Open drain circuit and push-pull output of stm32

           Open drain structure

         The "drain" mentioned in the concept of the so-called open-drain circuit refers to the drain of the MOS FET. Similarly, the "collector" in the open-collector circuit refers to the collector of the transistor. The open-drain circuit refers to a circuit with the drain of the 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:

              figure 1

 The circuits that form the open-drain form have the following characteristics:
1. Utilize the driving capability of the external circuit to reduce the internal driving 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. The IC only needs a very low gate driving current. As shown in Figure 1.
2. Multiple open-drain output pins can be connected to one line. Form 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 is 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 output capabilities. Only by adding other judgment circuits can the ability of bidirectional input and output be achieved.

 Notes in 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, an input pin is required to be driven by an open drain circuit. Then our common driving method is to use a transistor 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

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     The push-pull structure
             generally refers to two transistors that are controlled by two complementary signals respectively, and one transistor is always turned on while the other is turned off. To achieve line-and, an OC (open collector) gate circuit is required. If there are two transistors in the output stage, one is always turned on and the other is turned off, that is, the two transistors are connected in push-pull, and this circuit structure is called a push-pull circuit.

                Push-pull amplifier circuits are widely used in power amplifier circuits. In this circuit, two transistors are used to form a first-stage amplifier circuit. The two transistors respectively amplify the positive half-cycle and negative half-cycle of the input signal, that is, one transistor amplifies the positive half-cycle of the signal, and the other transistor amplifies the negative half-cycle of the signal. The half-cycle signals output by the two transistors are combined on the amplifier load to obtain an output signal of a complete cycle. 

　　In a push-pull amplifier circuit, when one transistor is in the on and amplifying state, the other transistor is in the off state. When the input signal changes to another half cycle, the transistor that was originally on and amplifying enters the cut-off state, and the transistor that was originally cut-off enters the on and amplifying state. The two transistors are constantly alternating between on and off, so it is called a push-pull amplifier.

 

        The push-pull circuit consists of two triodes or MOSFETs with the same parameters, which exist in the circuit in a push-pull manner. Each of them is responsible for the waveform amplification task of the positive and negative half cycles. When the circuit is working, only one of the two symmetrical power switch tubes is turned on at a time, so the conduction loss is small and the efficiency is high.

         The output can both sink current to the load and draw current from the load.