LEDs are devices driven by current, and their brightness is proportional to the forward current. There are two ways to control the forward current. The first method is to use the LED U-I curve to determine the voltage to be applied to the LED to produce the expected forward current. Its implementation method generally uses a voltage source and a ballast resistor. The figure illustrates this approach. This method has a number of disadvantages, such as any change in the LED forward voltage will cause changes in the LED current. If the rated forward voltage is 3.6V, the LED current is 20mA as shown in the figure. If the voltage becomes 4.0V, which is a specific voltage change due to temperature or manufacturing changes, then the forward current is reduced to 14mA. A 11% change in forward voltage results in a larger forward current change of 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 LED current adjustment method, which is to use a constant current power supply to drive the LED. The constant current power supply can eliminate the current change caused by the forward voltage change. Therefore, a constant LED brightness can be generated. Regardless of how the forward current changes, it is easy to generate a constant current power supply. It is only necessary to adjust the voltage through the current detection resistor without adjusting the output voltage of the power supply. The power supply reference voltage and current sense resistor value determine the LED current. When driving multiple LEDs, simply connect them in series to achieve a constant current in each LED. Driving parallel LEDs requires placing a ballast resistor in each LED string, which results in reduced efficiency and current mismatch.