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LMC555: Delayed reset circuit timing difference from simulation

Part Number: LMC555
Other Parts Discussed in Thread: TLC555

Hello,

I'm having trouble with my LMC555 timer not performing the way I expect. My simulation in PSpice for TI gives the results I expect, but on the board I see something similar but with different timing.

My requirements are to have a hardware reset 5 seconds (ish) after being placed on the charger. To do this, I used two 555 timers: one intended to delay for ~3 seconds, and one to assert RESET_N for about 3 seconds (it must be asserted for 2 - 3.8 seconds). The circuit I came up with is:

When simulated for 10 seconds, the output is:

This is what I expect. In my project (KiCAD), I have the following schematic:

But when I look at the signals with an oscilloscope, I see this:

(Yellow = RESET_N, Green = VUSB, Purple = output of delay stage)

The issues I see here are:

  1. The delay stage should be ~3 seconds, but is really only about 1 second
  2. The RESET_N should be asserted for ~2.9 seconds, but is really only a bit over 1 second

For the simulation I am using the TLC555 timer, whereas the board has the LMC555 timer on it, but these message boards indicate that the parts are very similar.

Is there something silly I'm doing here?

Thank you,

Jonathan

  • Hey Jonathan,

    What is the tolerance of the capacitors that you are using? Also what type of capacitors are you using in your application? The tolerance makes a big difference and also the type of capacitor used matters. The capacitor value will also change quite significantly for x5r and x7r across bias voltage. This is in addition to the tolerance. This is likely creating the discrepancy between expected vs measured behavior in the timing circuit. 

    Best Regards, 

    Chris Featherstone

  • Hello,

    The capacitors I'm using are the following:

    10uF (3 second delay circuit): https://octopart.com/cl05a106mq5nunc-samsung-22240478?r=sp (20% X5R)

    4.7uF (2.9 second pulse circuit): https://octopart.com/cl05a475kq5nrnc-samsung-21395527?r=sp (10% X5R)

    Just with back-of-the-envelope calculations, it would seem that even if the capacitors are off by the maximum amount, we'd not be so far off.

    Example:

    I'd like a 3 second delay. T = 1.1 x R x C, R = 274k, C = 10uF, T = 3.014 sec. Worst case, C = 0.8 * 10uF, T = 1.1 x 274k x 0.8 * 10uF = 2.4 sec. As measured, this delay is about 1.03 sec.

    Similarly, for the 2.9 second reset pulse circuit, the worst case should be T = 1.1 x 560k x 0.9 x 4.7uF = 2.6 seconds, but I'm seeing about 1.2 seconds.

    Am I missing something with these numbers? I can certainly tighten tolerances and use better capacitors if that's the issue.

    Thank you,

    Jonathan

  • Hey Jonathan,

    With capacitors there is the 20% initial tolerance. In addition to the 20% tolerance the value will drastically change with applied voltage as well. This can result in a very different capacitor value from the expected value. We don't make capacitors so I am showing an image below credit to Murata. I also provide a link to their article. There are a lot of pretty neat plots with experiments similar to this where engineers have compared various capacitor types. Note that the X5R can change value up to 60%. This is in addition to it's starting value that could be off from the theoretical by 20%. COG/NPO capacitors are typically what we would recommend for timing applications that require more precision. Temperature is also another factor that can change the expected value. Capacitor types are my first suspicion. One way to rule this out would be to try out COG/NPO type capacitors. Another option would be to use an LCR meter and vary the DC voltage across the capacitor while measuring it's value and plugging in the value into the equations to see if it lines up with the measured timing.

    https://article.murata.com/en-eu/article/voltage-characteristics-of-electrostatic-capacitance

    Best Regards, 

    Chris Featherstone

  • Hello,

    Thank you for this detailed response. I hadn't realized just how much capacitors can vary -- ouch! I'm not seeing any 0402 C0G MLCC parts, unfortunately, so replacing that would be tricky.

    Thank you very much, I'll take a deeper look at this.

    Cheers,

    Jonathan

  • Hey Jonathan, 

    No problem. Let me know if I can be of further assistance if any other issues arise during the debugging process. 

    Best Regards, 

    Chris Featherstone

  • We have 100 boards being made now; I'll have the capacitors swapped for this: https://octopart.com/f980j475mua-kyocera+avx-80851651?r=sp and see how that goes.