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TXB0104 - OE pulldown resistor value selection

jerrry
Prodigy 110 points
Other Parts Discussed in Thread: TXB0104, TXB0104-Q1

Hi,


I am using TXB0104 for shifting signals from VCCB = 3V3 to VCCA = 2V8.

While reading the datasheet there came up two questions.

1.  Is it ok to connect a 10k resistor between VCCA and OE? I want to have the output permanently enabled and I've heard that it isn't so good to connect VCC and OE directly.

2. In the Datasheet txb0104-q1.pdf p.14 section 9.2, there is shown a typical application with a resistor between OE and ground. What is the dimension of this resistor? I think it is the pull-down to set the OE to low, but when I need my OE always on high, I think I can leave him out.
Am I correct?
 So I just need the resistor from question1, right?

Thannks alot for your help and tipps.

Best Regards

Jerrry

  • 0
    TI__Expert 6335 points

    Jerry - My feedback is below:

    jerrry said:

    1.  Is it ok to connect a 10k resistor between VCCA and OE? I want to have the output permanently enabled and I've heard that it isn't so good to connect VCC and OE directly.

    For your application (i.e. TXB0104 is permanently enabled), it is ok to tie a 10k resistor between OE and VCCA. It is also acceptable to short OE and VCCA.

    jerrry said:

    2. In the Datasheet txb0104-q1.pdf p.14 section 9.2, there is shown a typical application with a resistor between OE and ground. What is the dimension of this resistor? I think it is the pull-down to set the OE to low, but when I need my OE always on high, I think I can leave him out.
    Am I correct?
     So I just need the resistor from question1, right?

    In your application, you may short OE to VCCA.

    The purpose of the pulldown resistor is to allow an external driver to set OE to the required VIH level that enables the TXB0104. 

    For example, let's say that VCCA = 3.30V, this means that VIH_OE >= (0.65)*(3.30V) = 2.145V

    If the external drive connected to OE can only source 20 uA of current, then the pulldown resistance must be >= (2.145V)/(20uA) = 107.25k

    If the external drive connected to OE can source 1 mA of current, then the pulldown resistance must be >= (2.145V)/(1mA) = 2.145k

    Best Regards,
    Nirav

  • 0 in reply to Nirav Patel
    Prodigy 110 points

    Hi Nirav,

    thank you fir your respinse. They help me alot.

    One last question I still have.

    Is it ik to use both resistors?

    So, the 10k between VCCA & OE and the pull-down between OE & GND too?

    Thanks in advance.

    Jérôme

  • 0 in reply to Nirav Patel
    Prodigy 110 points
    Its me again.

    i would upload an image of my setup but can't find a link.
    I am not sure about my driving current.
    VCCA is 2.8V and is sourced by a linear regulator (MPQ20056) which supports up to 250mA output current.
    OE is connected to VCCA about a 10k resistor as said in the startpost.
    If I now want to calculate the Rpd for the OE, do I use the 250mA from the linear regulator?

    VCCA*0.65 = 2.8V*0.65 = 1.82V
    Ioutmax = 250mA

    Rpd = 1.82V/0.25mA = 7.28 Ohms.

    Do I now this resistance or did I fail on identify my current?

    Best Regards

    Jérôme
  • 0 in reply to Nirav Patel
    Prodigy 110 points
    Hi again,

    I changed my setup. Just use the 10k resistance between VCCA and OE. There is no need for the resistor between OE and GND. I use the TXB0104 in "always enabled" mode.
    I am correct I think.

    Best regards

    Jerrry
  • 0 in reply to jerrry
    TI__Expert 6335 points

    Jerry,

    From what I understand, your application will never need to have OE = Low. If this is correct, then you may leave the 10k resistor between VCCA and OE so that the TXB0104 is permanently enabled.

    Best Regards,
    Nirav

  • 0 in reply to Nirav Patel
    Prodigy 110 points
    That's exactly what I mean.
    Thank you very much.

    Best wishes!
    Jérôme