Introduction to opendrain and opencollector

We often encounter the concept of open drain and open collector in circuit design. What exactly are these two concepts? The following gives you a detailed introduction.

The "drain" mentioned in the open-drain circuit concept refers to the drain of the MOSFET. Similarly, the "set" in the open circuit refers to the collector of the triode. An open drain circuit is a circuit that uses the drain of a MOSFET as an output.

The "drain" mentioned in the concept of the open drain circuit refers to the drain of the MOSFET. Similarly, the "set" in the open circuit refers to the collector of the triode. An open drain circuit is a circuit that uses the drain of a MOSFET as an output. A common use is to add a pull-up resistor to the circuit outside the drain. The complete open drain circuit should consist of an open-drain device and an open-drain pull-up resistor. As shown below:

1. Reduce the drive inside the IC by using the drive capability of the external circuit. When the internal MOSFET of the IC is turned on, the drive current flows from the external VCC through the Rpull-up, MOSFET to GND. Only a small gate drive current is required inside the IC.

2. It is possible to connect Pins of multiple open-drain outputs to one line. Form a "logical" relationship. As shown in Figure 1, when any of PIN_A, PIN_B, and PIN_C goes low, the logic on the open drain line is zero. This is also the principle that I2C, SMBus and other buses determine the bus occupancy status.

3. The transfer level can be changed by changing the voltage of the pull-up power supply. The logic level of the IC is determined by the power supply Vcc1, while the output high level is determined by Vcc2. This way we can control the output high logic with low logic.

4. Open-drain Pin is not connected to the external pull-up resistor, only low level can be output.

5. Standard open drain feet generally have only the ability to output. Add other judgment circuits to have the ability to input and output in both directions.

Open-collector/Open-drain is a technology for bidirectional (Bi-direcTIonally) communication using multiple cables between multiple devices.

as the picture shows

Because Collector's pin is not connected to any other device, it is called Open Collector. The case of BJT is called Open-Collector, and the case of MOSFET is called Open-drain. In theory, BJT is the same as MOSFET.

When the input has a high signal, the transistor is ON. At this time, Output and Groud form a path to make the Output output Low.

When the input has a low signal, the transistor is OFF, and the Output is in the Hi-inpedence state.

Hi-impedence means a state in which the impedance is high and Low and High cannot be distinguished.