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AM26C32: Power limiting resistor and fail-safe circuit for the AM26C32

Taichi Utsumi
Prodigy 10 points
Part Number: AM26C32

Tool/software:

I have some questions regarding the design of the AM26C32.

  1. I am considering an external fail-safe circuit. I plan to connect Vcc to the positive side and GND with a resistor to the negative side. I intend to calculate the resistor value using the internal pull-up/pull-down resistor of 288kΩ and the input impedance of 12kΩ. What is the tolerance range for the 288kΩ resistor?

  2. Is a power-limiting resistor necessary when implementing the fail-safe circuit? If so, what should the resistor value be?

"I would greatly appreciate your response.

  • 0
    Prodigy 10 points

    From the response of Parker Dodson.

    "Additionally, it is stated that the resistance value of the pull-up resistor can be determined using the following formula. What is the original formula, and what kind of transformations were applied to it?"

  • 0
    TI__Guru 72621 points
    Taichi Utsumi said:
    I am considering an external fail-safe circuit. I plan to connect Vcc to the positive side and GND with a resistor to the negative side. I intend to calculate the resistor value using the internal pull-up/pull-down resistor of 288kΩ and the input impedance of 12kΩ. What is the tolerance range for the 288kΩ resistor?

    I think the tolerance for this can be as large as 25% for the internal pull up and pull downs but normally the external fail safe resistors are so small that it you can basically ignore them. 

    Taichi Utsumi said:
    Is a power-limiting resistor necessary when implementing the fail-safe circuit? If so, what should the resistor value be?

    Customers normally put a small 10 ohm resistor infront of the inputs when they expect there could be a large transient event that could damage the device. It's not required but it depends on the application or expectation on if a large transient could appear. 10 ohms is normally what I see.

    Taichi Utsumi said:
    "Additionally, it is stated that the resistance value of the pull-up resistor can be determined using the following formula. What is the original formula, and what kind of transformations were applied to it?"

    https://www.ti.com/lit/an/slyt324/slyt324.pdf?ts=1754602842600&ref_url=https%253A%252F%252Fwww.google.com%252F

    This document walks through how to calculate external pull up and pull down resistances.

    I think the equation Parker derived was something he did himself. 

    -Bobby