Open collector/drain
- Thread starter Surendar
- Start date
A conventional logic circuit generally has a push-pull or 'totem pole' output that can actively drive the output to (near) 0V or the supply voltage.
If it's connecting to another circuit operating on the same voltage, this is generally what's needed.
If you want to drive a circuit with a different supply voltage, or something like a 12V relay from 5V logic, the fixed 'high' voltage output from a normal circuit is no good, you need something that works like a switch to 0V.
An open-collector style output has only one transistor, with the emitter to 0V and the collector is the ouput - hence 'open collector', there are no other connections, it's directly to the ouput pin. (Open Drain is the same thing with a FET rather than a bipolar transistor).
As it's in effect just a switch, you set the voltage when it's off. An example is a 5V TTL device feeding a CMOS circuit running on 12V. The TTL 'high' level would not be enough to guarantee the CMOS input seeing a clean signal, but if you use an open collector TTL output & use a pullup resistor to the 12V CMOS supply, you will get a nice clean 0v - 12V switching signal into the CMOS circuit.
Other applications are things like Darlington Driver ICs, which are effectively open-collector ouputs, though often with extra protection diodes etc.
I guess the 'pseudo open collector' could be a tri-state output switched between a low state & high impedence. This would not work for an output needing a higher voltage than the device, but could be used for parallel connections or with pullups to a lower voltage, e.g. 5V logic -> 3.3V logic.
If it's connecting to another circuit operating on the same voltage, this is generally what's needed.
If you want to drive a circuit with a different supply voltage, or something like a 12V relay from 5V logic, the fixed 'high' voltage output from a normal circuit is no good, you need something that works like a switch to 0V.
An open-collector style output has only one transistor, with the emitter to 0V and the collector is the ouput - hence 'open collector', there are no other connections, it's directly to the ouput pin. (Open Drain is the same thing with a FET rather than a bipolar transistor).
As it's in effect just a switch, you set the voltage when it's off. An example is a 5V TTL device feeding a CMOS circuit running on 12V. The TTL 'high' level would not be enough to guarantee the CMOS input seeing a clean signal, but if you use an open collector TTL output & use a pullup resistor to the 12V CMOS supply, you will get a nice clean 0v - 12V switching signal into the CMOS circuit.
Other applications are things like Darlington Driver ICs, which are effectively open-collector ouputs, though often with extra protection diodes etc.
I guess the 'pseudo open collector' could be a tri-state output switched between a low state & high impedence. This would not work for an output needing a higher voltage than the device, but could be used for parallel connections or with pullups to a lower voltage, e.g. 5V logic -> 3.3V logic.
The only thing I can add to rjenkins' reply is this:
Open collector/drain outputs are available in some digital logic gates as it allows the NOR'ing (positive logic) of the outputs from multiple logic gates by simply connecting the outputs of the open collector/drain gates together and then providing a common pull-up resistor.
hgmjr
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