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TMS320F280039C-Q1: Crystal Oscillator Load capacitance

Huihuang Chen
Expert 4010 points
Part Number: TMS320F280039C-Q1

Tool/software:

Hi Champ,

From the datasheet is CL1,CL2 is 12-24pF.

My customer want to the failure mode for below case:

1.CL1,CL2  is 6pF,

2.CL1 ,CL2 is 33pF.

  • 0
    TI__Expert 4010 points

    Any update on this post?

  • 0 in reply to Huihuang Chen
    TI__Expert 7965 points

    Hi Chen,

    Thanks for patience. Since most of the team is out of office this week please expect a delayed response.

    Crystal shunt capacitance (C0) and effective load capacitance (CL) greatly influence the negative resistance of the electrical oscillator. Rneg (negative resistance of the electrical oscillator) can be viewed as the overall gain of the circuit which affects the start-up time and oscillation sustenance of the crystal.  

    CL also affects ESR (effective series resistance). Higher the ESR, the lower the Q, and less likely the crystal will start up or maintain oscillation.

    If the CL is too high the crystal then Overall circuit gain is too low and Crystal Takes a Long Time to Start Up
    On the other hand, If the effective load capacitance is lower than the target, the crystal frequency will increase and vice versa

    The values stated in datasheet are for user guidance only, ultimately the user should  rely on crystal manufacturer completely to characterize the crystal with their board to ensure the crystal always starts up and maintains oscillation since the crystal and board parameters affect the performance significantly.

    Thanks

  • 0 in reply to Prarthan Bhatt
    TI__Expert 4010 points

    Prarthan,

    Hard to understand below content, could you explain detail.

    If the CL is too high the crystal then Overall circuit gain is too low 

    So If the CL is too high, the crystal frequency will decrease ?

    If CL is too high, the crystal already start up, during running stage , could cause the oscillation to stop or not ?

  • 0 in reply to Huihuang Chen
    TI__Expert 7965 points

    Hi Chen,

    Please refer to the section 6.12.3.4 XTAL Oscillator in datasheet.


    Huihuang Chen said:
    If the CL is too high the crystal then Overall circuit gain is too low 

    >>>Please read the whole statement that I made referenced below, overall circuit gain impacts the start-up time and continuous crystal oscillation sustenance.

    Prarthan Bhatt said:
    If the CL is too high the crystal then Overall circuit gain is too low and Crystal Takes a Long Time to Start Up

    Thanks

  • 0 in reply to Prarthan Bhatt
    TI__Expert 4010 points

    Prarthan,

    I don't think  i got the right answer , can i consider there are some mistype for  word  "If the CL is too high the crystal then Overall circuit gain is too low "?

    Can you correct  it if there have some mistype?

    Can you just reply on below question? instead let's me guess what your means!

    So If the CL is too high, the crystal frequency will decrease ?

    If CL is too high, the crystal already start up, during running stage , could cause the oscillation to stop or not ?

  • 0 in reply to Huihuang Chen
    TI__Expert 7965 points

    Hi Chen,

    Its not a mistype.

    When CL is too high, crystal might take a long time to start-up and even if it starts chances are there that it might not sustain

  • 0 in reply to Prarthan Bhatt
    TI__Expert 4010 points

    Prarthan,

    How about below question?

    So If the CL is too high, the crystal frequency will decrease ?