Open collector

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A simplified schematic of the externalized (open) collector of an integrated circuit (IC).

Open collector is a type of output on many integrated circuits (IC). Instead of outputting a signal of a specific voltage or current, the output signal is applied to the base of an internal npn transistor whose collector is externalized (open) on a pin of the IC. The emitter of the transistor is connected internally to the ground pin.

Open-collector is one of many electrical input/output standards in digital designs today.

1 Function
2 Applications of open-collector devices
3 See also
4 References

[edit] Function

The output essentially acts as either an open circuit (no connection to anything) or a short circuit to ground. This is then usually connected to an external pull-up resistor, which sets a higher voltage to the output when the transistor is open. When any transistor connected to this resistor is closed, the output is forced to 0 volts. Open-collector outputs can be useful for analog weighting, summing, limiting, etc., but such applications are not discussed here.

[edit] Applications of open-collector devices

One useful property is that the pull-up resistor need not be connected to a voltage similar to that of the chip supply (Vcc); a lower or higher voltage can be used instead. Open collector circuits are therefore sometimes used to interface different families of devices that have different operating logic voltage levels or to control external circuitry that requires a higher voltage level (e.g. a 12 V relay).

Another advantage is that more than one open-collector output can be connected to a single line. If all outputs attached to the line are in the high-impedance(i.e. logic 1) state, the pull-up resistor will hold the wire in a high voltage state. If 1 or more of the device outputs are in the ground( i.e. logic 0) state, they will sink current and pull the line voltage near ground.

By tying the output of several open collectors together, the common line becomes a "wired AND" (positive-true logic) or "wired OR" (negative-true logic) gate. A "wired AND" behaves like the boolean AND of the two (or more) gates in that it will be logic 1 whenever (all) are in the high impedance state, and 0 otherwise. A "wired OR" behaves like the boolean OR for negative-true logic, where the output is LOW if any one of its inputs is low.

One problem with Open-collector devices is power consumption, as they tend to require higher current minimums for correct operation. Even in the 'off' state, they often have a few nanoamps of leakage current (the exact amount varies with temperature.)

[edit] See also

Open drain

[edit] References

Open Collector Outputs.
Chapter 4: Circuits. Honeywell Solid State Technical Documentation.
Horowitz, Paul; Winfield Hill (1989). The Art of Electronics, Second Edition, Cambridge University Press.