Choosing the optimal LED backlight driver for portable MFF displays

White LEDs are rapidly replacing cold cathode florescent lamps (CCFL) for LCD back (edge or side)-lighting in larger media form factor (MFF) displays up to 19in. The backlights for these displays may require up to 100 LEDs. Determining the optimal parallel and series connection and dimming method for the LEDs, in order to prolong battery life without sacrificing display brightness quality, can be challenging. This article provides guidance on how to pick optimal white LED (WLED) drivers for MFF displays in order to achieve high efficiency (long battery life) and best brightness for the lowest total cost.

WHY WLEDS ARE REPLACING CCFL IN MFF DISPLAYS
The shift away from CCFL started with the European Union’s RoHS initiative, which has sought to purge several toxic substances including mercury, a major component in fluorescent lamps, from consumer products. But WLEDs have other advantages over CCFL, such as being a solid-state device; directional light source; operating at much lower voltages; easier to dim over a wider brightness range; and the brightness is more linear when dimming.

WLED’s directional lighting allows the use of smaller diffusers and light guides in displays, resulting in thinner panels and notebooks.

CHOOSING THE WLED DRIVER TOPOLOGY
A WLED’s brightness varies linearly when the current passes through it. For the best WLED current accuracy and uniform WLED brightness per string, an LED driver should regulate current the voltage through instead of across an LED. Figure 1 shows how any adjustable DC/DC converter is easily re-configured as a constant current source to drive multiple WLEDs in series. As long as its output is greater than the sum of the LEDs forward, voltage (VLED) drops.

By regulating VSENSE, the voltage across the current sense resistor (RSENSE) and not the output voltage (VO), the driver is essentially a constant current source – leaving its output voltage (VO) free to self-adjust for changes in VLED with current and temperature. WLEDs have voltage drops ranging from 3V to 4V, with the drop varying directly with the LED current and inversely with temperature. Therefore, the WLED driver’s output voltage must be capable of reaching at least the sum of the WLED string with the largest VLED drops at the maximum LED current per string.

Although the input voltage for the majority of backlight applications ranges from 3.6V to 48V DC, most MFF LCD backlight drivers use 7.2V to 21V stacked lithium-ion (Li-ion) cells to drive 24 to 100 LEDS. The number of WLEDs for various MFF panel sizes varies from 36 for 12.1in panels to 72 for 17in panels.

Regulating up to 72 LEDs in a single string using the configuration in Figure 1 results in voltages up to 72 x 4V = 288V. Consequently, most LED backlight drivers are based on boost converter cores. High-voltage, single-inductor boost converters are expensive and difficult to design because they require:

• higher voltage-rated and, therefore, larger and more expensive power FETs, similarly rated diode and output capacitors
• a boost controller capable of duty cycles (D=Vout/(Vout+Vin)) from 87.5–96 percent, which, assuming 1MHz switching frequency, results in on-times (tON) of 875–960ns, and very difficult to control minimum off time (tOFF) of 40ns
• a costly and space-consuming insulation barrier to prevent arcing to chassis,
• high-voltage handling and testing procedures
• additional consumer product safety ratings

They also produce more electromagnetic interference (EMI) due to higher common mode current, computed as ICM = CPAR*VOUT*fSW, where CPAR is the parasitic board capacitance from drain to earth ground and fSW is the boost converter switching frequency.

Moving to a fly-back topology instead of an inductor-based boost topology allows you to use a standard, lower cost boost controller IC, but with the added complication of a custom designed transformer. Therefore, to keep the IC and supporting passive component costs low, manufacturers of boost-based drivers with integrated FETs prefer to limit the driver output below 60V. A single LED string driven by such a boost converter would be limited to less than 20 LEDS, hardly enough to drive larger MFF panels. So, the converter in Figure 1 has several m strings in parallel, each with n LEDs and ballast resistors in the 10Ω range to help equalize the current through and voltage across each string. The closer the current is through and voltages across the WLEDs, the more uniform the color and brightness of each string will be.

It is difficult to size the ballast resistors in Figure 1 to provide perfect matching between strings. A better approach is to incorporate the boost converter and multiple current regulators (sinks) that literally pull the same current through the strings into a single driver IC (Figure 2). Drivers sense the voltage drop at each VIFBx pin and use the boost converter to provide just enough output power to keep the lowest VIFBx pin voltage (VIFBmin) above its current regulator’s maximum dropout voltage. The next question becomes, how to select n and m?

OPTIMIZING THE NUMBER OF n STRING LEDS PER m STRINGS
There are several factors to consider when selecting n and m for boost-based drivers:
• nMAX x VLEDmin < the boost converter’s maximum output voltage level .
• nMIN x VLEDmin > VINmax
• m determines the brightness requirement and sets the converter’s maximum load current, I_LOADmax=m x I_LEDmax.

Measured data confirms that for the same input voltage and ILED per string, a driver with an m=6 and n=12 (i.e., 12S6P) configuration is more efficient than the same converter with a 9S8P configuration. Why is that? While a detailed efficiency analysis of the boost converter and current regulators is beyond the scope of this article, one can intuit the answer.

Boost converter losses increase as the converter’s output power increases. The boost converter output power increases as VOUT increases and/or the output load increases. The boost converter output voltage increases as the number of n series LEDs increases and output load increases as the number of m strings increases (or as the current per string increases). The losses in the current regulators are simply each string’s current times the voltage at each current feedback pin, IFBx. Obviously, the regulator losses are higher as the current per string increases or for a driver with a large VIFB regulating voltage. As previously mentioned, driver’s as shown in Figure 2 regulate the boost converter so that the output voltage rises only to the sum of the VLEDs of the string having the WLEDs with the largest VLEDs plus VIFBmin. Because the voltages at the remaining VIFB’s are higher due to the LEDs in the remaining strings having lower voltage drops, the remaining current regulators waste power.

Statistically, there are an optimal number of m strings n LEDs per string to minimize that power loss and maximize the driver’s efficiency. Performing a statistical analysis of the losses in the current regulator by incorporating the mean, variance and standard deviation of the voltage drops in the LEDs reveals that the current regulator losses increase directly with the number of m strings, but only as the square root of the number of n LEDs per string.

Figure 3 shows the results of an efficiency model for the boost converter and current regulators for a specific driver.

While the results vary a bit with VIN, ILED and VIFB, it is apparent that backlights for most MFF panels will be most efficient with 4
DIMMING
As shown in Figure 4, the simplest method of dimming a WLED string is to apply a pulse-width modulation (PWM) signal at a fixed frequency with a duty cycle of D to the driver’s enable pin in Figure 1. The average WLED current is the PWM signal’s duty cycle times the LED maximum current ILED-max, i.e., ILED-avg = D x ILED-max. Because the maximum current through the LEDs is the same, PWM dimming results is a very linear change in brightness. Also, since an LED’s emitted spectrum of light varies with its voltage drop and the voltage drop varies with ILED, the LED backlight’s chromaticity, its colorfulness and hue is excellent when PWM dimming.

Nevertheless, ceramic output capacitor’s piezoelectric properties can cause a problem when PWM dimming is used. Specifically, as the capacitor charges and discharges at PWM signal frequencies in the audible range (20Hz–20kHz), it vibrates and the human ear hears the capacitor and PCB motion as a ringing or buzzing sound. The vibration is directly proportional to voltage amplitude and ceramic capacitor package size. Reducing the capacitor package size reduces the ringing. Moving to more m parallel strings of fewer n LEDs per string, thereby lowering the voltage on that capacitor, reduces the magnitude of the ringing. Also, recent drivers with current regulators simply turn off the current regulator and boost converter when PWM dimming, preventing the ceramic output capacitor from discharging fully during PWM dimming.

Initially, to avoid ceramic capacitor ringing, many panel makers moved to analog dimming, also shown in Figure 4. Analog dimming causes virtually no output ripple because an external signal adjusts the boost converter of Figure 1 or current regulators of the regulation point in Figure 2 and, therefore, the DC current level through the LEDs. Other benefits of analog dimming over PWM dimming include higher electrical efficiency, since the boost converter output voltage = VLEDs drops as ILED drops, and also higher optical-to-electrical efficiency, meaning more lumens for the same power consumed.

Analog dimming has some current accuracy problems when deep dimming because either the VREF voltage or the current sink voltage becomes too small to accurately control due to the offset voltage of the error amplifier. And, the brightness linearity and chromaticity are not as good as can be achieved with PWM dimming, especially when deep dimming. So, the optimal solution is to combine the PWM and analog dimming methods, termed mixed-mode dimming as illustrated in Figure 4.

Mixed-mode dimming uses the input PWM signal to implement analog dimming until just before the LED current drops low enough to significantly affect LED accuracy, linearity and chromaticity. In Figure 4, that current is reached when the PWM signal duty cycle (D) is 12.5 percent. At this minimum current level, the circuit begins using true PWM dimming. However, instead of turning on and off the maximum LED current through the current sinks at the input PWM signal’s duty cycle, the circuit translates the input duty cycle to the appropriate value for the minimum WLED current level achieved with analog dimming.

For instance, the TPS61195 is capable of driving up to m = 8 strings (in parallel), each with n = 10+WLEDs (in series). Through the SMBus interface, the TPS61195 also provides flexible dimming options so that the design engineer can dim the WLEDS using either pure PWM dimming or a mixed mode of analog and PWM dimming, according to the system requirement.

CONCLUSION
Experts predict that WLED’s will completely replace CCFL’s in MFF LCD panel backlight by 2011. Backlight driver manufacturers are continually improving the backlight drivers in order to meet the panel makers need for small solution size, maximum efficiency and flexible dimming. For example, the 4x4 QFN packaged TPS61195, driving eight strings of 12 WLEDs each from input voltages up to 21V and providing flexible dimming, meets these needs.
About the author
Jeff Falin is a Factory Applications Engineer with the High-Performance Analog Portable Power Applications group at Texas Instruments Inc. Jeff received his MSEE from the University of Tennessee with a concentration in IC design. He can be reached at ti_jfalin@list.ti.com.

Click here for the illustrations:

Figure 1, Figure 2, Figure 3, Figure 4, Figure 5

Caption
Figure 1: Adjustable output DC/DC converter providing constant current through WLED string(s).
Figure 2: Boost converter based backlight driver with integrated current sinks.
Figure 3: Total number of LEDs vs. total driver losses Vin=11V, VIFBx=0.4V, ILED=20mA.
Figure 4: Methods of dimming.
Figure 5: Example of a backlight driver using the TPS61195.