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LMK6C: LMK6C and LMK00105 schematic review

tee_user
Prodigy 180 points
Part Number: LMK6C
Other Parts Discussed in Thread: LMK00105,

hello,

ti suggest to use an external crystal oscillator instead an crystal for the lmk00105.

that is the reason why i changed the schematic, from crystal to the lmk6c.

can someone please check my schematic, is something wrong or i have missed?

thanks a lot

  • 0
    TI__Genius 12380 points

    Hello, 

    Thanks for sending over your schematic, see my feedback below:

    1. R257 can be removed or replaced with a 0 Ohm resistor since the LMK6C output impedance is already 50 Ohms 

    2. Please ensure that C211 is placed close to the VCC pin of LMK6C 

    3. Each VDD/VDDO pin of LMK00105 should have it's own bypass capacitor placed as close as possible to the device pin, so I would recommend adding 2 extra 100pF capacitors for the 2.5V supply rail. 

    4. The 22 Ohm series resistors on the LMK00105 output (R19,R20,R231,R232) are also not needed since the output impedance is 50 Ohms. 

    Besides that, I think everything else looks good. 

    Regards, 

    Connor

  • 0 in reply to Connor Lewis
    Prodigy 180 points

    Hello,

    thanks for the answer, sorry for the late reply.

    1) Resistor is removed

    2) is very close placed to lmk6c

    3) I added two more capacitors

    I have some follow-up questions:

    4) i don't need to be afraid of reflections without the resistors? (i may revise the circuit diagram, but i am not the creator).

    Thanks

  • 0 in reply to tee_user
    TI__Genius 12380 points

    Hello, 

    If the LMK00105 will be driving a trace or transmission line that has a 50 Ohm impedance, then the output driver is already well matched and I wouldn't expect reflections to be an issue. However, if the trace impedance is above 50 Ohms then adding series resistor would help with matching and could reduce reflections. See section 8.1.2 and 8.2.1.1 of the datasheet for reference: 

    Regards, 

    Connor