LED is a device driven by current, and its brightness is proportional to the forward current. There are two ways to control the forward current.
The first method is to use the LED V-I curve to determine the voltage applied to the LED to produce the expected forward current. Its realization method generally adopts a voltage power supply and a ballast resistor. As described below, this method has several shortcomings. Any change in LED forward voltage will cause a change in LED current. If the rated forward voltage is 3.6V, the current of the LED in Figure 1 is 20mA. If the voltage becomes 4.0V, which is a specific voltage change caused by temperature or manufacturing changes, the forward current is reduced to 14mA. An 11% change in forward voltage will result in a larger forward current change, up to 30%. In addition, depending on the available input voltage, the voltage drop and power consumption of the ballast resistor will waste power and reduce battery life.
The second method is also the preferred method of LED current adjustment is to use a constant current power supply to drive the LED.