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SN74AXC4T245: What is the resister values of internal pulldown resister connected to each signal pin?

Kazuya Nakai59
Guru 20620 points
Part Number: SN74AXC4T245
Other Parts Discussed in Thread: SN74AVC4T245

 Hello guys,

 One of my customers is considering using SN74AXC4T245 for their new products.

 They have the following questions. Could you please give me your reply.

 Q1, The follwoing URL says in case of SN74AXC4T245, very weak pulldown resister is connected to each signal pins.

        https://e2e.ti.com/support/logic/f/151/t/788628

        What is the resister values of internal pulldown resister connected to each signal pin?

        The customer is going to attach very small external pullup resister because some circuits input which connected to output pins is needed to be kept high level.

        So if the internal resister value is small, the external pullup needs to be small value to keep high level.

 Q2. Is the internal pulldown resister connected to GND anytime?

        Or is each pin connected to GND by the internal pulldown resister when either VCCA or VCCB is powered off only?

 Q3. What is "VCC Isolation" function?

       The device datasheet says that if either VCC Input is Below 100 mV, All I/Os Outputs are Disabled and Become High-Impedance.

      I think this function is implemented to SN74AVC4T245 too.

      Could you please tell me what is the different point? Is it the internal pulldown resister?

 Your reply would be much appreciated.

 Best regards,

 Kazuya.

  • 0
    Guru 268780 points

    1. This is not documented anywhere. The actual value appears to be about 100 kΩ.

    2. They appear to be active at all times.

    3. "High impedance" means that the output drivers are disabled (as if OE were high). The pulldowns at the input buffers are a separate feature.

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 24795 points

    Hello Kazuya-san,

    1. If they want the output state to default to high, it would be recommended to use the external pullup with a value <15kOhm (10kOhm is a good value)

    2. The pulldowns are active during power up and power down, and are present when the line is low, but they are disabled when the output is high so that it does not cause excessive leakage.

    3. The functionality is the same as the SN74AVC4T245 in that the I/Os will no longer actively drive a line high or low once one of the supplies is <100mV.

    Best,
    Michael

  • 0 in reply to Michael J Schultis
    Guru 20620 points

     Hello Michael,

     Thank you very much for your reply.

     Could I ask you a few questions as the follows?

     Q1. I think that when VCCA=3.3V, VCCB=0V, all output buffers are disabled and all pins are pulled down to GND.

           Is my understanding correct?

     Q2. If Q1 is correct, is the following correct too?

           I think that very small input current is flowed to GND from controller because the controller pulls the input pin of A-side

           up to high level (3.3V) and all pins are pulled down to GND.

     Q3. If Q2 is corrected, what is the input current? Is it same as "Ioff" as the maximum?

     Thank you again and best regards,

     Kazuya.

      

  • 0 in reply to Kazuya Nakai59
    TI__Mastermind 44340 points

    Hey Kazuya,

    Q1. Your understanding is correct here. Though, the control inputs would need external pull-downs to keep it from floating.

    Q2. No, this path should be removed if there is 3.3 V being applied to the I/O.

    Q2. Yes, the max current at this point should be Ioff (max).

  • 0 in reply to Dylan Hubbard
    Guru 20620 points

     Hello Dylan,

     Could I confirm you the following point?

                   The internal pull down circuit of all data pins are active when each pin is driven to low level and both VCC are powered on

                   (Examaple: VCCA=3.3V, VCCB=1.8V) and the device is in partial power down mode (VCCA=3.3V, VCCB=0V)

     Is this correct?

     Thank you and best regards,

     Kazuya.

     

  • 0 in reply to Kazuya Nakai59
    TI__Mastermind 44340 points

    Hey Kazuya,

    Yes that is correct, the pull-downs are active when the device is in Ioff (one of the supplies being 0 V) and the data pins are low. They will not be active when the device is enabled or when there is a high on the data pin.