Vendors of large-area and medium-area displays are switching from cold-cathode fluorescent lamps (CCFLs) to the more environmentally friendly LED backlights, as “green” technology permeates more and more systems. LED backlights are also thinner, consume less power, and are more reliable than CCFLs.

As designers face tighter environmental regulations, they focus on reducing the number of environmentally harmful substances in their systems. And, as consumers become more environmentally conscious, they demand replacement of the lead in integrated circuits and the mercury in CCFLs. Some of these changes are mandated by organizations such as the EEE, which developed the Restrictions on Hazardous Substances (RoHS) directive for electrical devices in the European Union. Thus, manufacturers of LCD panels and modules all over the world are actively looking for substitutes for CCFL backlights.

When compared with traditional CCFL backlighting, LED backlights meet many of the new environmental requirements simply by being hydrargyrum-free (free of mercury). They have many other advantages, such as good color reproduction, fast response speed, lower-power operation, improved safety (no glass to break or vapor to disperse), long life, and small size. Until now, the main drawback has been cost: matched, high-brightness LEDs have been much more expensive than CCFLs. But, that situation is changing, now that the sales volume of LED backlights has begun to increase. The effect of superior features in LED backlights is now driving their rapid adoption by display manufacturers.

Enlarge this picture Fig. 2. Use of an LDO regulator for driving the LED-backlight group in a medium-size LCD.

LED sources have replaced CCFLs in many applications requiring small-area backlights, and are now the mainstream choice for those applications. Similarly, the large increase in production of medium-size LCD screen modules for control panels, flat-screen monitors and televisions will lower the cost of LED backlights for those applications, again making them the mainstream choice for the larger LCD panels.

The driving scheme for medium-to-large LED backlights differs from that used for the small LCD screens found in mobile telephones and other portable products. To accommodate larger screens, the backlight must include several dozens, or even hundreds of high-brightness LEDs illuminated simultaneously. That configuration usually means higher driving voltage and higher current, more rigorous cooling requirements, more flexible controlling methods, and most important — adequate current matching and equal currents among the LEDs. Thus, LED backlighting for medium-LCD and large-size LCD screens imposes more stringent requirements on the driving circuitry.

Essential requirements

Enlarge this picture Fig. 3. Use of switched power for driving the LED-backlight group in a medium-size LCD.

To devise an effective power-driving circuit for LEDs, one must understand their essential photoelectric characteristics. LEDs are semiconductor devices. It is possible to fabricate about 30,000 5 mm LED die (chips) on a 3-inch epitaxial wafer, but testing, classifying, and packaging contribute to a reduction in the final number of usable LED products. The chips on a 3-inch epitaxial wafer can be sorted into various bins. (See Fig. 1.)

Chips on the dark blue field are the best, and chips on the middle fields are less stable, with the quality of chips on the outer field the worst on the wafer. Thus, the forward conducting voltage, wavelength, and brightness of LEDs from the same epitaxial wafer differ somewhat, providing a basis for sorting into various bins. Moreover, the fabrication processes of different manufacturers impose different environments for creating and cutting wafers and packaging the LEDs, thereby producing LED products with a range of characteristics.

LEDs are typically sorted and classified according to wavelength, brightness, and other parameters. When an LED conducts, its forward-current variation is much greater than the rate of variation for forward conducting voltage. The testing and classification for optical characteristics, therefore, is based mostly on a consistent nominal current value such as 20 mA or 350 mA. The range of allowable variation in forward conducting voltage is then noted for the brightness level specified. To guarantee equal brightness and chroma for similar LEDs, they must all have the same driving current.

To guarantee reliability, however, the drive current for LEDs must be lower than the nominal value. Moreover, this permitted nominal value decreases as the ambient temperature increases. An LED used for backlighting, therefore, must be driven with a known constant current. Otherwise, current that exceeds the maximum nominal value can affect the LED’s reliability.

Factory testing and classifying of LEDs is based on equal nominal current values. As a consequence, the LEDs purchased have similar optical characteristics around the nominal current value. LEDs are often dimmed by simply reducing their drive current. But, if those LEDs operate at a current far from their nominal value (for the purpose of dimming, perhaps), their optical characteristics will differ considerably, even if all have the same driving current. If two LEDs with nominal drive currents of 20 mA are driven at 5 mA, the total brightness diminishes, but the difference in their brightness levels increases greatly.

Medium-size LCD backlights must simultaneously illuminate dozens, even hundreds of high-brightness LEDs. If the forward currents deviate too much from the LEDs’ nominal value, the LEDs exhibit unequal brightness levels, even if those currents are all identical. To solve this problem, the drive current is often pulse-width modulated. Compared with direct analog control of the drive current, a PWM dimmer maintains LED currents at their nominal value, and adjusts the brightness via duty-cycle control of the PWM signal. As a result, all LEDs maintain a uniform appearance as their brightness changes.

Drive scheme

Enlarge this picture Fig. 4. Backlighting for this medium-size LCD panel includes switched power and an LDO driving eight 10-in-series strings of LEDs.

Medium-size LCDs (5 in. to 17 in.) are the displays most often found in portable DVDs, notebook computers, and GPS receivers. The LED driving strategy in these applications has developed according to the strategy for small-size LED backlighting, and is compatible with the current-driving circuits for CCFL backlights, as far as possible. One simply replaces the CCFL with an aluminum-based LED lighting bar, and replaces the complicated high-voltage AC driver with a simple low-voltage DC driver. The resulting cost is very close to that of a traditional CCFL backlighting module.

To extend the LED-backlighting drive scheme from small-size to medium-size LCD panels, one simply adds LEDs as required, and considers the series-parallel block of LEDs as an integral load. One then adjusts the brightness of the LCD screen by controlling the total current passing through this LED group.

A low-dropout voltage (LDO) regulator is commonly used to drive an LED group (See Fig. 2). It accepts the 12-VDC input required by a CCFL driver, and delivers an output current set by the external current-sense resistor, R1. The LDO shown (U1) can support a PWM dimmer while maintaining constant output current. U1 delivers output currents as high as 350 mA directly, and with the addition of an external bipolar-junction transistor, can deliver output currents as high as 2 A.

Another LDO from the same series as U1 (the MAX16804) integrates a PWM circuit on the CMOS chip. It simplifies circuit design by directly supporting analog dimmer signals. The biggest advantages of using an LDO to drive the LED group are simplicity and the absence of EMI. Another advantage of the LDO — it is easily fabricated along with the LED light group, and assembled directly within the LCD screen. Its main problem is lower efficiency, because the difference between the input and driving voltages appears directly across the LDO. When this difference is large, the efficiency is low, and the LDO becomes very hot. Even worse, one must choose LEDs with matched forward conducting voltages to minimize the effect of unequal current distribution in the parallel chains of series LEDs.

Replacing the LDO with switched power is the most direct method for improving the driving efficiency of the LED group. (See Fig. 3.) The main advantage of switched power is better efficiency. The input voltage of U1 (MAX16819) is 4.5 V to 28 V, which makes the LED backlighting module compatible with the original CCFL input voltage. One can also power U1 directly from a battery or AC adapter, which minimizes power switching links and improves the system efficiency. This scheme also supports PWM dimming directly. Like the LDO approach, however, one cannot eliminate the effect of unequal current distribution among the series strings, even by driving the LED group with switched power.

Thus, current distribution among series strings of LEDs cannot be made equal by controlling the current for the whole LED group, even by driving with the same voltage and choosing LEDs with similar electrical parameters (which would incur a higher cost for the LEDs). The series-LED string that conducts with the highest voltage would probably have insufficient current, and the string with the lowest voltage would probably conduct an excessive current that oversteps the nominal value. The result would then be unequal string illuminations and a short lifetime for some of the LEDs.

To avoid hidden trouble, one should configure the LED group to exploit the best features of both LDOs and switching regulators. One can obtain better efficiency by powering the whole LED group with a switching supply, and then place a series LDO in each branch to ensure equal currents in the branches. The 8-channel LED driver MAX16807, for example, equalizes the branch currents (to the level set by a single external resistor, 55 mA maximum) by including an LDO in the loop for each branch. These LDOs automatically adjust the load voltage for each branch. (See Fig. 4). The system processor controls each branch and the PWM dimmer’s duty cycle via a serial I2C bus interface. The output voltage can be as high as 36 V. Every loop can have as many as 10 white LEDs in series, so each group can include about 80 LEDs.

Summary

LED backlighting is becoming more and more popular, as the price of high-brightness LEDs decreases and their light efficiency increases. The LED-backlighting drivers for large-size and medium-size LCD panels are also making progress in response to market pressures. One consequence of these gradual changes is a trend toward replacing CCFL backlights with LED backlights. LED-driver circuits depend on the changing character of LEDs, so new circuits must continually emerge to take maximum advantage of the new LED devices.