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TPS65950 Data Sheet Questons on CXin and CXout

Brian Narveson
Prodigy 15 points
Other Parts Discussed in Thread: TPS65950

The following questions have been asked by a customer

1)  What is the significance of specifying the "Nominal load cap on each oscillator input CXIN & CXOUT" (Table 12-4 of the Data sheet)?


2)  Must I keep this Nominal load cap within 9 pF to 12.5 pF as specified in Table 15-1 of the data sheet?

3)  Within Figure 12-5 of the data sheet, is the internal capacitor on each input (Cint) shown?

4)  Are the Cint caps really 10 pF?

  • 0
    TI__Genius 15100 points

    These cap values are required and depend on the Cload spec of the crystal.
    If these values aren't corrent 32K osc will not generate a clk and system will not start.
    Generally Cint and Cpin are can be ignored. I haven't seen Cint specs in data sheets for most of the crystals.

    e.g. If Cload spec is 12.5pF then the capacitor value should be 25pF, can use the standard 22pF capacitor.

    Also see info in - http://community.ti.com/forums/p/2897/10797.aspx#10797


  • 0 in reply to Gandhar Dighe
    Prodigy 60 points

    Cint and Cpin are with respect to the TPS65950 and, according to the TPS65950 data sheet, are 10 pF (typ) and 1 pF (max), respectively.

    Why can Cint generally be ignored (Cpin is more understandable)?  The difference is 10 pF on each crystal input dependent on the use of Cint (i.e., considered or ignored (see below)).

    2 x 12.5 pF - (0) (ignoring Cint and Cpin) = 25 pF

    2 x 12.5 pF - (10 pF) (ignoring Cpin) = 15 pF

    What is the range for Cint (i.e., Min. and Max.)? 

     

  • 0 in reply to John Schaefer
    TI__Genius 15100 points

    Hi John,

    The calculation resulting in 15pF is correct. You will not have any issues if you use 15pF. We dont have a min and max range for Cint, you can consider Cint as a typical value.

    Hope this helps.

     

    Regards,

    Gandhar.