LED Backlighting for LCDs Requires Unique Drivers

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.

Bigger LCDs with White-LED Backlighting
Instead of CCFL tubes, you can use white LEDs for large LCD backlighting. LED backlight can be side lit (like in laptops) or direct backlight type in bigger computer displays and televisions. Usually, these series-driven LEDs are divided into multiple banks.

Compromise must be made between the total voltage needed to drive the series-connected LEDs and the number of banks (which determines the number of LED control pins on the driver) Also, more banks require a lower voltage for driving the LEDs, but routing and controlling of the LEDs is more complex.

The number of series LEDs in one string defines the required boost voltage. Typically, the boost voltage is adapted to the actual forward voltage of the LED strings to minimize the power dissipated in the driver circuit and also to maximize the LED-drive efficiency.

The constant current-driven LEDs may use PWM control to set the desired brightness. You can use any one of several serial interfaces (SMBus, SPI or I2C) or an external PWM interface for brightness control. Depending on the application, synchronization to video signals might also be necessary.

Larger displays require more LEDs for backlighting. This causes problems, especially with heat dissipation, mechanical design and power-rail voltage ripple if traditional PWM control is used.

Always take into account the heat dissipation of the LEDs and the driver circuit when designing the mechanical solution for the backlight module. Operating at high temperatures reduces the LEDs’ operational life and shifts their spectrum while reducing luminance. You can compensate for spectrum shift by changing the electro-optical transfer function of the LCD pixels, but this also reduces contrast and brightness.

Integrated temperature regulation in the driver circuit can prevent overheating of LEDs. This temperature regulation dims down the LEDs gradually when the trip point is reached to reduce thermal loading.

Using traditional PWM for brightness control causes a large peak current drawn from the input-voltage rail when all LEDs turn on simultaneously. To compensate for this, use large-value input/output capacitors for the driver-circuit boost converter; however, this may cause electromagnetic interference (EMI) and noise problems.

There are several possible phase-shift PWM schemes to overcome these EMI and noise issues. One simple solution is to delay the LED outputs (i.e., turn them on sequentially). This reduces the peak current and allows the use of smaller input/output capacitors, which reduces costs. Fig. 5 shows the affect of a phase-shifted PWM scheme and its effect on the power-rail voltage drop.

Use an external light sensor as circuit feedback to adjust the backlighting relative to ambient light. If the driver circuit has a light-sensor interface, then lighting control is automatic and the application processor does not have to control the brightness based on the lighting conditions.

You can use optical feedback to compensate for the differences in LEDs and temperature effects. Sensor response can also compensate for the LED aging, thus increasing the lifetime of the backlight.

You can also compensate for variations in the LCD panel, color filter and LEDs with an integrated calibration memory in the driver circuit. In practice, this means that the manufacturer measures brightness and then calibrates the LCD display, with the calibration curves stored in the driver-circuit memory.

Adaptive Lighting Control
Adaptive backlight driving provides better contrast, black level and power savings. This concept means that the backlight dims based on the video signal and also when RGB levels are low, which minimizes LCD leakage. Simultaneously, you must increase video signal brightness to preserve the original brightness. The video processor must be able to adjust the LCD’s brightness and control the backlight driver circuit, which will adjust the backlight brightness accordingly.

Adaptive backlight driving can involve the whole screen (0D dimming), or you can divide the panel into blocks that have their own brightness control (1D and 2D dimming). You can use 0D dimming with side-lit displays, but 1D and 2D dimming are intended for larger displays with direct backlighting.

With 1D dimming, divide the display backlight into lines that have separate controls. Best results are achieved with horizontal segments, because this corresponds to the brightness profile of pictures like landscapes. From the video data, the brightness of the segments is calculated and these levels are filtered spatially and temporarily.

For 2D dimming, divide the panel into smaller segments of the horizontal and vertical axis, which provides more accurate spatial control for the backlight. Compared with 1D dimming, you can obtain better power savings and contrast improvements with 2D control. However, because there are more segments in 2D, their control is more complicated.

Compared with white LED driving, RGB LEDs have special requirements as noted in small format LCD backlighting. First, the white point of the backlight has to be controlled somehow. The preferred approach is optical feedback (i.e., closed-loop compensation) with bigger LCD panels, because it can compensate the white point variation caused by additional parameters like differences in LEDs.

With bigger LCD screens, the mechanical constraints for the backlight are more manageable than smaller displays. This enables an easier light sensor assembly for the backlight module.

To help compensate individual color issues, red, green and blue LEDs are connected in series in their own strings so that each primary color can be controlled separately. You can also divide the LCD screen into segments, so each segment has its own drivers.

Problems arise due to the different LED forward voltages in different color LEDs. Red LEDs have significantly lower forward voltage compared with green and blue LEDs. To maintain good LED-drive efficiency, this leads to the need for having a different boost voltage for different-color LEDs.

You can use similar 0D/1D/2D adaptive dimming with RGB-LED backlight modules as in white-LED backlighting. With RGB backlighting this can be taken even further so that the backlight is no longer kept white, but its color is changed based on the video signal. Effectively, the backlight is used as a low-resolution primary display and the LCD is a secondary modulator. Especially with 2D-color dimming, the power savings and contrast improvements are very significant compared with traditional LED backlighting.