Driving LED Backlights in Large LCD Televisions

May 22, 2007
Selecting a driver architecture for white LEDs used in LCD backlighting requires consideration of the desired TV features, the available dc system voltages, and the manufacturing tolerances of the LEDs themselves.

There is an ongoing effort to replace conventional CCFL backlighting in large-area (40-in. or greater) LCD TVs with LED backlighting. The benefits LEDs provide include improved color gamut, adjustable color temperature, backlight blinking to compensate for motion artifacts of the LCD, longer reliability, and mercury-free components. Additionally, since LEDs are inherently point sources of light, a matrix of direct-emitting LEDs can use a local dimming driving scheme across the panel to greatly improve contrast ratio. This scheme dims, or turns off, LEDs in darker areas in the image, and turns on LEDs more fully in brighter areas of the image. This is a major benefit compared to global dimming, where the entire backlight is dimmed uniformly in response to the displayed image.
An important consideration for any LED backlight unit (BLU) is the driving scheme used for powering the LEDs. The architecture chosen for this scheme can have a dramatic impact on the TV’s overall cost and performance including it’s power consumption (energy efficiency), contrast ratio, motion artifacts, white point balance, and audible noise.

To maintain backlight uniformity in a large-area LCD display, currents in a large number of LED devices must be balanced with high accuracy. The most obvious way of balancing LED current is to connect them in series strings, where each string includes a large number of LEDs. Parallel connecting LEDs is usually complicated by a significant variation of their forward voltage, VF, at a given current, IF. For example, typical VF of a blue LED can vary between 3 V and 4 V even at room temperature. The problem is further aggravated by a significant temperature dependence of VF. However, the number of LEDs in each string is usually limited by a certain total voltage dictated by some practical considerations of power conversion or by safety requirements. Hence, multiple LED strings must be used. Current in each string must be controlled by a dedicated regulator, either linear or switching, that forces the LED current to meet a reference level common for all LED strings of a given color.

LED Driver Architectures

Switching LED drivers achieve the highest efficiency. Each series LED string requires a dedicated switching current regulator. The power architecture of the switching regulators depends on the number of LEDs in each string and on the supply voltage available in the TV. Two basic types of regulators are generally used: a stepup (boost) regulator and a stepdown (buck) regulator. Linear regulators are not very practical in most cases, since they need to dissipate a large amount of power to balance the LED string currents. However, mixed solutions exist that combine a single switching regulator with multiple linear current sinks. In this case, the switching regulator is used to regulate the dropout voltage across the linear sinks, thereby minimizing the power dissipation in them. Though being somewhat less efficient than switching regulators alone, this second class of BLU drivers can be cost effective in certain situations.

Linear drivers are usually paired with boost converters since the output LED string voltage is generally higher than the input voltage. This boost converter may be integrated or discrete, and has a typical efficiency in the low 90% range. Usually, output LED voltages are less than 50 V. Control circuitry monitors and selects the channel with the lowest headroom for regulation to maximize efficiency. Overall efficiency including boost and linear components varies depending on the range of variation in the LED forward voltages, but efficiencies of 80% are feasible (Fig.1).

Boost drivers such as that shown in Fig. 2 are popular because regulated 12-V and 24-V supplies are commonly available in existing TV platforms. To minimize the number of boost converters, and therefore system cost, the number of LEDs connected in series can be made quite large. Output LED string voltages are typically up to 200 V, with efficiencies in the low 90% range. Typical boost drivers can increase the input voltage up to ten times while achieving high efficiency.

Buck drivers may alternatively be used instead of boost drivers (Fig. 3). While lower supply voltages are common for smaller size TVs, they can cause a significant thermal problem in larger ones. High currents up to tens of amperes would need to be distributed across the panel to power such a low-voltage backlight unit. However, a supply voltage of 60 V or greater can be used to minimize the power loss in the wiring. In this case, a buck regulator can power each LED string.

Design Considerations: Linear Vs. Switching

For comparison purposes, we assume that a multi-output secondary supply for power-factor correction and isolation provides power to on-board circuitry, including the LED driver portion. Since this stage is present in all cases to be analyzed here, it does not impact the analysis, and can be ignored.

Linear drivers represent the simplest scheme for regulating constant current through LEDs. However, this simplicity comes at the expense of efficiency when compared to switching regulators. Not only is the linear driver itself inefficient, but since the output voltage VO remains constant across all strings, any variation in LED string voltage between strings degrades this efficiency further. This variation results from variations in VF among the individual LEDs due to manufacturing tolerances, and cannot be eliminated.

Fig. 4 illustrates this degradation in efficiency versus the variation in VF, by examining the chip temperature as the number of channels on-chip increases. This example uses an MQFP package having a JA of 51°C/W, 14 LEDs per channel, and a worst-case 85°C ambient temperature. The LED voltage spread identifies the LED forward-voltage binning required to keep the chip’s junction temperature below a certain value. Notice that the larger the spread in VF, the greater the chip temperature becomes due to larger voltages dropped across the linear regulating channels. Furthermore, adding channels to the IC is desired to minimize the number of boosts needed, but this requires tighter manufacturing binning for VF to reduce the total LED string voltage, and therefore reduce heat dissipation in the linear regulator stage.

For example, if the linear pass element chip can withstand a 120°C junction temperature, then the maximum permissible VF variation for six channels is 0.3 V. This reduces to 0.07 V for ten channels. For this reason, many multi-channel linear drivers on the market cannot use all channels simultaneously at their maximum current rating since they are ultimately limited by the power dissipation of the package. In general, binning LEDs for VF improves efficiency as the bin-width reduces, but adds to system cost. As the VF binning width approaches zero, the mixed boost-linear driver architecture’s overall efficiency approaches that of the boost-only regulator. p> Alternatively, boost drivers can power a much larger number of LEDs in series and eliminate inefficient linear regulators. The total forward voltage of the LED string can approach hundreds of volts, and brings a considerable efficiency improvement. If the maximum LED forward voltage is 3.6 V, for example, then 50 LEDs can be strung together for a maximum drop across the LEDs of 180 V. While this configuration is ideal for global dimming applications, it becomes expensive when used for local dimming where the number of LEDs per string is low.

A choice between the boost and the buck architectures may also be driven by cost considerations. Output current of a buck regulator is continuous. Hence, a simple constant peak-current control can be implemented with the buck driver to regulate the LED current. Moreover, the ac-ripple content in its output current is small, and therefore, little or no additional attenuation is required. Since output capacitance of the buck driver is small, the rise and fall transition times of the PWM dimming can be made as fast as the current slew rates of the output inductor. Thus, the buck topology permits a very simple and inexpensive dimming control circuit. Buck drivers are a good choice for local dimming applications compared to boost drivers for these reasons. Strings used for local dimming usually contain a relatively small number of LEDs, further justifying the use of buck architectures.

On the other hand, due to its open-loop nature, peak-current control can introduce significant output current errors caused by variation in the inductance value and propagation delays in the control circuit, especially at high switching frequencies. Closed-loop control of a buck driver requires high-side current sensing when its power switch is referenced to the ground potential.

Alternatively, a boost converter can be used for greater current accuracy. Closed-loop control is easily implemented with the boost driver since the output current signal is available at the ground potential. However, PWM dimming of a boost driver is more complex. The LED string must be disconnected from the output of the driver with a dedicated load disconnect switch for each PWM dimming cycle. Moreover, the error signal must be stored prior to the turn-off transition in order to prevent over-shooting of the LED current upon the subsequent turn-on. Hence, the control circuit of a boost driver can significantly affect the overall cost.

Fig. 5 illustrates the cost relationship between switch- (buck or boost) and linear- (boost plus linear) based architectures as a function of LED current, for a given panel size and power. For low LED currents, switchers are more expensive due to higher external passive component costs. As the LED current increases, the number of required LED strings decreases, lowering the current-control circuitry expenses for both solutions. However, increasing LED current eventually increases the linear-based cost due to the need for thermal management components. For very large LED currents, these costs become prohibitive. The high efficiencies afforded by switchers eliminate the need for thermal management components, and the intersection of the cost curves is empirically placed at 50 mA. Note that this analysis doesn’t account for power consumption in the sense of customer utility bills, which might be a consideration even when linear-based solutions appear most cost-effective.

A Spectrum of Solutions

The table summarizes the key attributes of the different LED driver architectures for LCD backlighting of large LCDs. Linear plus boost drivers are simple to design and cost-effective for LED currents below 50 mA, but provide poor efficiency for large-area TVs.

Switcher-based drivers are the most cost effective option when driving LEDs above 50 mA, where linear-based drivers require expensive thermal management components. Boost drivers provide accurate current control and are well suited for global dimming applications where LED string voltages can be hundreds of volts. Finally, buck drivers are useful for distributing high-voltage buses to minimize power loss in power-distribution wiring, and their open-loop control makes them the most cost effective for localized backlight dimming.

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