Driving LED Backlights in Large LCD Televisions

May 22, 2007 4:29 PM
By Ahmed Masood, Vice President of Marketing, Supertex, Sunnyvale, Calif.

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.