DCDC buck circuit refers to: converting DC voltage into low-voltage DC. The circuit is manifested as the output voltage is smaller than the input voltage after the input voltage passes through the DCDC buck circuit. The principle of the DCDC buck circuit is mainly to reduce the voltage by controlling the conduction time of the internal switch. The application scenarios of DCDC voltage are mainly in devices that require different voltages for power supply, such as: mobile phone chargers, portable computers, and mobile phones commonly seen in consumer electronics. Compared with LDO, the DCDC buck circuit has a higher conversion efficiency, generally up to 90%, and reduces heat generation. The DCDC circuit modulates the voltage according to the feedback, and the output voltage is stable. The disadvantage of the DCDC circuit is that it uses more devices and introduces more costs.
Simplified diagram analysis of the buck-type DCDC circuit:

When switch S1 is closed, the current passes through S1, inductor L1, supplies power to the load, and flows to the negative pole of the power supply. While supplying power, it also charges the inductor. Since D1 is a unidirectional conductivity of the diode, diode D1 will not work at this time. Characteristics of inductance: Inductor is an inductive device. Due to the existence of self-induced electromotive force, it will block the increase or decrease of current. The inductor current is gradual, not transient. The voltage on inductor L1 is the input voltage minus the voltage on the load, which is equal to L* (di/dton). When S2 is turned off, due to the characteristics of the L1 inductor device, the current of the inductor flows to the load, then flows to the freewheeling diode, and returns to the inductor to form a loop. The output voltage is equal to L*di/dtoff. Dividing these two expressions can get VO/VIN=don/dff; it is concluded that the output is less than or equal to the input voltage. The functions of each device in the above figure: capacitor C1 stabilizes the input voltage, inductor L1 is used to store or transfer energy to the load, capacitor C2 stabilizes the output voltage, and diode D1 is used to form a freewheeling loop when L1 is discharged.

Synchronous DCDC buck and asynchronous buck circuits
In the specification of DCDC power chips, there are generally two common rectification methods, synchronous rectification and asynchronous rectification. The rectification principle diagram is as follows:

The diode has unidirectional conductivity. The voltage drop of the silicon tube is 0.7V, the voltage drop of the germanium tube is about 0.3V, and the voltage drop of the Schottky tube is 0.4V.

As can be seen from the above figure, the difference between synchronous and asynchronous is:
Power consumption: The synchronous buck circuit uses a MOS tube with extremely low resistance in the freewheeling circuit, while the asynchronous buck circuit uses a diode. Since the internal resistance of the MOS tube is small, the synchronous rectification power consumption is low.
Cost: The price of the MOS tube is more expensive than the diode, so the synchronous buck circuit does not have an advantage in terms of cost.
Load: When lightly loaded, the working effect of synchronous rectification is better than that of asynchronous rectification. When asynchronous rectification works in discontinuous mode, it will generate a lot of harmonic noise.
DCDC buck circuit specification
The typical buck application diagram of LM2596 is as follows,

A chip step-down circuit

A chip boost circuit

How to quickly determine whether it is a boost or buck circuit through the DCDC power chip periphery? If the switch pin in the above figure is connected to an inductor externally, it is a buck circuit. If it is connected to a diode externally, it is probably a buck circuit. How should the external inductor be selected? The formula for the size of the inductor in a general buck circuit is:

VOUT is the output voltage, VIN is the input voltage, FS is the switching frequency, and IL is the ripple. Generally, there will be a recommended inductance value in the specification sheet, which can be referred to.