LED Backlighting for LCDs Requires Unique Drivers

May 12, 2008 11:31 AM
By Tomi Koskela, Applications Engineer, National Semiconductor, Oulu, Finland

Driver designs depend on the choice of RGB or white LEDs, LCD display size, color reproduction, power efficiency and system cost

A typical LCD consists of liquid crystal material with transparent electrodes and polarizing filters. Applying voltage across the liquid crystal layer allows light to pass through in varying amounts. Therefore, to illuminate a visible display the majority of LCDs employ an external light source, or backlighting. The backlighting subsystem requires driver circuits that provide the necessary controls to achieve optimum color reproduction.

In the past, cold-cathode fluorescent lamp (CCFL) backlights dominated the LCD displays in screens for computers and television sets. Currently, LED backlighting is the approach of choice because it exhibits better image quality while saving power. To achieve their full potential, however, LED backlighting requires sophisticated driving methods. Driving methods differ for white LED and RGB LED backlighting for small to large LCD displays.

Typically, a couple of white LEDs provide backlighting of small-panel LCD displays. As shown in Fig. 1, the LEDs are on the edge of the display, and a light-guide plate aids in achieving uniform backlighting.

White LEDs usually employ a constant-current drive using a pulse-width modulation (PWM) for dimming effects. You can drive the LEDs in either parallel or series. The series connection needs a relatively high-voltage boost converter to produce enough voltage to illuminate a large LED string. Along with easy control, series connections also simplify pc-board routing and enable optimum current matching between LEDs. Therefore, series connection is the preferred approach.

The emission spectrum of the backlight source and the transmission spectrum of its color filters determine the color gamut of the LCD display (i.e., the range of colors it produces). The problem with white LEDs is that their spectrum is not ideal for photographic reproduction because they are basically blue LEDs with a yellow phosphor on top. Their color spectrum has two peaks, one at blue and another at yellow. Fig. 2 shows the typical white versus RGB LED spectrum.

Pixels in LCD displays are divided into cells of three primary colors; red, green and blue. Pixel color is defined by mixing the primary colors. Color filters filter the right color to each cell.

Nevertheless, color filters waste a big part of the optical power and even after color filtering, the color spectrum passing through the LCD is not ideal. With white LED backlighting it is possible to produce up to 75% of National Television Standards Committee (NTSC) colors on an LCD.

When RGB LEDs are used for LCD display backlighting, the color reproduction can be adjusted to cover over 100% of the NTSC color gamut. This results in brighter colors and better picture quality. Fig. 3 shows the typical color gamut of different backlight technologies.

RGB Backlighting Compensation
To avoid shifting the white point, the driver must compensate for the so-called red shift in LED-emission wavelengths when temperature changes. The driver must also keep the light intensity adjusted correctly at any operating temperature. Compensation for these effects can be either closed loop or open loop.

With closed-loop compensation, an optical sensor measures the white point and intensity. With open-loop compensation, the temperature is measured and predefined compensation curves adjust the brightness balance.

Fig. 4 illustrates the principle of the open-loop color compensation. One example of an RGB backlight driver for small LCD displays employs the LP5520, an open-loop compensated LED driver. Its application involves five steps:

1. Measure the temperature compensation curves for the actual RGB LED type used
2. Program these curves into the LP5520’s internal EEPROM
3. Integrate the LP5520 into the LCD display module.
4. The module manufacturer programs compensation curves in production.
5. Use an RGB LED backlight optimized color filter.

There are three efficiency factors of LED backlight driving: boost efficiency, driving efficiency and optical efficiency. You can optimize boost efficiency with an adaptive boost mode that adjusts boost voltage based on the required headroom for the LED outputs. Meanwhile, LED driving efficiency is affected by two factors: PWM duty cycle and current ratio.

LED optical efficiency drops with lower PWM duty cycle values if power sourced to the LED is kept constant by adjusting the PWM duty cycle. This occurs because of the higher current needed with lower PWM values, which results in higher LED forward voltage. Driving RGB LEDs with the same boost voltage wastes red- LED-driver power because of the significantly lower forward voltage of red LEDs compared with that of green and blue LEDs.

When directly comparing the efficiencies of RGB-LED and white-LED backlights with the same color filter, results show 15% to 30% better efficiency for white-LED backlighting. This occurs mainly because of the better efficacy of white LEDs compared with RGB LEDs and also, to some extent, the better LED drive efficiency.

With RGB LEDs it is possible to use optimized color filters and this alone gives 20% to 40% improvement in RGB-LED backlight total efficiency. Improved red-LED driving could give an additional 10% to 15% improvement in efficiency. With RGB backlighting, it is possible to get better color gamut with additional optimized color filters. This also results in some power savings compared with white LEDs. Table 1 shows measurement results that compare RGB and white LED backlight efficiency using the LP5520 driver.

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