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TXB0104: TXB0104 with open drain input

Ruixin Feng
Intellectual 2435 points
Part Number: TXB0104
Other Parts Discussed in Thread: TXU0104, SN74LVC3G17, TXS0104E

Tool/software:

Hi team,

customer use TXB0104 for 5V to 3.3V level shift. The input signal is from optocoupler encoder and it's a open drain signal. I know that TXB0104 is not suggested to used in open drain signal, but customer feedback when they test, at first, TXB0104 can work for a while but after some time, it can't work normally. customer want us to help explain the reason. could you pls help with it? Thanks!

The encoder diagram is like below:

 

customer schematics:

Thanks!

Rayna

  • 0
    Guru 317480 points

    The reason is that the TXB does not work with open-drain signals; the pull-up does not have enough drive strength to override the TXB's output.

    Use the TXU0104 instead. For translating to a lower voltage, you could also use any buffer with overvoltage-tolerant inputs, e.g., SN74LVC3G17.

  • 0
    TI__Intellectual 2435 points

    Hi Clemens,

    I double checked with customer, the signal from optocoupler is not open drain signal like I2C. In optocoupler, there is a switch, when switching, it outputs high voltage and low voltage. And in customer application, it's a single direction application. To give enough drive strength, customer use a pull up resistor at input of TXB0104 to support 5V/3k=1.6mA drive current. is that correct?

    Rayna

  • 0 in reply to Ruixin Feng
    Guru 317480 points

    Your schematic shows phototransistor optocouplers with open-collector output.

    A 3 kΩ resistor is too weak; the TXB requires a drive strength of at least 2 mA near the switching threshold of 2.5 V.

    If you want a pin-compatible device, try the TXS0104E.