Excerpts on the principle and application of open-drain stage

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Excerpts on the principle and application of open-drain stage [Copy link]

In circuit design, we often encounter the concepts of open drain and open collector. Although I knew the basic usage when I was in school and never encountered any problems in design, a colleague asked me about this concept two days ago. I suddenly felt that I didn't understand it systematically. Recently, I took some time to summarize it. The
　　"drain" mentioned in the concept of open drain circuit refers to the drain of MOS FET. Similarly, the "collector" in the open collector circuit refers to the collector of the transistor. The open drain circuit refers to the circuit with the drain of MOS FET as the output. The general usage is to add a pull-up resistor to the circuit outside the drain. The complete open drain circuit should be composed of an open drain device and an open drain pull-up resistor. As shown in Figure 1: The circuit that constitutes the open
　　


drain form has the following characteristics:
1. Utilize the driving capability of the external circuit to reduce the internal drive of the IC. When the internal MOSFET of the IC is turned on, the driving current flows from the external VCC through R pull-up, MOSFET to GND. Only a very low gate drive current is required inside the IC. As shown in Figure 1.
2. Multiple open-drain output pins can be connected to one line. This forms an "AND logic" relationship. As shown in Figure 1, when any one of PIN_A, PIN_B, and PIN_C becomes low, the logic on the open-drain line becomes 0. This is also the principle of I2C, SMBus and other buses to determine the bus occupancy status.
3. The transmission level can be changed by changing the voltage of the pull-up power supply. As shown in Figure 2, the logic level of the IC is determined by the power supply Vcc1, and the output high level is determined by Vcc2. In this way, we can use low-level logic to control the output high-level logic.
4. If the open-drain pin is not connected to an external pull-up resistor, it can only output a low level.
5. The standard open-drain pin generally only has the ability to output. Adding other judgment circuits can have the ability of bidirectional input and output.





Things to note in the application:
1. The principles of open drain and open collector are similar. In many applications, we use open collector circuits instead of open drain circuits. For example, a certain input pin is required to be driven by an open drain circuit. Our common driving method is to use a triode to form an open collector circuit to drive it, which is convenient and cost-saving. As shown
in Figure 3. 2. The resistance value of the pull-up resistor R pull-up determines the speed of the edge of the logic level conversion. The larger the resistance value, the lower the speed and the lower the power consumption. Vice versa.