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TMS320F280039C: XTAL CL1/CL2 calculate question

Julia Li
Expert 4755 points

Part Number: TMS320F280039C

Tool/software:

Hi Champs,

In datasheet 6.12.3.4.4 How to Choose a Crystal it says:

3. Check that the load capacitance requirement of the crystal manufacturer is within 6 pF and 12 pF per specifications for 20 MHz

https://www.ti.com/lit/ds/symlink/tms320f280039-q1.pdf?ts=1750045762268&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252Fzh-cn%252FTMS320F280039-Q1

So I think CL's recommended value is 6-12pF, and customer choose 10pF.

CL = [CL1]/2 + Cstray

If we think Cstray=5pF, CL1 will be 10pF, but datasheet require CL1's value is 12-24pF.

If CL chooses 6pF, the result calculated for CL1 is 2pF, still over-spec.

So how should we explain the conflict between the CL requirement of 6-12pF and the CL1 requirement of 12-24pF to the customer?

How should customers choose the value of CL/CL1/CL2?

Thanks!

Julia

  • 0
    TI__Mastermind 45080 points

    Hi Julia,

    In the spec, there are two CL values depending on the max ESR of the crystal.  For 20MHz, the crystal load capacitance CL is right around 6pF if max ESR is 45 ohms and CL is right around 12pF if max ESR is 12pF.  If customer assumes a stray of 5pF, CL1=CL2=2pF for crystal with max ESR of 45ohms.  This will meet the 6pF total load cap requirement where CL = ((2pF)*(2pF))/(2pF+2pF) + 5pF = 6pF for a crystal with max ESR of 45ohms.  If crystal has max ESR of 50ohms, CL1=CL2=14pF.  In this case CL = ((14pF)*(14pF))/(14pF+14pF) + 5pF = 12pF.

    Regards,

    Joseph

  • 0 in reply to Joseph Casuga
    TI__Expert 4755 points

    Hi Joseph,

    But in datasheet it also requires CL1 and CL2's value as below. 2pF is not in this range. If max ESR is 45ohms, should customer use 2pF CL1 and CL2?

    How should the customer choose CL1? Currently, if the range of CL is 6-12pF, then the calculated range of CL1 is 2-12pF. Should the customer use the calculated value (2pF-12pF) or the value specified in the datasheet (12-24pF)?

    Joseph Casuga said:
    CL is right around 12pF if max ESR is 12pF

    I think this should be max ESR is 50 ohms?

    Thanks!

    Regards,

    Julia

  • +1 in reply to Julia Li
    TI__Mastermind 45080 points

    Hi Julia,

    Yes, statement below should be (sorry for the typo):

    CL is right around 12pF if max ESR is 50ohms.

    CL is the total load capacitance as seen by the crystal so this includes the stray capacitance plus the actual capacitances seen at X1 and X2 pins.  In the example I highlighted above where CL =  6pF (which is CL1=CL2=12pF including stray capacitance to get a total series capacitance of 6pF), the load capacitance at X1 and X2 should be 2pF to satisfy the equation.  I know it is confusing at first but think of CL as the total load capacitance which takes into account X1/X2 added capacitance for the actual components and the 5pF stray.

    Regards,

    Joseph