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LM393-N: Max. value of pull up resistor to allow output transistor to be in saturation when turned ON

Part Number: LM393-N
Other Parts Discussed in Thread: LM393

What is the highest value of pull  up resistor to 3.3V that will allow output transistor of LM393 comparator to be in saturation when output transistor is supposed to be ON (output collector voltage is low).

How should I calculate this value?



  • Hi Nitesh,

    The limit to the upper limit of the resistor value is the load resistance and the leakage current of the output stage, as well as load capacitence.

    The datasheet has a an "Output Leakage" or "High Level Output Current" specification in the electrical table. It shows a typical of 100pA and max of 1uA.

    So if you had a 3.3V supply and a 10M pullup, then the leakage current would cause a 1mV drop (100pA*10M= 1mV) across the resistor, which would make the "high" voltage 3.299V.

    But if you use the Max value of 1uA, 1uA * 10M = 10V! Obviously, this is too much current for this resistor value. So you have to balance the leakage current against your output high connection.

    The other limit is any loads on the output. Any load resistance will create a voltage divider with the pull-up resistor when the output is high, and the input currents of any gate inputs will cause a similar issue as the output leakage.

    Also, the larger the pull-up resistor, the more effect output capacitance causes on the rise-time as the capacitance is charged through the pull-up resistor.
  • Hi Paul, Thanks for the clear response. That greatly helps!
    Best regards, Nitish