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TXB0104: Can I use this Level Shifter for translating UART signals from 1.8V to 5V? What is the output impedance of pins in this case?

doner_t
Intellectual 645 points
Part Number: TXB0104
Other Parts Discussed in Thread: TXU0204

Hello, 

I have a three questions related to TXB0104 Level Shifter;

  1. Can I use this Level Shifter for translating UART signals? (TXD, RXD)
  2. Can this level shifter make translation process between VCCA=1.8V to VCCB=5.0V ? 
  3. If TXB0104 makes shifting from 1.8V to 5.0V, what is the output impedance of pins in this case? Will this output impedance make some reflection problems when I use a trace impedance 50ohm? Some output impedances are given for some other level translation scenarios in the page 17 of datasheet, but output impedance was not given for the case of level shifting from 1.8V to 5.0V. 

Best Regards,

  • 0
    Guru 316510 points

    Yes, the TXB can be used for some UART signals. But please note the restrictions described in sections 8.3.2, 8.3.3, and 8.3.5.

    The DC output impedance is 4 kΩ. When switching, the output impedance depends only on the output supply voltage, and the values on p. 17 are ranges. At 1.8 V, you have both 70 Ω and 50 Ω, so let's assume 60 Ω; at 5 V, you have 40 Ω.

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 45725 points

    Hi Doner,

    I recommend the TXU0204 device for this instead, its more suitable for translating the unidirectional signals of UART. If you're concerned with transmission line effects, I would have to assume you are intending on having a long trace/cable length and the TXU0204 device would be better suited for that.

    Here is an FAQ covering impedance matching.

  • 0 in reply to Clemens Ladisch
    Intellectual 645 points

    Thank you. In fact, I was worried about the whether TXB can shift signals from 1.8V to 5V. But now I think that it can translate the signals from 1.8V to 5V without problems, right?

    Output impedance of IO pins for 1.8V ( 60 ohm) and 5V (40ohm) sides are seen not much different from 50ohm PCB trace. So I dont expect reflections problems. Thank you. 

    Best Regards,

  • 0 in reply to Dylan Hubbard
    Intellectual 645 points

    Dear , 

    Thank you for your answer. 

    I was only worried about the output impedance would be very different from 50 ohm PCB trace impedance, when the case of level shifting from 1.8V to 5V. But It seem that output impedance of IO pins for 1.8V ( 60 ohm) and 5V (40ohm) sides are seen not much different from 50ohm PCB trace. Trace length will probably not exceed the 15cm. I have read impedance matching page, thank you. It is very useful. 

    Best Regards, 

  • 0 in reply to doner_t
    TI__Mastermind 45725 points

    Hey Doner,

    I'd recommend reviewing this section:

    The outputs are a little more complex than what you are expecting due to the AC acceleration. TXU0204 provides a more traditional buffered output with similar impedance values and would be more robust in this circuit.