Driving The Backlight: CCFLs Or LEDs?

The typical LCD backlight can be one or more cold cathode fluorescent lamps (CCFLs) or an array of light-emitting diodes (LEDs). An example of each is shown in Figure 1. The quality of the backlight image depends heavily on the backlight driver. In this article, we will discuss the considerations one can make for CCFLs and LEDs, as well as how to power both kinds of backlight.

GENERAL CCFL CONSIDERATIONS
CCFL backlights are the most common backlight technology and are used in displays ranging from 5.7 to 23 in. or more diagonal. They can have from one to 24 or more lamps mounted along the edge of the LCD or spaced uniformly over the entire back of the display.

Typically, brightness is controlled by modulating the CCFL current or lamp duty cycle. The basic driver is a dc-to-ac inverter powered by 5 to 48 V dc.

GENERAL LED CONSIDERATIONS
LEDs are already used in a wide range of smaller displays. For larger displays, because of their higher power consumption and, in some cases, their mercury content, CCFL backlights are beginning to be replaced by LED backlighting. LEDs may be arranged along the edges of the LCD or as a matrix over the back of the LCD assembly. The LED devices may be arranged in series or parallel. Either configuration will provide uniform LCD lighting. The LED strings can be arranged in parallel using a series resistor in each string to provide string-to-string current balancing as well as lighting redundancy.

While CCFL backlights typically provide white light, LED backlights may supply either white light or a mixture of red, green, and blue. LEDs emit light when biased in the forward direction. For quality performance, a constant current driver is required to compensate for LED voltage drops and changes with temperature. This ensures stable light output.

Unlike a CCFL, LED backlights don’t require high ac voltages; therefore, they don’t require an inverter. The basic LED driver is powered by 5 to 48 V dc and employs dc-dc boost to provide voltage to a constant-current driver that drives the LED string.

CCFL DRIVER CIRCUITS
Inverter product circuits can be divided into two groups: those with lower output power, which use power transistors as the primary circuit switching device, and those with higher output power, which use FETs.

The transformer steps up the input voltage. Design considerations include power, copper losses, and core material.

Figure 2 shows one type of CCFL driver in detail. Working backward from secondary to primary, a ballast (or secondary) capacitor, C2, reduces the voltage to the CCFL at the time the CCFL starts and the output current begins to increase. The relationships among the starting voltage (V_S), the voltage drop across the CCFL (V_R), and the voltage drop across the secondary capacitor (V_C), are defined by: V_S² = V_R² + V_C². The value for the secondary capacitor value depends on the output current and output frequency. Increasing the capacitance increases the output current and reduces the frequency.

On the primary side, capacitor C1 fine-tunes the inverter output-current level and output operating frequency once the secondary load is defined, the secondary capacitor is chosen, and the number of transformer primary and secondary turns is decided upon. C1 “rolls off” the output current and frequency, which were determined by the values of the components on the secondary side.

Base-current-limiting resistor R1 establishes enough transistor base current to guarantee transistor saturation. Meanwhile, the choke circuit reduces ripple in the input current as the transistors switch the primary windings. The choke also increases the current rise time at inverter turn-on. The goal is to reduce peak inrush current and choke audible noise from the choke.

Trading off inductance, physical size, saturation current, IR losses, and power losses make selecting an inductor somewhat challenging. Be aware that long input-current rise times can reduce the effectiveness of pulse-width modulation (PWM) dimming at low duty cycle. Also, careless choke selection can produce saddles or worse in the rising input current, and that will adversely affect low duty-cycle PWM dimming as well as CCFL startup.

All inverters with high power outputs or those that incorporate on-board dimming should use input bypass capacitors to reduce input voltage ripple. Without them, each time the inverter power devices switch, the resulting current increase will cause an input voltage decrease.

LED DRIVER CIRCUITS
The design in Figure 3 represents a constant current chopper driver that provides a dc current with 10% ripple to an LED string used to edge light an LCD. The pass switching device is a P-channel FET that provides the current to the LED string and, in conjunction with the inductor, sense resistor, and boost voltage, establishes the chopping current and frequency.

The dc-dc boost stage is a closed-loop boost supply that provides sufficient voltage to drive current to the LED string with at least 2 V of headroom. The part of the diagram designated Section A shows a comparator and associated resistors that form a positive hysteresis circuit. It compares the voltage across the sense resistor to a known reference. Section B in the diagram shows another comparator and associated resistors that buffer the Section A output to ensure proper hysteresis and provide drive to the pass device.

Section C in the diagram supplies LED on/off and dimming control. The +ENABLE input turns the backlight on or off, and +PW pulse-width modulates the chopper driver on and off for dimming. Implementation can be quite compact (Fig. 4).

THERMAL CONSIDERATIONS
The ambient temperature in which the LCD operates is a key consideration for the backlight-driver designer. CCFL starting or strike voltage is inversely proportional to temperature. Figure 5a shows a typical relationship between CCFL strike voltage and temperature, and Figure 5b illustrates CCFL brightness variation with increasing lamp current.

The time a CCFL needs to reach specified brightness is also inversely proportional to temperature. Mission-critical applications that require rapid brightness increase may need the inverter to provide a higher boost current for a short time to enhance CCFL warm-up and to accelerate the time to required brightness. However, as helpful as higher CCFL current is to lamp warm-up, sustained high current can saturate the lamps. It may also produce an actual decrease in brightness along with elevated lamp temperature and accompanying decreased lamp life. Rated lamp current for most CCFLs is between 3 and 8 mA rms.

LED backlights are less sensitive to low temperatures. Slight changes in LED electrical characteristics and turn-on time at lower temperatures don’t demand any special driver design considerations.

High application temperatures also affect driver design. In fact, above all other variables, high temperature has the most significant impact on CCFL driver function and reliability.

Copper and core losses in the transformers for CCFL drivers can be significant heat sources. Transformers typically operate at temperatures as much as 30ºC above the local environment. Copper and core losses can be minimized by tailoring the driver design to the CCFL, which sustains voltage and current.

High application temperatures are also important to LED backlights. However, the focus here is on the temperature of the LED itself, not the driver components. Recent advances in LED technology, packaging, and materials have generated dramatic increases in LED brightness. The challenge for LED backlights is to get the heat out of the LED device itself and then out of the display assembly. The key design point is to keep the LED junction temperature below 100ºC to ensure reliability.

DIMMING
LCD applications requiring a wide range of brightness are ever increasing. The driver must be able to deliver high brightness for daylight vision and low brightness for night vision. Brightness control across this wide requirement range must be smooth and free of flicker.

Analog dimming of CCFL backlights, wherein the driver output current is modulated to change lamp brightness, provides coarse dimming to about 30% of full brightness—not enough dynamic range for most application requirements. Furthermore, analog dimming can stress the oscillator transistors and reduce inverter reliability.

PWM dimming brings significantly better dimming control. In this type of dimming, the CCFL or LED is pulsed on and off at a fixed frequency, and the duty cycle is modulated to provide variable brightness. Typically, CCFL backlights are modulated at frequencies between 100 and 500 Hz. Low-level brightness control of CCFL backlights with four or more lamps can be enhanced by selective enable techniques, in which the lamps are sequenced off as brightness is reduced.

Also, the best way to dim LED backlights is via PWM dimming. Much wider dimming ratios can be achieved with LED backlights because the basic switching time of an LED is measured in nanoseconds compared to milliseconds for a CCFL.

INPUT VOLTAGE
Most LCD backlight drivers run off a 12-V-dc input, although applications may range from 5 to 48 V dc. CCFL control loops may be open or closed. Open-loop designs require regulated input supplies, because strike voltage and output current vary with input voltages. Closed-loop CCFL drivers provide constant strike voltage and current over a range of input voltages. As a result, they’re more desirable in applications that don’t have a regulated input. Typically, battery-powered applications fall into this category.

For LED drivers, V_CC must be greater than the minimum that’s required to power the LED string and sense resistor. The dc-dc boost stage must be closed loop to provide a relatively stable V_CC with no- or full-load conditions.

While obvious major differences exist between CCFL and LED backlighting of LCDs, the driver designer must observe certain similarities and basic principles. These include allowing for the backlight’s ambient temperature, paying particular attention to low temperatures for CCFLs, and high temperatures for LEDs.

The key challenge for CCFL backlights concerns CCFL packaging and driver layout due to high voltages, while the key challenge for LED backlights revolves around the load packaging due to thermal management. Either technology is best served by PWM as opposed to analog dimming. Though this article focuses on the major considerations, other factors, of course, must be considered in optimizing the driver design for application-specific demands, cost control, manufacturability, and reliability.