LED driver circuit design considerations for small LCD backlights

Over the past few years, small color LCD displays have been integrated into an ever-widening range of products. Color displays were once considered a luxury feature of mobile phones, but now they are a standard feature even in entry-level phones. Fortunately, the economies of scale in the mobile phone industry (with nearly 1 billion mobile phones shipped worldwide each year) have reduced the cost of LCD color displays and made them attractive for integration into other products, whether it is portable medical devices, universal entertainment remote controls, digital photo frames/image viewers, educational toys, or the latest WiFi-enabled VoIP cordless phones.

Color LCD displays require white backlighting so that users can see them properly in any lighting environment. This backlight subsystem includes an array of high-brightness white light-emitting diodes (LEDs), a diffuser to spread the light, and a backlight driver to regulate the available power into a constant current to drive the LEDs. A 1- to 1.5-inch display may contain 2 to 4 LEDs, while a 3.5-inch display may easily contain 6 to 10 LEDs. For LEDs, light output is proportional to current, and because LEDs have very steep current-voltage (IV) curves, it is important to closely match the current through the LEDs to ensure balanced backlighting, as the LEDs are typically distributed on one side of the LCD display. Software control is also required to allow the user to adjust the brightness and compensate for ambient lighting conditions. Depending on the current flowing through the LEDs, the LED color point may drift. Therefore, it is common to set the LED current to a fixed value and pulse-width modulate the LEDs to reduce the average light output. There are a number of factors that need to be considered to integrate small color LCD displays into handheld product designs and achieve the right balance of cost, performance, and battery life.

Battery-powered products require optimized LED driver circuit architectures that deal with multiple challenges such as space constraints, the need for high efficiency, and battery voltage variations—which can be higher or lower than the LED forward voltage. There are two common topologies, namely, a charge pump architecture/constant current source architecture with parallel configuration of LEDs and an inductor boost architecture with series configuration of LEDs. Both solutions have trade-offs to consider, such as the boost architecture ensures that all LEDs flow through the same current but require an inductor for energy conversion, while the charge pump architecture uses small capacitors for energy conversion, but all LEDs are arranged in parallel so closely that current matching becomes a difficult problem for balanced backlighting. Figure 1 shows examples of these two architectures.
PCbfans.cn Tips Please see the figure below:


PCbfans.cn Tips Please see the figure below:

Figure 1: Charge pump and inductor LED driver circuit diagram

The following points should be done when designing

1. Evaluate the approximate use time of the display

When selecting a white light LED driver, it is necessary to consider the frequency of display use. If the display will be backlit for a long time, having a high-efficiency converter is crucial to battery life. Larger displays require more LEDs, and applications with longer display usage will benefit from the more energy-efficient boost topology. On the contrary, if the display is only used for short-term backlighting, efficiency may not be a key design parameter.

2. Consider LED Selection Carefully

LED technology continues to improve rapidly, with manufacturers using new materials, manufacturing techniques, and LED designs to deliver greater light output for the same amount of current, so that a display that required four LEDs to backlight a few years ago may now be backlit by two LEDs. Furthermore, larger displays, 4 to 7 inches in size, that were typically backlit by cold cathode fluorescent lamps (CCFLs) are now being backlit by LEDs. In addition, LED forward voltages are trending lower. Therefore, it is important to consider not only the driver efficiency on the driver manufacturer's data sheet curve, but also the driver efficiency based on the selected LED. Table 1 lists some important specifications for several LEDs, showing how they differ in terms of forward voltage and light brightness. It should be noted that the wide range of forward voltages means that the driver efficiency should be evaluated using the LED specification limits.

3. Pay attention to wiring

Even if the individual LEDs are driven with very low currents of 10 to 20 mA, the peak current through the converter is significantly higher. This is especially true for inductive topologies, where the peak switch current can be 10 to 20 times the average LED current. Therefore, appropriate low-loss wiring techniques need to be used. For charge pump topologies, capacitors should be placed close to the driver to minimize loop area to avoid radiated switching noise. For inductive boost converters, input and output capacitors and inductors should be placed close to the driver. In addition, the current setting resistor (Rfb) should be connected directly to the chip ground, because the error between the internal reference and sense voltages will directly affect the LED current accuracy.

4. Test your product in real-world conditions

Consider how the display will perform under ambient high-brightness lighting conditions and ensure that the software dimming control has enough dynamic range to adequately dim the display under the expected lighting conditions.

The following issues should be avoided

1. Forgetting to consider boundaries and failure modes

Errors always happen. If the LED is open or shorted to ground, how should the driver handle this problem? For inductive boost drivers, if the LED string is open, the output will surge because the constant current will charge the output capacitor, requiring overvoltage protection, but this function may or may not be integrated into the driver. This may be a problem in factory testing because the display may not be installed during some test steps. In addition, it is important to evaluate the inrush conditions when the product is turned on, because the large current consumption during this period may reduce the battery voltage below the minimum operating threshold. Using soft start and/or software sequencing of different circuit blocks can minimize this problem. 2. Focusing only

on peak efficiency

Since the backlight brightness can be adjusted by the user, it is necessary to consider the driver efficiency during the time when the display backlight is expected to be on most of the time. When evaluating driver efficiency, the expected operating conditions of the LEDs, the battery voltage range, and the forward voltage variation need to be considered. Inductive drivers have better peak efficiency and are more tolerant to input and output voltage variations.

3. Ignore external component selection

In all design cases, low equivalent series resistance (ESR) X5R or X7R ceramic capacitors should be considered to minimize losses. Also in the inductive case, the forward voltage drop of the Schottky rectifier (if external) and the inductor ESR will affect efficiency. For example, using an inductor with a 0.3Ω ESR to drive 5 LEDs in series at 20mA will be 5% more efficient than using an inductor with a 1.3Ω ESR. Of course, this is not without cost, as the lower the inductor ESR, the larger the size of the inductor to provide the same inductance value. Fortunately, there are many new small-size inductors that allow the LED driver to be placed under the display.