SCEA147 September   2024 TXB0104 , TXB0302 , TXB0304

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2TXB-Type Architecture Differences
    1. 2.1 TXB0104 and TXB0304 Comparison
    2. 2.2 Recommended Device Selection
  6. 3Common Design Challenges
    1. 3.1 Distributed Load (Transmission Line Effect)
      1. 3.1.1 Cable Length Impact - Bench Findings
    2. 3.2 Impedance Matching
      1. 3.2.1 Bench Setup and Results
    3. 3.3 Lumped Load
  7. 4Case Study
  8. 5Summary
  9. 6References

Summary

  • Devices with one-shots (such as TXB) are sensitive to reflections
    • To reduce reflections, use shorter connectors/ design with shorter PCB traces.
    • Properly design for appropriate impedance matching, dependent on the system's transmission line.
  • TXB0304 can be mistaken as a weaker device (than TXB0104 or competitor devices).
    • This is due to TXB0304's stronger output impedance with a faster rise/fall time.
    • Such faster transition times are more sensitive to transmission line effects.
  • As an example, Section 3 shows TXB0304 with stronger transient drive strength - rise time < 1.3ns (with overshoot) and TXB0104 rise time up to 3.7ns with little to no overshoot.

Design Considerations:

  • Design with minimum PCB traces/connectors to reduce likelihood of transmission line effects
  • Section 3 shows the TXB0104 and TXB0304 both used with a 50 Ω transmission line
  • 0Ω series resistors can be populated at the outputs of TXB-Type Devices and swapped out later for dampening purposes
  • Appropriate impedance matching is recommended
  • TXB0104 (with a 1.8 V output impedance- 70 Ω ) matched closer to the 50 Ω transmission line then the TXB0304 with 5 Ω
  • The device with more impedance mismatch yielded more reflections
  • Using a typical series dampening resister to match closer to the transmission line resolves the reflections. For more information, see [FAQ] Can I estimate appropriate dampening resistor value for level-shifter outputs?