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DS90UB960-Q1: Power Supply Decoupling Capacitor Noise Frequencies

Liban Hussein1
Prodigy 460 points
Part Number: DS90UB960-Q1

Hello,

I wanted to inquire regarding the power supply decoupling caps used for the DS90UB960. There are recommendations for their values (ranging from .01 uF-10uF), but I need to calculate the frequencies where noise is generated as well as those frequencies in which the capacitors actually operate. Although I have the supply noise values, values like the ESR and ESL are still missing from the datasheet, making it a bit more difficult to find these frequencies.

Link to Datasheet for 960 Deserializer:

http://www.ti.com/lit/ds/symlink/ds90ub960-q1.pdf

Thanks,

Liban H.

  • 0
    TI__Mastermind 30515 points

    Hello Liban,

    thanks for detailed question on power supply circuit in ub960.

    As you talked here, the power supply circuit is one complicate design. Yes, you are right you need consider the components' parameters, also you need consider the on-board power device noise, such as switching noise, ground bounding noise etc., to select the better components with proper ESR/ESL, filtering the supply noise close to UB960's supply pins. In TI's d/s, we provide one refer. circuit design on power supply pins, covering from low freq. and high freq. capacitors to bypass the potential noise in our supply pins, pls note this just is used for reference. The customer can design more high performance or more cost effective power supply circuit for UB960, but anyway the key point is that the on-board power supply noise should meet our d/s request as mentioned in page8.

    best regards,

    Steven

  • 0 in reply to Steven(Shenzhen) Shi
    TI__Mastermind 30515 points

    Also, as ub960 is used for high speed serial link, the less power supply noise, the high reliable the high speed serial communication.

    best regards,

    Steven

  • 0 in reply to Steven(Shenzhen) Shi
    TI__Prodigy 460 points

    Hi Steven,

    I appreciate the response. Although your answer clarifies the issue of capacitor selection(and that the datasheet requirements for the on-board power supply noise), I'm having trouble finding and calculating these frequencies for the decoupling capacitors.

    If I could get a better sense of how I can go about finding the frequencies where noise occurs, as well as the operating frequencies of the capacitor, that would help greatly. I've tried looking at the capacitor specifications from websites like DigiKey to get a general sense of the operating frequencies, but have no idea how that calculation can be done using information from the datasheet.

    Thanks,

    Liban H.

  • 0 in reply to Liban Hussein1
    TI__Mastermind 30515 points

    Liban,

    I try to understand your issue, please check below answers:

    UB960 is the sink for power supply design, why ub960 has power supply noise request is that the noise would impact the internal high speed module operation, so we use "PI" filter circuit to reduce the power noise.

    For the noise freq., as I mentioned, the general freq. components include the ripple from the on-board DC-DC, also the on-board digital crosstalk, ground bounding, but all these noises are dependent on your board design. For TI, we recommend the 10u to reject the potential on-board low freq. noise, and 0.01uF/1nf to filter the potential on-board high freq. cross-talk noise.

    does this answer your question?

    best regards,

    Steven

  • 0 in reply to Steven(Shenzhen) Shi
    TI__Prodigy 460 points

    Hi Steven,

    This clarifies things very nicely. This explanation suits the need I have, but I wanted to see if I could find specific values for the 960. You mentioned that these noises are dependent on the board design and that TI recommends specific decoupling capacitors for specific reasons.

    This might be a long shot, but if I wanted to use the 10uF and the .01uF, is there any way I could calculate the noise generated for different supplies like VDDIO, VDDL1, etc? Or would more information be required on my part?

    Thanks,

    Liban H.

  • 0 in reply to Liban Hussein1
    TI__Mastermind 30515 points

    Liban,

    I attached one example on FPD-Link's power supply design which shares the same design as UB960 for your reference.

    https://www.ti.com/tool/PMP9758?keyMatch=power supply noise filter&tisearch=Search-EN-Everything#technicaldocuments

    pls note: To get good results for output voltage noise of the converters (this is due to many analog devices have power supply noise request), it is important to select appropriate components for the critical passive components of the step-down converter. Critical components are the capacitor on the input and the output. At the input it is most important to use capacitors which are still behaving like capacitors at the corner frequency of the switching edges of the on-board converter. It is good practice to use different sizes of capacitors in parallel to achieve filtering at a wide bandwidth. For the inductor, it is important to have a properly shielded structure to avoid interference of the inductor current with other parts of the circuit.

    regards,

    Steven