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BQ32002: Crystal resonator selection

hiroshi.yamada
Intellectual 575 points

Part Number: BQ32002

We are currently using the NDK crystal resonator NX3215SE-32.768K-STD-MUA-17.

According to the results of a matching test conducted by the manufacturer (NDK), the required negative resistance is 180 kohm, whereas our circuit only achieves approximately 40 kohm, which is insufficient to ensure proper operation.

We are using 18 pF external load capacitors for the crystal. Even when these capacitors are removed, the negative resistance only increases to about 130 kohm, which is still below the required level.

Do you have any recommended crystal resonators that are better matched to the BQ32002D?

Currently, the operating conditions are as follows:

  • Drive level: 0.1 µW
  • Frequency deviation: −89 dF/F (×10⁻⁶)
  • +1
    TI__Guru** 122400 points

    Hi There,

    Usually customers pick their crystal, we don't have recommendation on this. 

  • 0 in reply to Noel Fung
    Intellectual 575 points

    Hi Noel,

    Thank you for your previous response.

    I shared the information with the manufacturer, NDK.

    After reviewing it, NDK requested that we confirm once again whether there is any way to modify or optimize the effective load capacitance through the configuration or settings of the BQ32002.

    In particular, they are referring to the 12 pF load capacitance specified in the datasheet.

    Could this 12 pF load capacitance increase under certain conditions or due to any external/internal factors?

    If there is such a possibility, could you please advise on the potential causes and any recommended corrective actions or countermeasures?

    Thank you for your support.

    Best regards,

    Hiroshi Yamada

  • 0 in reply to hiroshi.yamada
    Intellectual 575 points

    Hi Noel,

    I wanted to follow up on this issue. Do you have any updates?

    I would appreciate it if you could reply in the comments when you have a chance.

    Thank you in advance for your help.

    Hiroshi Yamada

  • 0 in reply to hiroshi.yamada
    TI__Guru** 122400 points

    Hi Yamada-san,

    The only thing we can try is the calibration register, CAL_CFG.

    Set S = 1 or 0, set CAL to some values.

  • 0 in reply to Noel Fung
    Intellectual 575 points

    Hi Noel,

    Thank you for your suggestion.

    However, the issue is not frequency calibration.

    Our concern is the oscillator start-up margin.

    According to the NDK crystal specification, a negative resistance of at least 180 kohm is required.

    We measured the negative resistance and found it to be insufficient.

    To increase the negative resistance, we removed the external load capacitors connected to the crystal pins.

    However, even without the external capacitors, the negative resistance only reaches approximately 130 kohm.

    Since this is still below the crystal manufacturer's recommendation, we would like to know whether there is any method to reduce the oscillator's internal load capacitance or otherwise increase the negative resistance.

    We would appreciate your comments.

    Hiroshi Yamada

  • +1 in reply to hiroshi.yamada
    TI__Guru** 122400 points

    HI Yamada-san,

    That's why I asked you to try changing the calibration register to see if that help alternate the negative resistance.

    Other than this register, there is nothing we can do to change the device's characteristic. 

  • 0 in reply to Noel Fung
    Intellectual 575 points

    Hi Noel,

    Thank you for your additional comments.

    I understand your explanation.

    We will evaluate the suggested register settings and check whether they have any impact on the negative resistance.

    Thank you for your support.

    Hiroshi Yamada