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Original question:

SN65DP159: Pullup / Pulldown resistor of Control pins

SN65DP159: Resister value of Control pin

Y.Ottey
Mastermind 6145 points
Part Number: SN65DP159

Dear all,

I would like to ask a question about resister value of Control pin like the following picture.

Regarding the resistance value in the part surrounded by a red frame,

our customer says that he wants to use a pull-up / pull-down resistance value different from 65kΩ.

In a previous E2E post, it was stated that it would be used in the range of 65kΩ ± 10%.

If a resistor with a value outside this range is used as a pull-up / pull-down resistor, what is the effect on the operation?

If you only use pull-up and pull-down, I think 65kΩ is too big.

For example, in the figure above, when Vcc = 3.3V and you want to input only High with a pull-up resistor of 65kΩ, check the input current (IIH) and the threshold value (VIH) that becomes High when the data sheet is

High level. When 30μA is input, a voltage drop of 30μA × 65kΩ = 1.95V occurs in the resistance part.

Then, the input voltage becomes 3.3V-1.95V = 1.35V, and it is not input at High.

Best Regards,

Y.Ottey

  • 0
    TI__Guru**** 176231 points

    Y.Ottey

    There is a note in the datasheet on this about the voltage seen at the pins versus the values listed in the datasheet. This Note is listed under the table in 7.3 Recommended Operating Condition:

    These values are based upon a microcontroller driving the control pins. The pullup/pulldown/floating resistor configuration will set the internal bias to the proper voltage level which will not match the values shown here.


    The pullup/pulldown needs to be 65 +/-10%.

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Mastermind 6145 points

    Dear all,

    Thank you for your reply.

    I would like to ask a more question about your answer.

    If the diriving of the micro controller increases and the current flowing through the control pin increases, is it okay to reduce the pull-up resistance value?

    For example, when pull-up resistor 65kΩ is connected to the control pin and Vcc = 3.3V and the current flowing to the control pin is 2μA, the control pin voltage is 3.3V- (2μA × 65kΩ) = 3.17V. . In this case, VIH exceeds 2.6V, so it becomes High.

    If the current flowing through the control pin is 20μA, is there any problem in changing the pull-up resistor to 6.5kΩ?

    I would appreciate it very much if you could reply as soon as you can.

    Regards,

    Y.Ottey

  • 0 in reply to Y.Ottey
    Mastermind 6145 points

    Dear all,

    Our customers wold like to receive a quick response.

    I would appreciate it very much if you could reply as soon as you can.

    Regards,

    Y.Ottey

  • 0 in reply to Y.Ottey
    TI__Guru**** 176231 points

    Y.Ottey

    No, your calculation assumes the circuit is open internally, but there is an circuit that internally bias the input signal to the proper voltage level. If switching from 2uA to 20uA, then the external pullup resistor needs to be changed from 65k to ~35k to maintain the same input voltage level.

    Thanks

    David