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LMK00334-Q1: If I don't use REFout output pin, can I reduce decoupling capacitors for Vccoc.

Koichi Kajihara
Prodigy 50 points
Part Number: LMK00334-Q1
Other Parts Discussed in Thread: LMK00334

I plan to use LMK00334RTVTQ1 for as clock buffer for PCIe2.0.
The datasheet teach me that I shold place 1-uF to 10-uF decoupling capacitors for each supply pin.
However, my production boards don't have sufficient implementation space.
If I don't use REFout output pin, can I reduce decoupling capacitors for Vccoc.
Please teach me about the impact of the method.

Thanks.

  • 0
    TI__Mastermind 33370 points

    Hello Kajihara-san,

    If the REFout output is not used, you may place a 0.1µF decoupling capacitor for VCCOC alone. The wording of the datasheet is unclear, so let me clarify: 0.1µF is required close to each VCC pin, but 1µF and 10µF can be common to all supplies of the same voltage, as long as they are somewhere near the device.

    In practice, the impedance of 1µF and 10µF capacitors is usually only helpful to reject noise below 10MHz. Here are some situations where 1µF and 10µF are useful:

    1. If the VCC of the buffer is derived from a switching supply, 1µF and 10µF at the LMK00334 helps reject ripple
    2. If the VCC of the buffer is derived from an LDO, 1µF and 10µF at the LDO can help improve ripple rejection of the whole supply rail (as long as the LDO is stable with this capacitance); if one of the other criteria in the list is met, 1µF or 10µF at the LMK00334 may still be useful
    3. If the frequency of the LMK00334 I/O is <10MHz, 1µF and 10µF at the LMK00334 supply pins helps prevent buffer noise from contaminating the VCC rail for other devices
    4. If other devices on the VCC rail have digital switching noise below 10MHz, 1µF and 10µF at the LMK00334 helps reject ripple

    If you are confident that your supply and ground connections to the LMK00334 have good noise rejection below 10MHz, the 1µF and 10µF near the device are ultimately not going to contribute much to the design and can be omitted to reduce footprint. 

    Regards,