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DC control
LEDs are current-driven devices whose brightness is proportional to the forward current. There are two ways to control the forward current. The first method is to use the LEDV-I curve to determine the voltage that needs to be applied to the LED to produce the expected forward current. The implementation method generally uses a voltage supply and a ballast resistor. Figure 1 illustrates this approach. As described below, this method has several shortcomings. Any change in the forward voltage of the LED will result in a change in the LE 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 pressure change caused by temperature or manufacturing changes, the forward current is reduced to 14mA. A 11% change in forward voltage results in a larger forward current change of up to 30%. In addition, depending on the available input voltage, the ballast resistor’s voltage drop and power dissipation can waste power and reduce battery life.

The second method, and the preferred method of LED current regulation, uses a constant current source to drive the LEDs. The constant current supply eliminates current changes caused by forward voltage changes. This produces a constant LED brightness regardless of the forward current. It is easy to generate a constant current power supply. It is only necessary to adjust the voltage across the current sense resistor without adjusting the output voltage of the power supply. Figure 2 illustrates this approach. The power reference voltage and current sense resistor values ??determine the LED current. Driven

When multiple LEDs are connected, they can be connected in series to achieve a constant current in each LED. Driving a parallel LED requires placing a ballast resistor in each LED string, which can result in reduced efficiency and current mismatch. Battery life is critical in portable applications. LED drivers must be efficient if they are practical. The efficiency measurement of an LED driver is different from the efficiency measurement of a typical power supply. A typical power efficiency measurement is defined as the output power divided by the input power. For LED drivers, the output power is not a relevant parameter. What is important is the input power value required to produce the desired LED brightness. This can be done simply by making the LED power