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TMS320F28035-EP: TMS320F28035MPNTEP quartz crystal load capacitance

Asha C
Intellectual 385 points
Part Number: TMS320F28035-EP

Tool/software:

Hello,

As per datasheet , for 20MHz crystal - Load capacitance suggested is 8pF (considering stray capacitance as 2pF with CL1 , CL2=12pF)

But i do not find crystal which had load capacitance of 8pF and -55c temperature rating . Could you please provide your feedback on this.

I could find only one part ECS-200-CDX-0998 which is having  10pF load capacitance and -60c rated. Please provide the feedback.

Thanks!

  • 0
    TI__Mastermind 37760 points

    Hi Asha,

    CL specified in the datasheets are for the individual pins.  This is how its is specified in the 28035 datasheet and crystal manufacturers also specify the load cap for one pin.  ECS-200-CDX-0998 would meet the 28035 XTAL requirement since CL1=12pF = PCB stray of 2pF + CL of ECS-200-CDX-0998 of 10pF.

    Regards,

    Joseph

  • 0
    Intellectual 385 points

    But according to the formula CL(crystal)=(CL1*CL2)/(CL1+CL2) + Cstray of PCB(2pF)

    Then considering CL1=CL2=12pF , we get crystal load capacitance(CL) as 8pF. But i am not finding crystal with 8pF load capacitance which is -55c rated.

    Please provide your feedback on this!

  • 0 in reply to Asha C
    TI__Mastermind 37760 points

    Hi Asha,

    Please note that temperature range of F29035 is only down to -40C.  Are there crystals that meet the specs at -40C?

    Regards,

    Joseph

  • 0 in reply to Joseph Casuga
    Intellectual 385 points

    Hello Joseph,

    We are using TMS320F28035MPNTEP which is -55 degree rated. This is the reason we need to choose crystal with -55c rating.

  • 0 in reply to Asha C
    TI__Mastermind 37760 points

    Hi Asha,

    Sorry, I did not initially catch that you were using the extended temp range.  Unfortunately we do not have a recommended PN for the crystal at extended temp rating.  What you probably could do is check out if ECS-200-CDX-0998 will work for your system.  CLs are placed as typical values with some tolerances.  A small variation in CL may not impact the resonance frequency that much but the best way is to prove that this crystal still works in your system.

    Regards,

    Joseph