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Pull up resistor array for the input or output of TXB0108

Berat Atmaca
Prodigy 30 points
Other Parts Discussed in Thread: LSF0108, TXS0108E, SN74LV4T125

My aim is to design an adapter for the connection of two devices. The schematics is attached. I got the PCB a few days ago and completed the soldering. Now, I am testing it. For digital signals, there is a voltage drop at the output of isolator (Just 0.3V at the output). Should I use a pull up resistor array for this? I think voltage level translator has high impedance.


Do you know what is the specific features to understand the necessity of the pullup or pull down resistors? How is the value is determined?

  • 0
    TI__Guru* 96681 points
    Thanks for your post. The appropriate apps engineer has been notified and will respond shortly.
  • 0
    TI__Mastermind 19545 points

    Hello Berat,

    What is the output impedance of the opto-coupler?  Is it open drain?  open collector?  push-pull?  If you let me know the part number I can look it up.  The TXB can't have less than 50k resistors for pull ups.  If you need a stronger pull-up you may have to look at the TXS family of parts.

    -Francis

  • 0 in reply to fhoude
    Prodigy 30 points

    I couldnt see it on datasheet. Its name is ACSL-6400 from Avago technologies.

    PS: I am also looking for other options but I get stuck between TXS0108e and LSF0108. Both supports open drain systems but I couldn't see any difference except for a few pin configuration.

  • 0 in reply to Berat Atmaca
    TI__Mastermind 19545 points
    I think you are going to need to look at different parts. When looking at the ACSL-6400 opto-isolator I saw that this part needs quite a bit of current to forward bias the primary side diode properly such that the secondary side transistor is turned on fully. The TXS0108 has limited drive capability. I think you will have to go with two SN74LV4T125. The SN74LV4T125 has 4 inputs and has the ability to drive higher currents that are needed to bias the optio isolator diodes.