Setup: A digital output from a device is connected to an output buffer to drive an optocoupler. If device power is lost the output will go in high-impedance mode.
Problem: I want to make sure that the buffer output is low when device power is lost. How do I dimension a pulldown resistor for the input of the 74HC244N buffer?
Answer
10 kΩ is a typical value for a pullup/pulldown, and the 500 µA loss at 5 V is usually not a problem; the optocoupler's LED will usually need a multiple of that. For low power applications you can increase the value, and the upper limit is determined by the 74HC244's input leakage current. The datasheet says that is maximum 1 µA, then a 1 MΩ resistor may cause a 1 V drop across it. That's a rather large value, still OK at 5 V supply voltage, but I would choose a lower value.
A 100 kΩ resistor will cause a 50 µA leakage from the device's output, and a maximum input voltage of 100 mV when floating. This looks like a good solution.
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