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ISO6740: Input pull down resistor tolerance

Kazuki Itoh
Genius 10155 points
Part Number: ISO6740

Hi team,

Could you please let me know the input pull-down resistor tolerance value of ISO674xF?

My customer would like to check it to make sure if the input is correctly pulled down with the external resistors.

Regards,

Itoh

  • 0
    TI__Mastermind 24915 points

    Hello Itoh-san,

    Can you share what they are trying to do with the external resistor? Are they trying to pull the line up? Or trying to make sure that the line is pulled down?

    If they are trying to pull the line down, then the internal pulldown resistor will pull the line down by default, so an external pulldown is not necessary. 

    However, if they are trying to pull the line high, then they would need a pullup resistor that overcomes the input pulldown resistance.

    To determine the best pullup resistor, its recommended to look at the IIH specification in the datasheet. 

    This tells you what the worst case leakage into the input pin is, and based on this you can calculate the appropriate pullup value based on the desired voltage at the input in the static pullup condition.

    Hope this helps, let me know if there is anything else that needs clarification.

    Best,

    Michael

  • 0 in reply to Michael J Schultis
    TI__Genius 10155 points

    Hi Michael-san,

    In my customer's system, MCU connected to the input pulled high when MCU is being reset.

    So my customer wants to check if external pull-down resistor is needed to make sure pull down.

    Again, could you let me know the tolerance of internal pull-down resistor?

    Regards,

    Itoh

  • 0 in reply to Kazuki Itoh
    TI__Mastermind 24915 points

    Hello Itoh-san,

    I would have to inquire internally as to what the anticipated tolerance is. Based on the datasheet input leakage, the lowest pulldown value that it could be would be 500kOHm - 5V with 10uA of leakage. So that would be a 66% tolerance, although the tolerance is probably much closer to 10-20%. Assuming 66% tolerance then the highest value it could be would be 2.5MOhm. 

    We could calculate based on this very conservative estimate if that works.

    Do we know what the MCU typical pullup value is? and maybe the tolerance of that resistor.

    I would think if its an intentional pullup it would almost certainly be <1.5Mohm, which would mean in order to have the line remain low, you would need to have a pulldown on the line for this condition.

    Best,

    Michael

  • 0 in reply to Michael J Schultis
    TI__Genius 10155 points

    Hi Michael-san,

    Thank you for your response.

    I'm afraid to say but 66% looks too much.

    Could you please check the maximum pull-down resitance value with the design team?

    Regards,

    Itoh

  • 0 in reply to Kazuki Itoh
    TI__Mastermind 24915 points

    Thank you Itoh-san,

    Let me inquire and get back to you when I have more information.

    Michael

  • +1 in reply to Michael J Schultis
    TI__Mastermind 24915 points

    Hello Itoh-san,

    The tolerance of the 1.5Mohm resistor is ±20%, so you can use this in your calculations for whether or not an external pulldown may be necessary.

    Best,

    Michael