LEDs light up LCD TVs

No one likes a recession, with the possible exception of Hollywood. And why is that? The answer is simple, if not obvious, box office receipts go up in an economic downturn.


Another form of entertainment that is seeing a positive impact on its sales during the downturn is the LED backlit flat-panel liquid crystal display television. 

White LEDs for backlighting

To understand the driving force for white LED backlit flat-panel LCD TVs, it is important to have some idea as to why they should be used over other forms of lighting, such as cold cathode florescent lamps (CCFL).

Large-panel LCD TVs have traditionally used CCFL backlighting. However, these CCFL backlit televisions have a variety of shortcomings ranging from fast motion blur to inaccurate colour reproduction.

With the current generation of LCD TVs, true blacks cannot be attained and the dynamic range of all colours leaves room for improvement.

For example, most LCD TVs can only offer contrast ratios between 450 and 650cd/m2. The primary problem with these HDTVs is their inability to completely turn off or locally dim the CCFL backlighting.

Conversely, with high-brightness (HB) LED backlighting, an array of LEDs can be divided into many individual dimming clusters. Each cluster usually consists of between eight and 10, 30 to 50mA LEDs which can be dimmed independently via a single pin offering local dimming.

Such a design offers contrast ratios almost an order of magnitude higher (greater than 4,000cd/m sq.) than CCFL designs. Additionally, by adjusting the brightness of the backlighting LED clusters with wide dimming ratios, more mid-tone colours can be replicated, offering a more vivid picture.

Furthermore, being able to completely turn off the LEDs locally allows for a reduction of motion blur. By turning the LEDs off between frames, the blur associated with fast-moving objects is virtually eliminated.

The LEDs’ very fast response rate is critical in resolving this fast motion blur issue commonly encountered by CCFL backlit LCD TVs.

Another driving force for the elimination of the CCFL is the desire to eliminate the toxic mercury vapour found in this type of lighting. 

So, what supports such an impressive growth potential for LED backlighting?

First, the current generation of LEDs is more than twice as efficient as fluorescent lamps, thus reducing the electrical power necessary to deliver the required light output dramatically (measured in lumens). As LEDs are developed further, their efficacy at producing lumens from electrical power will continue to increase and is projected to double within the next few years.

Secondly, LEDs have 100,000-plus hours of lifetime compared to fluorescent bulbs, which only last up to 10,000 hours.

How to drive an LED

Driving an LED requires careful design, since overdriving them can produce thermal dissipation issues which can significantly degrade light output and also the useful life of an LED.

This has led analogue IC manufacturers to design specialised LED driver ICs to power LEDs correctly, regardless of their current rating, input voltage source and array string topologies. To understand the obstacles for the design and manufacture of these LED driver ICs, it is necessary to understand what a white LED requires to produce light.

A white LED must be driven by a constant current source so that the white point of the light does not shift. Furthermore, since the white LED is a diode, its internal forward voltage (Vf) drop has to be overcome. This Vf varies with the current rating of the white LED and will also change with temperature.

A typical 20mA white LED has a Vf that varies between 2.5V and 3.9V over the entire operating temperature range. Most applications use more than one white LED and can have these LEDs configured in parallel, in series, or a combination of both – for example, parallel strings of LEDs in series.

This means that white LED driver ICs must be capable of delivering sufficient current and voltage for the specific configuration of LEDs, and in a conversion topology which satisfies both the input voltage range and required output voltage and current requirements.

Dimming considerations

Traditionally, the dimming of LEDs has been done by adjusting the forward current flowing through the LEDs via a DC signal or with a filtered PWM signal. Reducing the LED current adjusts the intensity of the LED light output; however, a change in forward current also changes the colour of the LED as the chromaticity of the LED changes with the current.

Many applications, including LCD HDTV backlighting, cannot tolerate any shift in the colour of the LED. Wide dimming ranges are needed in these applications because of the different light variations in the ambient environment and the fact that the human eye is sensitive to minor changes in light intensity. Controlling the intensity of the LED by applying a PWM signal allows dimming of the LED without changing the colour.

One approach is to dim an LED via a PWM signal. It essentially involves turning the LED on and off at full current at the PWM frequency. The human eye has a limit of 50 to 60 frames per second.

By increasing the PWM frequency to 80Hz to 100Hz, for example, the eye can be deceived into believing that the pulsed light source is continuously on. Additionally, by modulating the duty cycle (amount of “on-time”) the intensity of the LED can be controlled.

The colour of the LED remains unchanged in this scheme since the LED current value is either zero or a constant value. Many LCD HDTV designers require dimming ratios of upward of 3,000:1 to adjust to a wide array of ambient lighting conditions.

Figure 1 shows the 3000:1 PWM dimming waveform and a very square looking LED current waveform. Even at a mere 3.3?s on-time, a 20mA LED current turns on and off in sync with the 100Hz PWM signal. 

Higher PWM dimming ratios are achievable with lower PWM frequencies, but 100Hz guarantees that there is no visible flicker.

LED driver ICs must be capable of delivering sufficient current and voltage for many different types of LED configurations, with a conversion topology that satisfies both the input voltage range and required output voltage and current requirements.

Thus, an LED driver IC needs to have the following features to satisfy designers’ needs:
? Wide input voltage range
? Wide output voltage range
? High efficiency conversion
? Tightly regulated LED current matching
? High PWM dimming ratios
? Low-noise, constant-frequency operation
? Small footprint with minimal external components.

Author is Tony Armstrong, director of product marketing for power products at Linear Technology

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