LED driver circuit design tips for small LCD backlights

Color LCD displays require white backlighting to ensure that the display is viewable in any lighting environment. The backlight system consists of an array of high- brightness white light-emitting diodes ( LEDs ), a diffuser to diffuse the light, and a backlight driver that regulates the available power to a constant current to drive the LEDs. A 1- to 1.5-inch display might contain 2 to 4 LEDs, while a 3.5-inch display might 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 that the current through the LEDs is closely matched to ensure balanced backlighting, as the LEDs are typically distributed on one side of the LCD display. Software controls are also required to allow the user to adjust brightness and compensate for ambient lighting conditions. Depending on the current flowing through the LEDs, the color point of the LEDs can drift. Therefore, it is common to set the LED current to a fixed value and pulse-width modulate the LED to reduce the average light output. There are a number of factors to consider when integrating a small color LCD display into a handheld product design and achieving the right balance of cost, performance, and battery life.

Battery-powered products require optimized LED driver circuit architectures that deal with concurrent challenges such as space constraints, the need for high efficiency, and battery voltage variations—which can be higher or lower than the forward voltage of the LED. Two common topologies are charge pump architectures/constant current sources with LEDs in parallel and inductor boost architectures with LEDs in series. Both approaches have tradeoffs to consider, such as boost architectures that ensure the same current through all LEDs but require inductors for energy conversion, while charge pump architectures use small capacitors for energy conversion but all LEDs are so closely connected in parallel that current matching becomes a difficult problem for balanced backlighting. Figure 1 shows examples of these two architectures.

Figure 1: Charge pump and inductor LED driver circuit diagram.

The following points should be made when designing

1. Estimation of the approximate usage time of the display

When selecting a white LED driver, consider how often the display will be used. If the display will be backlit for long periods of time, having a highly efficient converter is critical to battery life. Larger displays require more LEDs, and applications where the display is used for longer periods of time will benefit from a more efficient boost topology. Conversely, if the display is only backlit for short periods of time, efficiency may not be a critical design parameter.

2. Carefully consider LED selection

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 for backlighting a few years ago may now be backlit by two LEDs. Furthermore, larger displays, 4 to 7 inches, that were typically backlit by cold cathode fluorescent lamps ( CCFLs ) are now being backlit by LEDs. In addition, the forward voltage of LEDs is trending lower. Therefore, it is important to consider driver efficiency not only on the driver manufacturer's data sheet curve, but also based on the evaluation of the driver with the selected LED. Table 1 lists some important specifications for several LEDs, showing the differences in forward voltage and light brightness. It should be noted that the forward voltage range varies widely, which means that the efficiency of the driver should be evaluated at the extremes of the LED specification.

Table: Characteristic parameters of several different LEDs.

3. Pay attention to wiring

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

4. Test your product in real environment

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

The following problems should be avoided

1. Forgetting to consider boundaries and failure modes

Errors will always happen, if the LED is open or shorted to ground, how should the driver handle this? For an inductive boost driver, if the LED string is open, the output will surge as the constant current charges the output capacitor, requiring overvoltage protection, which may or may not be integrated into the driver. This can be a problem during factory testing , as the display may not be installed during some test steps. It is also important to evaluate the surge conditions when the product is turned on, as the large current draw during this time 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 is user-adjustable, it is necessary to consider the efficiency of the driver during the time that the display backlight is expected to be on most of the time. When evaluating the efficiency of the driver, it is necessary to consider the expected operating conditions of the LEDs, the battery voltage range, and the forward voltage variation. Inductive drivers have better peak efficiency and are more tolerant to input and output voltage variations.

3. Ignore external components selection

In all cases, consider using low equivalent series resistance (ESR) X5R or X7R ceramic capacitors to minimize losses. Also in the case of inductors, the forward voltage drop of the Schottky rectifier (if external) and the ESR of the inductor 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 an inductor with a 1.3Ω ESR. Of course, this is not without cost, as the lower the ESR of the inductor, the larger the size required to provide the same inductance value. Fortunately, there are many new small-size inductors available today that allow the LED driver to be placed underneath the display.