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Original question:

TXB0304: Float unused data pins?

TXB0304: input drive strength

John Garrett
Expert 4220 points

Part Number: TXB0304
Other Parts Discussed in Thread: TXB0104, SN74AXC4T245

I have a customer who is concerned about meeting the following drive strength requirements:

Input Driver Requirements

Typical IIN vs VIN characteristics of the TXB0304 are shown in Figure 2. For proper operation, the device driving the data I/Os of the TXB0304 must have drive strength of at least ±3 mA.

 Their FPGA IOs are only good for 2mA. Please recommend a driver?

Thanks, John Garrett, TI-AFA

  • 0
    TI__Genius 16735 points

    Hello,

    What are the key parameters you are looking for? The TXB0104 can be used instead of the TXB0304 with a lower drive strength if that is a possibility for you.

    If this is truly a bidirectional signal then a unidirectional driver will not work. If each line is not bidirectional, you might want to use a device like the SN74AXC4T245.

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    TI__Expert 4220 points

    Thanks Karan!

    One other issue has come up wrt simulation:

    A simulation was done for TXB0304 and the ringing is really bad. I can provide the simple schematic and the ringing waveform in a separate email. If you change R1 to be higher, the ringing is better  but the rising edge is still not monotonic and has a very long rise time.  The trace is set to be really short in the simulation but it could be much longer on the customers board.  So a few questions:

    1.  Is this ringing expected?

    2.  If so, what can be done to reduce it?

    3.  How long a trace can this part drive?

     

    Thanks, John Garrett   TI-AFA

  • 0 in reply to John Garrett
    TI__Genius 16735 points

    Hi John,

    As mentioned, the TXB devices don't have much drive strength. The ringing occurs because the device weakly drives the outputs and if there are any reflections, then the one-shot gets triggered and the device starts changing direction when it shouldn't. This causes the ringing to occur.

    PCB signal trace-lengths must be kept short enough so that the round-trip delay of any reflection is less than the one-shot duration, approximately 10 ns, ensuring that any reflection encounters low impedance at the source driver.

    Short trace-lengths should be used to avoid excessive loading.The TXB device is autobidirectional device which doesn’t have enough drive strength to overcome excessive resistive / cap loading so traces should be kept as short as possible.

    Please provide me with the schematic but I am assuming R1 is a series resistor and its most probably improving impedance matching when increased.

    Thanks!
    -Karan

  • 0 in reply to Karan Kotadia
    TI__Expert 4220 points

    Thanks Karan,

    You are correct in that R1 is a series resistor added.

    Since the TXB devices require short trace-lengths (how short?!,) I guess the most pertinent question would be what part can we recommend, with similar functionality, for longer trace lengths?

    Thanks again & regards,

    John Garrett,  TI-AFA

  • 0 in reply to John Garrett
    TI__Genius 16735 points

    Hi John,

    There aren't any auto bidirectional translators with a stronger drive than the TXB family. Your only option is to use a direction controlled translator like something in the AXC family like the SN74AXC4T245.

    Thanks!
    -Karan