k, C = 100nF