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TXB0104: Seeing unusual behavior

Harsh b
Intellectual 430 points
Part Number: TXB0104
Other Parts Discussed in Thread: TS3DDR3812, TXS0104E

Tool/software:

Hey Team,

In the circuit we utilized for one of our industrial sensor read kits, we incorporated a TXB device as a bidirectional buffer. According to the datasheet, we can use pull-up or pull-down resistors greater than 50K, so we’ve implemented a 100K pull-up on the SEL_LINE pin.

Now, when the switch is in the OFF position, we're not observing any voltage on the "SEL_LINE" or "SEL_LINE_OUT" pins. Given that we have a pull-up in place, we would expect to see a 3.3V output, right? What's happening here? Is there any internal Pull up or Pull down on TS3DDr device?

1. Seeing this on multiple Boards

2. No assembly issue

  • 0
    TI__Mastermind 23651 points

    Hi Harsh,

    Yes your understanding is correct. When OE of TXB0104 is driven low to GND, we can expect the IOs to be in HIGH-Z state with external 100kohm pullup with determine the voltage on the SEL_LINE without any influence on SEL_LINE_OUT side. What is the expected vs. the actual voltage you are measuring? What is the state of the TS3DDR3812? 

    Harsh b said:
    Now, when the switch is in the OFF position, we're not observing any voltage on the "SEL_LINE" or "SEL_LINE_OUT" pins. Given that we have a pull-up in place, we would expect to see a 3.3V output, right? What's happening here? Is there any internal Pull up or Pull down on TS3DDr device?

    Looping in the correct team supporting this device to help clarify. 

    Regards,

    Jack

  • 0 in reply to Jack Guan
    TI__Genius 14410 points

    Hi Harsh,

    Happy to help here with the TS3DDR device.

    Can you tell me more about "we're not observing any voltage on the "SEL_LINE" or "SEL_LINE_OUT" pins"?

    Like is the voltage reading you are seeing  on the sel line 0V?

    Please let me know and we can figure this out.

    Regards,

    kameron

  • 0
    Guru 272430 points

    The TXB is not a buffer; its outputs have a drive strength if 0.02 mA. (In order to allow bidirectional signals, the input and output buffers on both sides are always active, and external devices are allowed to override the voltage at the pin.)

    A pull-up of 50 kΩ or more is weaker than the TXB's output and has no effect.
    A pull-down of 1 kΩ might not be strong enough because it cannot sink 2 mA at the switching threshold near 1.65 V; see section 8.3.2 of the datasheet. It should be 820 Ω or less.

    The TXB is not suitable for this application. The TXS0104E would work better (and it already has internal pull-ups).