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SN74CB3Q3345: Cin range confirm

Max Guan
Intellectual 1350 points
Part Number: SN74CB3Q3345
Other Parts Discussed in Thread: TS3A27518E, SN74CBT3125

team,

In notebook application we noticed that for SPI bus switch Intel spec the Cin as <3pF. sn74cb3q3345 has a typical of 2.5pF while 3pF as the maximum. 

Will it be suitable for the application here? And by the way is the 2.5-3.5 variation comes from part to part difference or some specific application parameters related?

Thanks

Max

  • 0
    TI__Intellectual 1350 points

    And also I notice TS3A27518E could be an alternative as a Mux switch with Cin typical of 2pF in datasheet. 

    Would you also help to comments which one is more suitable for the application here please?

    Thanks

  • 0 in reply to Max Guan
    TI__Genius 10974 points

    Hi Max,

    I would like to get more information on this SPI application:

    1) What is the frequency SPI will be running at?

    2) Is this 3.3V or 5V SPI?

    3) How many devices do you need to control with the SPI bus?

    4) "CIN" refers to the input capacitance of the control inputs, not the SPI signal inputs. I am assuming the Intel spec refers to the multiplexer inputs where the SPI signals will be fed into (so CON would be the parameter of interest). In this case, these devices range from 3.5pF typical (3Q3345) to 21.5pF (27518E).

    If you could confirm the above, I can help recommend the best part that would work here.

    Thanks!
    Bryan

  • 0 in reply to Bryan Marshall
    TI__Intellectual 1350 points

    Hi Bryan,

    Appreciate your reply. 

    For you questions (1) and (3) I'm not able to give a accurate answer now as I will need further checking with my customer. 

    (1) It's the data rate for the SPI ROM for the EC and Bios for the PC application. Not sure if this could help you figure out the data rate. 

    (2) 3.3V is used

    (3) Should be 2.

    If possible please consider the parameters of 3Q3345 as preferred and help to recommend some device with smaller Cin spec and that would be helpful. 

    Thanks

    Max

  • 0 in reply to Max Guan
    TI__Genius 10974 points

    Hi Max,

    I would need confirmation on the datarate/frequency to give the best possible device here. Also, can you confirm you are constrained by the CON parameter (input capacitance for the mux channel) and not the CIN parameter (input capacitance for logic pins).

    Please let me know about these inquiries and anything else you hear back from the customer and we can go from there.

    Thanks!

    Bryan

  • 0 in reply to Bryan Marshall
    TI__Intellectual 1350 points

    Hi Bryan,

    The SPI clock rate support 20M/33M/50MHz.

    Customer is not really sure about Intel definition on CIN (marked FET component criteria). 

    SN74CBT3125 is used on Intel reference design so I would assume it meet intel requirement and we only need a 8-channel version of it. 

    Thanks

    Max

  • 0 in reply to Max Guan
    TI__Genius 10974 points

    Hi Max,

    Thanks for providing these details!

    If the reference design used the SN74CBT3125, then the SN74CB3Q3345 would be suitable as the 8-channel 1:1 alternative. Has very low CON/CIN and can meet the bandwidth requirement without issue (500MHz for this device).

    Lastly, the variation you see from the capacitance parameters come from the manufacturing process. Every device will be slightly different, the maximum value just states how much variation is possible over the process.

    Thanks!

    Bryan