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LMC555: Timing Variations

Part Number: LMC555

Hi Team,

My customer is using the LMC555C for a 10 second timer one shot use case. In their schematic they have the RC pulled up to OUT instead of VCC like our application circuit shows. They are using a 8.66M and 1uF combination for the time constant.

1. Is this a valid design to have the pull up to OUT or will that lead to variations?

2. They tried changing to 866k and 10uF which actually changed the timer average response to ~14 seconds. Is this possibly related to the above point? Is there a recommendation on magnitude for the R?

Thanks!
Reed

  • Hi Reed,

    RC tied to out is usually a 50% duty cycle a-stable setup (not one shot, see figure 17 in the data sheet). If pin 2 (trigger) is connected separately, it is a one shot setup that has a much longer discharge time. If there is a re trigger within about 20 seconds of the last end of pulse then that pulse will be shorter because the timing cap isn’t back down to zero voltage yet.  

    In the second scenario, it's likely that because of the tolerance of the parts, the RC constant is not actually 10 seconds.

    Best,

    Katlynne Jones. 

  • Hi Katlynne,

    Thank you for the reply! 

    Okay understood. The schematic matches the Figure 3 setup exactly except for R_A connects to the output instead of V+. Is there any instability that could come from this? Should the customer change to match figure 3?

    -Reed