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SN74CB3T16211: About pullup current source

Ryoma Ohashi
Expert 3020 points
Part Number: SN74CB3T16211

Hi team,

My customer has some questions about this deivice. In the datasheet, it says as below:

The I/O port of this device has a pullup current source that maintains the output voltage at VCC when the device

is ON, and the input is greater than or equal to VCC – 1. Because of the pullup current source, the output voltage

level may be less than VCC when the operating frequency is low and the I/O port is connected to a pulldown

resistor. In order to maintain the output voltage at VCC, a pullup resistor must be connected to VCC instead of a

pulldown resistor to ground.

[Question]

(1) Is it correct to understand that the IC have a pull-up current source on either port that keeps the output voltage at VCC? In other words, when the switch is turned on, both the left and right sides are pulled up.
(2) Could you tell us the value for the pull-up resistor?  I want to know roughly how much it is (thousands of k Ω, a few k Ω, etc.).
(3) When turned off, it the output not connected to the oull up current source that maintain the VCC? In other words, it doesn't pull up when switched off?

Regards,

Ohashi

  • +1
    TI__Genius 10552 points

    HI Ohashi,

    (1) Yes, it is correct to understand that the device has a pullup current source on either port that keeps the output voltage at VCC when the switch is ON. This means that both the A and B ports are pulled up to VCC when the corresponding OE input is low, enabling the switch. This feature allows the device to support voltage translation between different logic levels.

    (2) The value of the pullup resistor depends on the operating frequency, the load capacitance, and the desired rise time of the signal. Generally something like a 3.3Kohm pull up resistor is sufficient for this application. 

    (3) When turned off, the output is not connected to the pullup current source that maintains VCC. In other words, it does not pull up when switched off. Instead, it behaves like an open circuit with high impedance. The data sheet specifies that the device has an OFF-state leakage current of 10uA max, which means that there is minimal current flow between the ports when the switch is OFF. The data sheet also states that the device has an OFF-state output capacitance of 5 pF typical, which means that there is some parasitic capacitance between the ports when the switch is OFF.

    Regards,

    Alex

  • 0 in reply to Alex Vish
    TI__Expert 3020 points

    Hi Alex,

    Thanks for your answer. It really helps.

    Ryoma Ohashi said:
    (2) Could you tell us the value for the pull-up resistor?  I want to know roughly how much it is (thousands of k Ω, a few k Ω, etc.).
    Alex Vish said:
    (2) The value of the pullup resistor depends on the operating frequency, the load capacitance, and the desired rise time of the signal. Generally something like a 3.3Kohm pull up resistor is sufficient for this application. 

    For this question, I was meant to ask the value of the internal pull up resistor that is integrated inside the IC. Could you tell us the value for the internal pull up resistor value?

    Regards,

    Ohashi

  • +1 in reply to Ryoma Ohashi
    TI__Genius 10552 points

    Hi Ohashi,

    So that I am clear on the question: are you referring to where the datasheet mentions "pull up current source"? This is not done using a pull up resistor. Instead, this is done via a transistor. This pullup current source will vary based on your output voltage. Let me know if I am misunderstanding here. 

    Thanks!

    Alex