First of all, we need to understand what is a bootstrap capacitor?
The DCDC BUCK chip has a pin called BOOT (or BST), as shown in the figure below. When designing the external circuit, a capacitor, usually 0.1uF, needs to be added between the BOOT and SW pins and connected to the driving end of the DCDC high-end MOS tube. This capacitor is called a bootstrap capacitor.

 In the synchronous structure, MOS tubes can be used as switch tubes in two ways.

1. No external bootstrap circuit is required: the upper tube is PMOS and the lower tube is NMOS. Driving the upper tube PMOS does not need to generate a voltage higher than VIN, and no BOOT capacitor is required.

 2. External bootstrap circuit is required: both upper and lower tubes are NMOS

 Why use a bootstrap circuit?

This is because MOS is used to build a bridge circuit in some circuits. The conduction condition of the lower tube NMOS is easy to achieve. It can be turned on after the voltage Vgs between the gate G and the source S exceeds Vgs (th). Vgs (th) is usually relatively low, so it is easy to achieve.

 As for the upper tube Q1, the source S has a certain output. It should be noted that when the upper tube is turned on, the voltage Vds between the drain D and the source S is very small. If you want to directly drive the gate G and meet the condition of Vgs>Vgs(th), you need to add a very high voltage between the gate G and the ground, which is difficult to control.

With the bootstrap circuit, a high voltage can be easily generated on the upper tube gate G to drive the upper tube MOS.

During the entire working process, the bootstrap circuit uses the characteristic that the voltage across the capacitor cannot change suddenly. In the PWM cycle of the Buck circuit, it alternates between charging and discharging processes to achieve continuous driving of the high-side MOSFET, thereby ensuring that the Buck switch circuit can work normally.