Q is a mos transistor, and its selection depends on the peak current and VCC of the mos transistor.

The principle needs to understand the buck circuit first.

Let's take a brief look.

u？ A pulse signal is generated and connected to the control stage (gate) of Q (regarded as a switch). Q is turned on when the pulse signal is at high level, and turned off when the pulse signal is at low level.

When Q is turned on, VCC is connected to the LED through the connector, and then a series inductor conducts the current to the ground. Due to the inductance, the current rises with a certain slope during the time when Q is on, and di/dit = u/L. This current is converted into a voltage signal through 0. 1R and fed back to u? ，U？ When the current reaches the set value, the pulse signal of this pulse cycle is switched from high level to low level, and then this process is repeated in the next cycle.

When Q is turned off, the current in the LED inductor series circuit will continue to flow through the diode Q 1, waiting for the high signal of the next pulse period.

In the reciprocating process, there will be a constant set current flowing through the LED, which is a triangular wave. Q rises when it is turned on and falls when it is turned off, and its slope is determined by the values of VCC and L.