Application of Linear WLED Driver in LCD Backlight

Improving energy efficiency is one of the most prevalent challenges in today's world. The consumer electronics industry is constantly moving towards greater efficiency, driven by new and increasingly stringent energy standards and rising consumer awareness. The need for greater efficiency is driving innovative companies to develop smarter power management technologies. One of the fastest growing areas is display backlighting. Whether it's in a mobile phone, MP3 player, portable game console or GPS system, the light behind the LCD screen makes it all more colorful.

Powering these screens is like many engineering challenges, with a variety of solutions depending on the specific application. In the portable display backlighting market, a newer, smarter solution is set to revolutionize the way LCD screens are illuminated. This article will discuss the more common solutions in the market today and introduce alternative solutions that are easier to use, require fewer external components, are lower cost, and have higher efficiency.

Boost WLED Driver

Lithium-ion batteries are the most common source of power in the portable market. A typical lithium-ion battery has a voltage of about 4.2V when fully charged. The battery discharges as current is consumed, but about 80% of the battery life is within the 3.9V to 3.5V range. The LEDs only need to be forward biased at about 3.3V to emit light. A boost LED driver can power multiple LEDs simultaneously by boosting a lower input voltage (from a lithium-ion battery) to a higher output voltage (across the LEDs). The higher output voltage is used to forward bias the series-connected string of LEDs. The boost driver can drive multiple LEDs at once, depending on the voltage it can carry. For example, if the switching transistor is rated for 24V, then the boost LED driver can easily forward bias six LEDs in series (6x3.3VLED≈20V). Figure 1 shows a typical boost LED driver with six series-connected LEDs.

Figure 1. Typical boost LED driver circuit for six LEDs.

A typical boost LED driver circuit requires a total of four external components, namely, an input capacitor (CIN), an output capacitor (COUT), an inductor (L), and a set resistor (RSET). As shown in Figure 1, the LED current is obtained by dividing the voltage at FB by the resistor (RSET). The boost LED driver is basically a modified boost regulator that can reduce the power loss through the RSET resistor with a lower feedback voltage. Similar to the boost regulator, this topology uses an inductor and an internal power transistor to transfer and transmit energy to the output (OUT). Therefore, it also inherits the disadvantages of the boost regulator, such as low efficiency at light loads, switching noise, and sometimes audible noise problems caused by the piezoelectric effect (noise generated by the high alternating voltage across the ceramic capacitor). Figure 2 shows the efficiency curve of a typical boost LED driver.

Figure 2 Efficiency of a boost LED driver.

As shown in Figure 2, a typical boost LED driver used to drive six LEDs has a peak efficiency of approximately 80%. This value can vary depending on the inductor used. Larger inductors generally provide higher efficiency, but at the expense of larger size and cost. When the LED current is low (during dimming), efficiency is reduced due to switching losses, which is often a disadvantage of using a boost LED driver. Low efficiency can potentially waste energy depending on the application, but it also leaves room for improvement. If a boost LED driver is chosen as a solution, but is used in the lower current region (less than 20mA) most of the time, the system will become inefficient. Assuming that the system designer uses the boost LED driver at full brightness most of the time, its efficiency can maintain 80% for most of the life of the lithium-ion battery, as shown in Figure 2. Selecting the right LED driver for a given application is challenging, but failure to consider all parameters can also damage precious battery life in portable systems.

Charge Pump WLED Driver

Another type of LED driver currently on the market is a charge pump LED driver. The circuit is shown in Figure 3.

Figure 3: Charge pump LED driver.

A charge pump LED driver drives individual LEDs in parallel. A typical charge pump requires a total of five external components, namely, an input capacitor (C1), an output capacitor (C2), two charge pump capacitors (Cx, Cy), and a set resistor (RSET). One advantage of a charge pump over a boost regulator is that it uses two charge pump capacitors instead of a large external inductor. Another advantage is its efficiency at light loads. Since there are no switching losses at light loads, the charge pump can maintain high efficiency over a wide range of loads. When the lithium-ion battery voltage is high, the charge pump is in bypass mode. In this mode, the input voltage (VIN) is connected to the output (OUT) through the internal transistor of the charge pump. When the battery voltage is lower than the forward voltage required to forward bias the LED, the charge pump is activated. By charging the capacitors (Cx and Cy) in series and then connecting them in parallel to provide energy, the voltage at the output is increased by 50%. This charge pump method allows the LED to be fully biased even if the battery voltage is lower than the LED forward voltage. However, this is done by switching multiple internal switches, and non-ideal switches cause energy losses. For example, most of the time, the charge pump is in bypass mode, where the input voltage is connected to the output through an internal switch. As long as the LED is on, there will be energy loss in the switch, even if the charge pump is not active. When the charge pump is working, it is extremely inefficient due to energy losses in the process of charging and discharging the capacitor and in the switch. Since the life of a lithium-ion battery is from 3.9V to 3.5V, it is futile to pump voltage when the battery life is almost exhausted. The inherent problem of charge pump LED drivers is that they waste energy in bypass mode and are inefficient in the final boost stage. The inefficiency of one solution in a given application will lead people to develop another solution without such defects.

Linear WLED Drivers

The MicrEL linear LED driver family (MIC2841A, MIC2842A, MIC2843A, MIC2844A, MIC2845A, MIC2846A) is a highly efficient, low-cost and easy-to-use solution designed specifically for driving LEDs in the portable display backlight market. Figure 4 shows a typical linear LED driver circuit.