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TXB0104: Transition point is different when ramping up and rolling down the input signal.

[FAE] Andrew Wang
Genius 10685 points
Part Number: TXB0104
Other Parts Discussed in Thread: SN74LVC1G34

Hi Team,

Customer uses voltage divider as below to convert signal. 

Customer found an strange thing would like to check with us.

Taking example of green line below.

Ramping up the input from 0V at B4. Up tp around 14V, the output A4 will become "1", at mean time the voltage dividing point (mid of R1814 & R1836) will change from 3.xV to 5V.

Then rolling down input towards to 0V, when B4 is 3~14V, the A4 remains at "1" , until B4 is below 2V, then A4 change to "0".

Customer would like to understand if this behaviour is expected? And how it works internally?

Thanks!

Andrew

  • 0
    TI__Guru* 97046 points

    Hi Andrew,

    Pull-up and pull-down resistors should not be attached to the TXB series of translators.

    This is the internal structure of the device (available in the datasheet):

    Note that there are 4kohm resistors in series with the output drivers (highlighted in yellow). This is to allow the I/O pins to be over-driven with ~3mA of drive strength.  Unfortunately, this also means that any external resistors will cause additional voltage division.

    Is the signal they are translating unidirectional? ie does it always go from B4 to A4? If so, there are many other options that will work better than the TXB0104.

  • 0 in reply to Emrys Maier
    TI__Genius 10685 points
    Hi,

    It's uni-directional.
    Customer wants to know if they want to stay with this device, can they modify the circuit so this strange behavior can be minimized?
    If there is no way we can fix it with this device, can you suggest one device that is easy to do such application?
    Thanks!

    Andrew
  • 0 in reply to [FAE] Andrew Wang
    TI__Guru* 97046 points
    For a single channel, I would recommend using the SN74LVC1G34. The inputs are over-voltage tolerant to 5.5V and the supply can be set to 1.8V for a 1.8V output. You can basically use any buffer to accomplish down-translation.

    The ULN2003 could be used for a direct translation (although the device acts as an inverter, so that would have to be taken into account).

    This down-translation can also be achieved by using a zener diode to ground and series resistor to the 30V input -- when the input is 0V, the zener will be 0V, when the input is 30V, the zener will clamp to it's rated voltage (4.7V could do the trick))

    If they are set on using the TXB, I would think any engineer could easily do the circuit analysis given the above circuit diagram (buffer with series 4kohm output resistor) to determine the correct resistor values. I can help out if they need it, but I would highly recommend changing parts to one of the above options.