What does "open collector" mean?


#1

If you are just starting out with electronics a term you may come across early on is “open collector”. It is very common in integrated circuits for output pins to be open collector. A datasheet for an IC will either state this for an output pin or show a functional circuit diagram with the output pin being internally connected to the otherwise “open collector” of an NPN transistor.

Open collector outputs require a pull-up resistor (R in the image above) for the output to be able to properly “output high”. The pull-up resistor is connected between the output pin and the output voltage (Vcc in the image above) that is desired for a high state. When the internal NPN of the IC is off, R “pulls up” the output pin to Vcc and at that point only a very small amount of “leakage current” should flow through the NPN transistor. When the internal NPN of the IC is on, it “pulls down” the output pin to very nearly GND and the current flow is set by Ohm’s Law (I = Vcc/R).

The value of R should be large enough to limit current sufficiently so the output NPN isn’t damaged (refer to IC datasheet ratings), but not so large such that it isn’t much smaller (generally orders of magnitude smaller) than the combination of any other impedances connected to the same output pin (usually just called “high impedance”). Similarly, Vcc must be within the datasheet given voltage ratings of the output pin.


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