LED is a device driven by the current, its brightness and forward current is proportional relationship.
There are two ways to control the forward current. The first method is to use the LED V-i curve to determine the voltage required for the led to produce the desired forward current. Its realization method generally uses a voltage power supply and a town-flow resistor. As described below, there are several drawbacks to this method. Any change in the LED forward voltage will lead to a change in the LED current. If the rated forward voltage is 3.6V, the current of the LED in Figure 1 is 20mA. If the voltage changes to 4.0V, which is a specific pressure change due to temperature or manufacturing change, the forward current is reduced to 14mA. A positive voltage change of 11% will result in a greater positive current change of 30%. In addition, according to the available input voltage, the voltage drop and power dissipation of the town-flow resistor will waste power and reduce battery life.