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Original question:

LMH1983: PLL1 Loop Filter Capacitor Variance

LMH1983: Unlock issue depending on the loop filter capacitance

Yoshikazu Kawasaki
Intellectual 505 points
Part Number: LMH1983

Hello,

My customer seems to have the same issue as reported before by this.

https://e2e.ti.com/support/clock-and-timing/f/clock-timing-forum/945591/lmh1983-pll1-loop-filter-capacitor-variance

My customer has been using the same PCB with LMH1983 for >7 years on several products, but they have never had unlock issue in the past.  They've started using the same PCB on a new product since 2019, but they've never had unlock issue until quite recently.  It seems if they swapped the 47uF tantalum to a another one, it has fixed the unlock issue, so I'm asking them if the unlock issue will go with the capacitor or the combination of capacitor and LMH1983, but do you have any more information from the above post?  What items (i.e. capacitance tolerance, ESR etc) would be required for the 47uF capacitor?

Best Regards,

Yoshikazu Kawasaki

  • 0
    TI__Mastermind 28530 points

    Kawasaki-san,

    I apologize for your customers troubles.

    Can you confirm that the output frequency is locked but the lock detect is reporting an unlock condition?  My understanding is these problems are associated with the leakage of tantalum capacitors.

    What is the voltage rating of the tantalum capacitor?  A higher rating may help improve the leakage.

    Which PLL is unlocked?

    What is the charge pump gain setting?  Can that be increased?

    73,
    Timothy

  • 0
    Intellectual 505 points

    Hello Timothy-san,

    Thank you very much for your quick reply.

    It seems the FW of their actual products doesn't have access to the registers, so they see NO_LOCK pin if it is locked or not.  They use tantalum capacitor of 10V ratings.  They think PLL1 is unlocked because they saw stable output of video clock(148.35MHz).  They can icnrease the gain settings of the cahrge pump if they enable the FW access to the registers, but their current one is 0.000262238.

    They confirmed the issue goes with the tantalum capacitor.  If the temperature is higher, the issue happens more often than room temperature, so it may be related to the leakage current as you mentioned.  Do you think the root cause would be the leakage current of the tantalum capacitor?  What would you need more from the customer?

    Best Regards,

    Yoshikauz Kawasaki

  • 0 in reply to Yoshikazu Kawasaki
    Intellectual 505 points

    Hello Timothy-san,

    My customer would like to fix the issue by replacing the tantalum capacitor with lower leakage current, but do you agree with their solution?  Note that their current one has leakage current of 24uA and the one they'll use does 4.7uA.

    Do you have requirement for the leakage current of the 47uF tantalum capacitor?

    Would you please tell me the requirements of the 47uF tantalum capacitor other than the leakage current?

    Best Regards,

    Yoshikazu Kawasaki

  • 0 in reply to Yoshikazu Kawasaki
    TI__Mastermind 28530 points

    Hello Kawasaki-san,

    I don't have data on the leakage current, but I think this is a good approach.  However we have also had good success in resolving these types of issues by increasing the lock window detect and increasing charge pump value.  So both techniques could be combined.

    Please also refer to this thread.

    73,
    Timothy