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LMK05318: LF pin capacitor

MINKYUNG KIM
Expert 1250 points
Part Number: LMK05318
Other Parts Discussed in Thread: LMK04828,

Hi team,

I wonder the purpose of the external capacitor on LF1/2 pin. The loop filter is internally programmable, isn't it?

A device with dual APLL loop, such as LMK04828, has a output degrading issue due to the change in loop filter value with environment, because it has external RC filter. Is the degradation from the same reason expected due to the external capacitor on LF1/2 ? Thank you. 

  • 0
    TI__Genius 11900 points

    Hello Minkyung,

    The LMK05318 has a partially integrated LF within the IC. The internal LF components can be set via registers, however the device also requires an external C2 capacitor as shown in the image you are referring to.

    When you say output degradation two things come to mind, phase noise differences with changes in the environment such as temperature and instable loop filters.

    If you follow the recommendations on the LMK05318 datasheet (see below) and use X5R and X7R capacitors I do not expect either of these issues will be a problem.

    "External Loop Filter Capacitor for APLL1 and APLL2. Place a nearby capacitor on each pin. For LF1, 0.47-µF is suggested for typical APLL1 loop bandwidths around 2.5 kHz. For LF2, 0.1-µF is suggested for typical APLL2 loop bandwidth around 500 kHz."

    Thanks,

    Vibhu