LED TV: Technology Overview and the DLP® Advantage

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Abstract
This white paper will discuss Light
Emitting Diode (LED) technology and
its impact on television applications. It
will highlight the advantages and
challenges for these applications and
will explore the specific advantages that
LED technology has for DLP® product
applications.
Introduction
The LED has become a pivotal
illumination technology with a wide
variety of applications. Since their initial
invention, LEDs have been used in many
diverse applications such as watches,
calculators, remote controls, indicator
lights, and backlights for many common
gadgets and household devices. The
technology is advancing at a rapid pace
and new applications continue to emerge
as the brightness and efficiency of LEDs
increase.
LED History
From the early 1900s, scientists have
been discovering ways to generate light
from various materials. In 1907, Henry
Joseph Round discovered that light
could be generated from a sample of
Silicon Carbide (SiC). For the next 50
years, scientists continued to discover
the light emitting properties that exist
with some compounds. In the 1950s,
studies around the properties of Gallium
Arsenide (GaAs) paved the way for the
first official LED discoveries that soon
followed.1
LED research began in the early
1960’s, primarily at Bell Labs, Hewlett
Packard (HP), IBM, Monsanto, and
RCA. Gallium-Aresenide-Phosphide
(GaAsP) provided the basis for the first
commercially available red LEDs in
1968 by HP and Monsanto. In the early
1970s, the use of LEDs exploded with
new applications such as calculators and
watches by companies like Texas
Instruments (TI), HP, and Sinclair. Other
applications such as indicator lights and
alphanumeric displays soon became the
mainstream use for LEDs and continued
to be so for many years.2
LED Technology Background
As the name implies, an LED is a
diode that emits light. The diode is the
most basic semiconductor whose
purpose is to conduct electrical current
with some form of controlled variability.
The diode in its simplest form is
comprised of poor conducting materials
that have been modified (or “doped”) to
increase the amount of free electrons that
are available. High electron materials
(referred to as N-type materials) are
combined with low electron materials
(referred to as P-type materials) to form
a junction for these free electrons to
flow. This junction is often referred to as
the PN junction.
An LED is a PN junction diode
semiconductor that emits photons when
voltage is applied. This process of
photon emission is called injection
electroluminescence and occurs when
electrons move from the N-type material
to fill the lower energy holes that exist in
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the P-type material. When the high
energy electrons fall into these holes,
they lose some of their energy which
results in the generation of photons. The
materials used for the P-type and N-type
layers along with the size of the gap
between them determine the wavelength
and overall energy level of the light that
is produced.
Many materials have been developed
for manufacturing LEDs. Aluminum-
Gallium-Arsenide (AlGaAs),
Aluminum-Indium-Gallium-Phosphide
(AlInGaP), and Indium-Gallium-Nitride
(InGaN) are commonly used for present
LED architectures. “AlInGaP” is
typically used for Red and Yellow dies
while “InGaN” is used for Blue and
Green. These materials efficiently
produce photons that have wavelengths
in the visible spectrum. These materials
in combination with new manufacturing
architectures have enabled the
production of very bright LEDs that are
beginning to find their way into general
lighting and automotive applications.
Some architectures have begun utilizing
additional phosphor compounds to
generate white light and are now
beginning to compete with common
incandescent and fluorescent lighting -
with much lower power and much longer
lifetimes.
The worldwide production of LEDs
has risen to about 4 billion units per
month. Manufacturing in Taiwan, Japan,
and the U.S. comprises the most
significant volumes with Taiwan leading
with about one half of that volume
overall. Much of the manufacturing
involves the packaging of the LED die
with a limited number of manufacturers
creating the actual LED die material.
Figure 1 illustrates the market size for
low brightness and high brightness
LEDs as a function of the total LED
market.3
[upload=jpg]UploadFile/2007-5/07511@52RD_1.1.JPG[/upload]

LED Technology Breakthroughs
Recent innovations in the
manufacturing of the die material and
packaging have resulted in ultra high
brightness capabilities. The use of new
materials for the substrate have allowed
for improved thermal conductivity which
allows for higher power consumption
and net light output. This increase in
light output has enabled new
applications for LEDs such as
automotive lighting, traffic signals, and
more recently, television displays. An
example of these new structures is
illustrated in Figure 2.
[upload=jpg]UploadFile/2007-5/07511@52RD_1.2.JPG[/upload]
Significant improvements in the
production of Aluminum-Indium-
Gallium-Phosphide (AlInGaP) and
Indium-Gallium-Nitride structures have
allowed for improved brightness in
green and blue specifically. Additional
colors such as amber and cyan are also
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being developed at a rapid pace. These
improvements enable system designs
that can produce better color fidelity at
near equivalent brightness to common
lamp-based technologies with longer
lifetimes. Additional performance
enhancements include system level
features like instant on, no mercury, no
color refresh artifacts, dynamically
adjustable brightness, and improved
color gamuts. Figure 3 illustrates the
gamut area for LED illumination as
compared to the common reference
standard (Rec. 709).
[upload=jpg]UploadFile/2007-5/07511@52RD_1.3.JPG[/upload]
LED illumination provides a much
larger color gamut (as much as 40% or
more than the HDTV color standard
[Rec. 709]), providing more accurate
color fidelity. These performance
attributes can be quite appealing for
television applications where long life
and excellent color fidelity are required.
As LEDs continue to advance, their
impact on television applications could
be significant. Figure 4 illustrates the
evolution of LEDs and their potential
brightness efficiency in the coming
years.4
[upload=jpg]UploadFile/2007-5/07511@52RD_1.4.JPG[/upload]
LED Technology Challenges
Controlling the thermal stability of the
LED die is critical to the performance
and stability of LED illumination and
reliability. The LED architecture
inherently produces light from all sides
and surfaces of the PN structure in a
lambertian distribution (uniform
distribution into a 180 degree
hemisphere). While this might seem
efficient, most of this light is actually
absorbed into adjacent die, the mounting
substrate, or other surfaces of the LED
assembly. This absorption results in an
increased thermal loading of the entire
LED assembly. This heat must be
addressed to obtain maximum light
output and reliability. Additionally, for
applications that require imaging of the
light energy to a small display device
(e.g. DLP® HDTV), any light that is
emitted outside of the system etendue is
not useable and only adds to the heat and
overall power loading. Controlling this
absorption, shaping the light to match
the system etendue, and maximizing the
thermal efficiency to extract heat from
the die are all critical to increasing the
light output and usability of the LEDs.
0
0
LED
Rec. 709
For traditional applications, LEDs are
commonly driven in CW (continuous
wave – 100% duty cycle) mode. For
high brightness applications, however,
this is not as desirable. Since the average
temperature of the PN junction
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determines both the light output and
lifetime of the LED, it is often more
efficient to drive the LEDs with a
smaller duty cycle. With a smaller duty
cycle, the LEDs can potentially be
driven to higher current loads to increase
the overall light output while
maintaining a lower average temperature
of the PN junction. The challenge with
this, however, is that the driver circuitry
must be able to generate fast switching
waveforms, switching large currents in
as short a time as only a few
microseconds. This certainly presents
some challenges for the design of the
LED power driver. But, solutions have
already been developed with
performance that easily meets these
requirements.
Another challenge that results from
higher thermal loading is that of color
shift. As the PN junction changes
temperature, the output wavelength of
the light can shift by as much as 10nm or
more. This color shift obviously impacts
the color point for that color, but also
impacts the white point for the system
since each of the colors are mixed to
create white. Fundamentally, to stabilize
this color shift, the LEDs must either be
run at a lower power or maintain
extreme thermal stability. However, with
the implementation of some form of
system feedback and proper power
control algorithms, the stability of the
white could be preserved while
maintaining high brightness efficiency.
DLP® TV with LED Illumination
TI has developed a DLP® HDTV
system to take advantage of LED
illumination with brightness
performance that is nearly equivalent to
lamp based systems. By utilizing the
latest generation of high brightness
LEDs and implementing a unique
feedback system, it is now possible for
DLP® HDTV designs to enjoy the
benefits of LED illumination. Figure 5
illustrates the basic optical configuration
of this system.
[upload=jpg]UploadFile/2007-5/07511@52RD_1.5.JPG[/upload]
Utilizing a unique feedback algorithm,
TI has demonstrated that any color shift
variations that affect the white point can
be controlled to a tolerance beyond what
the eye can detect.
The current DLP® products
implementation with LED technology
utilizes a TI DSP component to process
system information in real time, offering
superior stability over a wide range of
operating temperatures while
maximizing brightness and reliability.
DLP® Products Performance Advantages
The rapid switching capabilities of
LED technology match perfectly with
the fast switching properties of DLP®
technology. By taking advantage of the
high speed capabilities of the DMD and
LEDs, it is now possible to utilize color
refresh rates that are much higher than
what exists with today’s designs. It is
also possible to randomize the color
order. Ultimately, images can be created
with higher bit depth, better motion
fidelity, and higher brightness. By
increasing the switching frequency of
the LEDs, it is possible to drive them
with increased power while minimizing
the thermal loading of the PN junction.
These fast switching capabilities of
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DLP® technology take advantage of the
new LED colors that are becoming
available, providing much more
flexibility for multiple color
configurations using a single DMD
device. With a DLP® system, the LEDs
do not require polarization, reflecting the
light precisely off of the DMD mirror
surface. The light is used efficiently,
only when it is needed. This maximizes
brightness and system efficiency while
reducing heat. The net result is a lower
system cost with higher brightness and
larger color gamuts that far exceed those
possible by traditional systems utilizing
other common illumination sources.
Conclusion
As LED technology developments
continue to improve brightness and
reliability, LED illumination may
become more of a mainstream light
source for many future applications.
Future developments will be able to take
further advantage of the fast LED
switching time to improve video
performance, enhance contrast without
opto-mechanical components, and create
adjustable color gamuts that far exceed
the possibilities of traditional
illumination sources. New products will
soon benefit from these fundamental
capabilities providing new, unique
designs that offer instant on, better
colors, and overall better picture using
the speed of DLP® micromirror arrays.
With the advantages of LED and DLP®
technologies working together, it is
expected that DLP® HDTVs will provide
even better performance with better
reliability far exceeding any existing
DLP® HDTV product.
References
1 Web Article, “A brief history of the
Light Emitting Diode (LED)”,
http://www.wavicle.biz/led_history.html,
Wavicle LED Lighting Technology,
2002.
2 “LEDs Are Still Popular (and
Improving) after All These Years”,
http://www.maximic.
com/appnotes.cfm/appnote_number/1
883, Dallas Semiconductor / MAXIMIC,
Application Note 1883, February
2003.
3 LEDs 2005, October 2005, San Diego,
California, USA.
4 LUMILEDS, Nanoscience and Solid
State Lighting, Department of Energy
Nanosummit, M.G. Craford, June 2004,
Washington, D.C., USA.[br]<p align=right><font color=red>+3 RD币</font></p>