open drain vs open collector

Guri Bee

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21-10-2024

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Open Drain vs Open Collector: Demystifying the Differences

When delving into the world of digital electronics, you might encounter the terms "open drain" and "open collector". These terms refer to specific output configurations for transistors within integrated circuits (ICs). While they might sound similar, their functionalities and applications differ significantly. This article explores these differences, providing clarity and practical examples for understanding their uses.

Understanding the Basics

Both open drain and open collector outputs rely on transistors to control the flow of current. These transistors act like electronically controlled switches, allowing current to flow when activated (turned "on") and blocking current when deactivated (turned "off").

Open Drain Outputs:

• Structure: In an open drain output, the transistor's collector terminal is left unconnected (hence the name "open drain").
• Functionality: To turn the output on, the transistor conducts, allowing current to flow to ground through an external pull-up resistor. To turn the output off, the transistor stops conducting, preventing current flow.
• Behavior: The output voltage is pulled high (logical "1") by the external pull-up resistor when the transistor is off. When the transistor is on, the output voltage is pulled low (logical "0").

Open Collector Outputs:

• Structure: Similar to open drain, the open collector output also leaves the transistor's collector terminal unconnected.
• Functionality: To turn the output on, the transistor conducts, allowing current to flow to ground. To turn the output off, the transistor stops conducting.
• Behavior: The output voltage is pulled high (logical "1") by an external pull-up resistor when the transistor is off. When the transistor is on, the output voltage is pulled low (logical "0").

Key Differences

• Transistor Type: Open drain outputs typically use MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors), while open collector outputs use BJTs (Bipolar Junction Transistors).
• Current Flow: Open drain outputs allow current to flow to ground through the external pull-up resistor, while open collector outputs allow current to flow to ground directly.
• Voltage Levels: The voltage levels in both configurations are determined by the external pull-up resistor.

Applications

Both open drain and open collector outputs have specific applications based on their unique characteristics:

Open Drain Outputs:

• Wired-OR Logic: This configuration allows multiple open drain outputs to be connected together, with the output logic being the OR function of all individual outputs. This is useful in scenarios where multiple devices need to share a single output line, such as in I2C and SPI communication protocols.
• Voltage Level Shifting: Open drain outputs can be used to convert logic levels from high voltage to low voltage or vice versa, by choosing an appropriate external pull-up resistor.
• High-Side Switches: Open drain outputs can be used as high-side switches, where the output controls the flow of current to a load, using the pull-up resistor.

Open Collector Outputs:

• Wired-AND Logic: Open collector outputs can be connected together with a single pull-up resistor, and the resulting output logic is the AND function of all individual outputs.
• High-Current Applications: Open collector outputs are often used in applications requiring high current, due to the inherent properties of BJTs.
• Low-Side Switches: Open collector outputs are commonly used as low-side switches, where the output controls the flow of current from a voltage source to a load.

Practical Example:

Imagine a scenario where you want to control a LED with multiple buttons. Using an open drain output, you can connect the buttons to an IC's open drain pins. Each button, when pressed, will turn its respective output low, allowing current to flow through the LED. If multiple buttons are pressed simultaneously, the LED will still turn on, demonstrating the "wired-OR" functionality.

Conclusion

Open drain and open collector outputs offer unique functionalities and are essential components in various electronic systems. Understanding their differences and applications allows for more informed design decisions, enabling the implementation of complex circuitry and efficient control of electronic components.

Note: This article is based on information gathered from various sources, including Github discussions. Please refer to the original sources for further details and technical specifications.

References:

• Open Drain vs Open Collector
• Open Collector and Open Drain Outputs
• Open Drain and Open Collector Outputs