Today, many portable electronic products require a backlight LED driver solution with the following features: DC control, high efficiency, PWM dimming, overvoltage protection, load disconnection, small size, and ease of use. This article explores the characteristics of each LED driver and how to implement them. Finally, a typical circuit with each feature will be described.
DC control
LEDs are current-driven devices whose brightness is proportional to forward current. There are two ways to control the forward current. The first method is to use the LED VI curve to determine the voltage that is required to be applied to the LED to produce the desired 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 LED forward voltage will result in 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 becomes 4.0V, which is a specific pressure change caused by temperature or manufacturing changes, the forward current is reduced to 14 mA. 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.
Figure 1: Voltage supply with ballast resistors leads to reduced efficiency and forward current mismatch
The second method, which is also 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. When driving multiple LEDs, it is only necessary to connect them 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.
Figure 2: Constant current power supply driving the LED
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