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AM3352: Crystal OSC0 - Parameters - ESR and Behaviors

RNJ
Prodigy 30 points
Part Number: AM3352

Hi Texas team,


We have a custom board with an AM3352 and I was checking some changes that we made across the last years in this project and noted that we disrespect a datasheet parameter after changing a 24MHz crystal (reduced footprint for improving placement). We didn't realize that doing that we changed the ESR of the device due to the small package.


We never received any negative reports after this change. But am I wondering what are the expected behaviors of not respecting the ESR specification of an oscillator circuit? We did some SI analysis and noted a slower star-up time, but nothing that matters, from 250us to 600us. But after the circuit is in a steady state the values and measures are equals.

I searched for more cases with this same question and the best approach was founded on this topic but not finish it. e2e.ti.com/.../tms320f28377s-about-crystal-equivalent-series-resistance-esr-requirements

My main question is, could this affect the MPU in some strange behavior (PLL not fixing, drift frequencies, or anything worse)? Or it's more about efficiency and start-up timing?


To let you guys know my specific case we have:

The ESR from most manufacturers for the size (2.5x3.2mm) that we updated is around 60Ohms.

Our older board have an crystal with ESR max < 50 and; Our updated board with ESR max < 60.

Our capacitance used in both cases are 18pF, We noted that reducing that to 12pF the start-up time is improved (reduced).

  • +1
    TI__Guru 59285 points

    The biggest concern is not having enough gain margin to reliably start oscillation when the higher ESR crystal is combined with the other crystal circuit components and internal oscillator.

    There is a good chance you have enough gain margin if this combination of components has been used for a while without receiving any reports of the oscillator not starting.

    In most cases the actual ESR of a crystal is much smaller than the published value. This is especially true when the ESR value you are using is taken from a data sheet that covers an entire family of crystals. The max ESR value published in a crystal family data sheet is typically inflated to cover the entire range of crystals. You may be able to reach out to the crystal manufacture and request a part number specific datasheet to confirm the max ESR of your crystal.

    You can test the gain margin by placing a resistor in series with the crystal, which increases the ESR seen my the entire circuit. We typically recommend confirming reliable start of oscillation across operating conditions when the series resistor inserted increases the ESR to at least 3x the expected ESR of the crystal. 

    Load capacitor values should be selected based on the load expected by the crystal. Crystal are cut to oscillate at the specified frequency with a specific capacitive load. The frequency of oscillation will be pulled a few PPM from the specified frequency if the load you apply is not correct. Most crystal manufactures provide a service where they evaluate your product with their crystal and make recommendations if your initial component selection is not optimum. I think the fee they charge is reasonable for any high volume product where you want to be confident that you clock solution is robust.

    Regards,
    Paul