What is the significance of push-pull and open drain controls in circuits?

A "normal" digital output actively drives both high and low. Open drain or open collector outputs only actively drive low, and go to high impedance when the signal is meant to be high. Some reasons the latter can be useful:

1. Multiple outputs may be driving a bus. When devices only drive the bus low and a passive pullup drives it high, there are no electrical problems with collisions. The IIC bus is an example of this. Look at the protocol spec and see how the "anyone that drives it low wins" electrical result was used to advantage.
2. The output can often go to a higher voltage than the supply when high. Since the only thing connected to the output is a collector or drain of a transistor, the voltage limit is whatever that transistor can withstand. With outputs that are also actively driven high, there is generally static discharge protection circuitry that clips the output to the supply voltage.

posted over 4 years ago

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Olin Lathrop‭

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And to add to Olin Lathrop's answer, case 1. might be called "multidrop" or "party line" or "dot-or" connection. IIC bus as a good example of WHY we might use this case.

Case 2. allows what might be called "level shifting". For example a digital system using a 3.3V power supply might be used to control a 5 V system.

An open-drain output of a 3.3 V digital system can control a 5 V input if the 3.3 V open drain output can withstand the 5V when it is in the high state. If so, we might say it's "5 V tolerant". If we connect that 5 V input with a resistor to its own 5 V supply, it will be in its high state. This is called a "pull up resistor".

Then if we also connect that 5 V tolerant open drain output from the 3.3 V system to this same point, and connect the two systems' ground points, then the 3.3 V system can control the 5 V input. The low state is near zero volts because the 3.3 V system pulls it low, the high state is near 5 V because the pull up resistor connects it to the 5 V of that system.

There are many examples of using this connection to control 5 V output devices on 3.3 V powered computer systems.

posted about 4 years ago

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DrBearEE‭

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