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SN74LVC2G07: Pull-up resistor value at output

Antony Lin
Genius 13480 points
Part Number: SN74LVC2G07

Hi,

With SN74LVC2G07 output connected as below, how to calculate if the pull-up resistor value is ok or not?  Thanks!

Antony

  • 0
    Guru 316220 points

    These are not pull-up resistors but current-limiting resistors.

    They look as if you wanted to get 20 mA. But the LED forward voltage has large tolerances, and you did not take the buffer's output impedance (max. 25 Ω at VCC = 3.3 V, 11 Ω typical) into account. And the LED intensity has large tolerances, too, so there is no reason to aim for exactly 20 mA.

    64.9 Ω is from the E48 series, which is more precision than you'd bother to use. Just use 56 Ω or 68 Ω.

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 24975 points
    Hello Antony,

    I agree with Clemens here. There is no need to ensure a very precise current, and additionally, there is some series resistance due to the output stage of the SN74LVC2G07 which will affect the current draw.
    Really the only key is to make sure that the current that you are using to drive the LEDs does not exceed the maximum current that the device can sink.

    Best,
    Michael