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 3.3μs 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