LED TV: Technology Overview and the DLP Advantage - DJ Segler Texas Instruments Inc.
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LED TV: Technology Overview and
the DLP® Advantage
DJ Segler
Texas Instruments Inc.
DLP® Products
LED TV: Technology Overview and the DLP® Advantage
DJ Segler, Texas Instruments Incorporated, DLP® Products
Abstract LED research began in the early
This white paper will discuss Light 1960’s, primarily at Bell Labs, Hewlett
Emitting Diode (LED) technology and Packard (HP), IBM, Monsanto, and
its impact on television applications. It RCA. Gallium-Aresenide-Phosphide
will highlight the advantages and (GaAsP) provided the basis for the first
challenges for these applications and commercially available red LEDs in
will explore the specific advantages that 1968 by HP and Monsanto. In the early
LED technology has for DLP® product 1970s, the use of LEDs exploded with
applications. new applications such as calculators and
watches by companies like Texas
Instruments (TI), HP, and Sinclair. Other
Introduction applications such as indicator lights and
The LED has become a pivotal alphanumeric displays soon became the
illumination technology with a wide mainstream use for LEDs and continued
variety of applications. Since their initial to be so for many years.2
invention, LEDs have been used in many
diverse applications such as watches,
calculators, remote controls, indicator LED Technology Background
lights, and backlights for many common As the name implies, an LED is a
gadgets and household devices. The diode that emits light. The diode is the
technology is advancing at a rapid pace most basic semiconductor whose
and new applications continue to emerge purpose is to conduct electrical current
as the brightness and efficiency of LEDs with some form of controlled variability.
increase. The diode in its simplest form is
comprised of poor conducting materials
that have been modified (or “doped”) to
LED History increase the amount of free electrons that
From the early 1900s, scientists have are available. High electron materials
been discovering ways to generate light (referred to as N-type materials) are
from various materials. In 1907, Henry combined with low electron materials
Joseph Round discovered that light (referred to as P-type materials) to form
could be generated from a sample of a junction for these free electrons to
Silicon Carbide (SiC). For the next 50 flow. This junction is often referred to as
years, scientists continued to discover the PN junction.
the light emitting properties that exist An LED is a PN junction diode
with some compounds. In the 1950s, semiconductor that emits photons when
studies around the properties of Gallium voltage is applied. This process of
Arsenide (GaAs) paved the way for the photon emission is called injection
first official LED discoveries that soon electroluminescence and occurs when
followed.1 electrons move from the N-type material
to fill the lower energy holes that exist in
2the P-type material. When the high LEDs as a function of the total LED
energy electrons fall into these holes, market.3
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
Low Brightness
between them determine the wavelength 36%
and overall energy level of the light that
is produced.
High Brightness
Many materials have been developed 64%
for manufacturing LEDs. Aluminum-
Gallium-Arsenide (AlGaAs), ** Total LED Market: $5.74 Billion
Aluminum-Indium-Gallium-Phosphide
(AlInGaP), and Indium-Gallium-Nitride Figure 1 - LED Market Segments
(InGaN) are commonly used for present
LED architectures. “AlInGaP” is LED Technology Breakthroughs
typically used for Red and Yellow dies Recent innovations in the
while “InGaN” is used for Blue and manufacturing of the die material and
Green. These materials efficiently packaging have resulted in ultra high
produce photons that have wavelengths brightness capabilities. The use of new
in the visible spectrum. These materials materials for the substrate have allowed
in combination with new manufacturing for improved thermal conductivity which
architectures have enabled the allows for higher power consumption
production of very bright LEDs that are and net light output. This increase in
beginning to find their way into general light output has enabled new
lighting and automotive applications. applications for LEDs such as
Some architectures have begun utilizing automotive lighting, traffic signals, and
additional phosphor compounds to more recently, television displays. An
generate white light and are now example of these new structures is
beginning to compete with common illustrated in Figure 2.
incandescent and fluorescent lighting -
with much lower power and much longer
lifetimes. Al2O3
The worldwide production of LEDs Quantum Wells
N Layer - GaN
has risen to about 4 billion units per Reflective Layer
P Layer - GaN
month. Manufacturing in Taiwan, Japan,
and the U.S. comprises the most
significant volumes with Taiwan leading Thermally Conductive Substrate
with about one half of that volume Figure 2 - Basic LED Structure
overall. Much of the manufacturing
involves the packaging of the LED die Significant improvements in the
with a limited number of manufacturers production of Aluminum-Indium-
creating the actual LED die material. Gallium-Phosphide (AlInGaP) and
Figure 1 illustrates the market size for Indium-Gallium-Nitride structures have
low brightness and high brightness allowed for improved brightness in
green and blue specifically. Additional
colors such as amber and cyan are also
3being developed at a rapid pace. These 200
Incandescent Lamps
180
improvements enable system designs
Efficiency (Lm / W)
160 Flourescent Lamps
that can produce better color fidelity at 140
High Pressure Arc
120
near equivalent brightness to common 100
Lamps
Light Emitting
80
lamp-based technologies with longer 60
Diodes
lifetimes. Additional performance 40
20
enhancements include system level 0
features like instant on, no mercury, no 1940 1960 1980 2000 2020
color refresh artifacts, dynamically Year
adjustable brightness, and improved Figure 4 – Lighting Technology Evolution
color gamuts. Figure 3 illustrates the
gamut area for LED illumination as
compared to the common reference LED Technology Challenges
standard (Rec. 709). Controlling the thermal stability of the
LED die is critical to the performance
and stability of LED illumination and
LED reliability. The LED architecture
Rec. 709
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
0 substrate, or other surfaces of the LED
0
assembly. This absorption results in an
increased thermal loading of the entire
Figure 3 – LED Color Gamut
LED assembly. This heat must be
addressed to obtain maximum light
LED illumination provides a much
output and reliability. Additionally, for
larger color gamut (as much as 40% or
applications that require imaging of the
more than the HDTV color standard
light energy to a small display device
[Rec. 709]), providing more accurate
(e.g. DLP® HDTV), any light that is
color fidelity. These performance
emitted outside of the system etendue is
attributes can be quite appealing for
not useable and only adds to the heat and
television applications where long life
overall power loading. Controlling this
and excellent color fidelity are required.
absorption, shaping the light to match
As LEDs continue to advance, their
the system etendue, and maximizing the
impact on television applications could
thermal efficiency to extract heat from
be significant. Figure 4 illustrates the
the die are all critical to increasing the
evolution of LEDs and their potential
light output and usability of the LEDs.
brightness efficiency in the coming
For traditional applications, LEDs are
years.4
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
4determines both the light output and feedback system, it is now possible for
lifetime of the LED, it is often more DLP® HDTV designs to enjoy the
efficient to drive the LEDs with a benefits of LED illumination. Figure 5
smaller duty cycle. With a smaller duty illustrates the basic optical configuration
cycle, the LEDs can potentially be of this system.
driven to higher current loads to increase
the overall light output while Collimating Lenses Dichroic Filters
Optical Integrator
maintaining a lower average temperature DMD
of the PN junction. The challenge with
this, however, is that the driver circuitry Blue Array TIR Prism
must be able to generate fast switching Green Array Red Array Condenser Lens
waveforms, switching large currents in
as short a time as only a few Projection Lens
microseconds. This certainly presents
some challenges for the design of the Figure 5 – DLP® HDTV LED Optical Architecture
P
LED power driver. But, solutions have
already been developed with Utilizing a unique feedback algorithm,
performance that easily meets these TI has demonstrated that any color shift
requirements. variations that affect the white point can
Another challenge that results from be controlled to a tolerance beyond what
higher thermal loading is that of color the eye can detect.
shift. As the PN junction changes The current DLP® products
temperature, the output wavelength of implementation with LED technology
the light can shift by as much as 10nm or utilizes a TI DSP component to process
more. This color shift obviously impacts system information in real time, offering
the color point for that color, but also superior stability over a wide range of
impacts the white point for the system operating temperatures while
since each of the colors are mixed to maximizing brightness and reliability.
create white. Fundamentally, to stabilize
this color shift, the LEDs must either be DLP® Products Performance Advantages
run at a lower power or maintain The rapid switching capabilities of
extreme thermal stability. However, with LED technology match perfectly with
the implementation of some form of the fast switching properties of DLP®
system feedback and proper power technology. By taking advantage of the
control algorithms, the stability of the high speed capabilities of the DMD and
white could be preserved while LEDs, it is now possible to utilize color
maintaining high brightness efficiency. refresh rates that are much higher than
what exists with today’s designs. It is
DLP® TV with LED Illumination also possible to randomize the color
TI has developed a DLP® HDTV order. Ultimately, images can be created
system to take advantage of LED with higher bit depth, better motion
illumination with brightness fidelity, and higher brightness. By
performance that is nearly equivalent to increasing the switching frequency of
lamp based systems. By utilizing the the LEDs, it is possible to drive them
latest generation of high brightness with increased power while minimizing
LEDs and implementing a unique the thermal loading of the PN junction.
These fast switching capabilities of
5DLP® technology take advantage of the
new LED colors that are becoming References
available, providing much more
1
flexibility for multiple color Web Article, “A brief history of the
configurations using a single DMD Light Emitting Diode (LED)”,
device. With a DLP® system, the LEDs http://www.wavicle.biz/led_history.html,
do not require polarization, reflecting the Wavicle LED Lighting Technology,
light precisely off of the DMD mirror 2002.
surface. The light is used efficiently,
2
only when it is needed. This maximizes “LEDs Are Still Popular (and
brightness and system efficiency while Improving) after All These Years”,
reducing heat. The net result is a lower http://www.maxim-
system cost with higher brightness and ic.com/appnotes.cfm/appnote_number/1
larger color gamuts that far exceed those 883, Dallas Semiconductor / MAXIM-
possible by traditional systems utilizing IC, Application Note 1883, February
other common illumination sources. 2003.
3
Conclusion LEDs 2005, October 2005, San Diego,
As LED technology developments California, USA.
continue to improve brightness and
4
reliability, LED illumination may LUMILEDS, Nanoscience and Solid
become more of a mainstream light State Lighting, Department of Energy
source for many future applications. Nanosummit, M.G. Craford, June 2004,
Future developments will be able to take Washington, D.C., USA.
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.
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