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SN74AXC4T774-Q1: Data pin impedance & Current inquiry

Evan Wang
Genius 9550 points
Part Number: SN74AXC4T774-Q1

Hi team,

May I know how to evaluate the following parameter in our SN74AXC4T774-Q1?

1. IIH(?)

2. IIL(?)

3. Input Impedance(?)

4. Output Impedance:

     4-1. (VCCB - VOH)/IOH = Output high impedance-> This value is consistent.(O)

     4-2. VOL/IOL = Output low impedance but the value is inconsistent.(?)

Since this IC is directional controlled so both Ax pins and Bx pins are all push-pull CMOS structure?

Looking forward to your reply.

Thank you.

  • 0
    Guru 316940 points

    1./2. This is specified as II.

    3. This can be computed as VI / II. But the input currents are leakage currents, and the inputs do not behave like resistors, so this value would not be very useful.

    4. What do you men with "inconsistent"?

    5. All outputs are push/pull outputs. The direction pins disable the outputs on one side. (The input buffers on both sides are always active.)

  • 0 in reply to Clemens Ladisch
    TI__Genius 9550 points

    Hi Clemens,

    Clemens Ladisch said:

    1./2. This is specified as II.

    You mean just leverage the II as input current no matter what input pin voltage is?

    Clemens Ladisch said:

    4. What do you men with "inconsistent"?

    I mean the value of VOL/IOL as below:

    Thank you.

  • 0 in reply to Evan Wang
    Guru 316940 points

    In theory, the input current of CMOS devices is zero. In practice, the input current is just a leakage current and depends mainly on the temperature. (The leakage flows mostly through the ESD protection structures, so there is not even a guaranteed relationship between the input voltage and the leakage current's direction.)

    The gate-to-source voltage of the MOSFETs is the supply voltage, so a higher supply will result in a higher drive strength, i.e., a lower output impedance.