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Level shift question for SD card device.

David Wang32
Prodigy 120 points
Other Parts Discussed in Thread: LSF0108, TXS0206, TXS0206-29, TXS0206A

Our customer choose LSF0108 for SD card application, because the datasheet has SDIO application.

But when testing, they found the waveform of 3.3V SD_CLK is not good enough. Like picture.

Although it doesn't affect the function, they are afraid that it may cause problem after MP.

Does any reason cause the problem? 

The length of their design is as below:

             VCCA                              VCCB
CPU --------------> Level shift --------------> SD CARD connector
               8.4"                                   0.8"

They consider to use TXS0206 for option design, but there is no layout guide. Does TI have TXS0206YFPR layout guide?

Please help to advice, thanks.

  • 0
    TI__Intellectual 2530 points
    Hi
    I will answer the TXS0206 part. There is no specific layout guide but we can put one together if needed.
    As stated in the datasheet:
    TI recommends careful PCB layout practices with short PCB trace lengths to avoid excessive capacitive loading
    and to ensure that proper O.S. triggering takes place. PCB signal trace-lengths should be kept short enough
    such that the round trip delay of any reflection is less than the one-shot duration. This improves signal integrity
    by ensuring that any reflection sees a low impedance at the driver. The O.S. circuits have been designed to stay
    on for approximately 30 ns. The maximum capacitance of the lumped load that can be driven also depends
    directly on the one-shot duration. With very heavy capacitive loads, the one-shot can time-out before the signal is
    driven fully to the positive rail. The O.S. duration has been set to best optimize trade-offs between dynamic ICC,
    load driving capability, and maximum bit-rate considerations. Both PCB trace length and connectors add to the
    capacitance that the TXS0206-29 SDIO output sees, so it is recommended that this lumped-load capacitance be
    considered and kept below 50 pF to avoid O.S. retriggering, bus contention, output signal oscillations, or other
    adverse system-level affects.ThanksThanks
  • 0 in reply to Jennifer Joseph
    Prodigy 120 points

    Hi Jennifer, 

    Thanks for your response!

    So can you provide TXS0206 layout guide?

    Our customer need the layout guide, thanks.

    By the way, what is difference between TXS0206A and TXS0206?

    I know TXS0206-29 has internal LDO.

  • 0
    TI__Guru* 96852 points

    David,

    The LSF0108 is a simple FET switch based translation solution.  The output signal will follow the input signal up to where the FET shuts off (Vccb - Vgs), then the pullup will passively drive the output high.  The slow ramp seen on the high side of this waveform is the RC curve on the output line.

    If you wanted to stick with the LSF solution, there are two solutions.

    1.  Reduce the output capacitance (ie shorter traces, smaller capacitive load)

    2. Reduce the pullup resistance.

    Of course, either of those could cause other problems for you in the circuit.  The best solution for you might be to just go with another part.

  • 0 in reply to David Wang32
    TI__Intellectual 2530 points
    Hi David
    Yes, I can put one together but it will be a couple of days. Lets target 5/22 deadline. The guidelines will be similar to what was mentioned before, but I will attempt at a more comprehensive guideline.
    TXS0206 has an integrated EMI filter. TXS0206A does not have the EMI filter.
    Thanks
  • 0 in reply to Jennifer Joseph
    TI__Intellectual 2530 points

    Hi

     

    Please see attached for PCB guidelines. Please let me know if anything else is needed.

    Thanks.

     

    /cfs-file/__key/communityserver-discussions-components-files/151/0654.TXS0206_2D00_Layout.pdf

  • 0 in reply to Jennifer Joseph
    Prodigy 120 points

    Hi Jennifer,

    Thanks for your information.

    Is there any specific trace length limit on clkA and clkB side if using TXS0206?

    As I know, TXS0206 A version is newer than non A version, why the TXS0206 spec is better than TXS0206A?

    Is there any reason release the non EMI version?