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Clayton Huff
L01 – Dr. Stephen Ralph
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Technical Review of High Speed LED Drivers
Introduction
High speed LED drivers are becoming more prominent in the digital industry due
to increased speeds of data transfer. The term "high speed" in the market sense refers to
data rates greater than one Mbps. There is a switching speed and light intensity tradeoff
that hinders the design for some applications. This paper focuses on commercial
applications for high speed LED drivers, how the underlying technology works, and the
hardware needed to implement the drivers.
High Speed LED Drivers in Commercial Applications
LED drivers are used in home and commercial electronics such as cellular
phones, LCD TVs, and other lighting applications. The digital industry wants to increase
the use LEDs over laser diodes and light bulbs because of their low power consumption
and cost effectiveness. Analogic Tech has created new family of flash LED drivers that
manage power consumption and provided the proper light switching for higher resolution
video on mobile phones [1]. These drivers provide high current/high brightness at two
MHz switching speed. Research is being conducted on the use of LEDs for free space
optical communications. There are not many free space optical links using LEDs, if any,
on the market, but research has been going on since the 1980's. Sumitomo Electric
Industries created a shunt LED driver circuit using GaAs semiconductors that was
successfully tested at bit rates of 400 Mbps over a few centimeters [2]. The shunt driver
circuit is frequently used in current FSO research. The bit rates produced by high speed
LED drivers would satisfy speed requirements for smaller communication networks
(Mbps range), but not for the larger tier networks (Gbps range).
The Technology Behind High Speed LED Drivers
Driver Function
The LED driver controls the voltage across the diode and either turns the diode
"on" or "off". The LED turns "on" when a forward bias greater than the turn on voltage
is applied, and the diode begins to emit light. The driver must be designed to produce a
large enough voltage so that the diode will give off the desired intensity. When the driver
turns the LED "off" the voltage should adjust the diode to barely conduct. This is
necessary because it takes far too long to turn on a diode once it has been completely
turned off. The photon intensity from the diode when it is barely conducting must be
negligible when designing a product.
Shunt Driver Circuit
The shunt driver circuit simply puts the LED in parallel with the driver output.
This circuit is patented because the old LED drivers had the diode in series with the
driver output, and while in parallel the rise and fall times of signals are much faster [3].
The output of the driver consists of a high speed switching transistor. The carriers built
up in the junction of the diode are swept out quickly through the shunt connection to the
transistor. When the transistor starts conducting it reverses the direction of minority
carriers and recombines electron-hole pairs much quicker than natural recombination.
Essentially the diode is "on" when the transistor stops conducting and vice versa. The
shunt driver circuit is accompanied by an AC coupled peaking circuit that consists of
resistors, capacitors, and inductors. This circuit varies with different designs but is
mainly used to increase signal integrity, reduce jitter, and decrease the extinction ratio.
Maxim designed a high speed LED driver circuit with a data rate of 155 Mbps and
discovered that the fall time was slower than the rise time [4]. This is due to dynamic
resistive changes in the diode to the drive current. The eye chart for the Maxim driver
showed good signal integrity with minimal jitter at 155 Mbps, but if the speed is
increased the problem with dynamic resistance must be fixed.
Hardware Needed to for a High Speed LED Driver
A high speed LED driver requires a circuit consisting of analog components.
There are many packaged LED driver ICs available on the market, but many of them do
not operate at speeds higher than 100 Mbps. ON semiconductor makes a chip that can
get up to 250 Mbps and only uses +5 V for PECL and -5.2 V for ECL [5]. This chip
would be good to use in hand-held devices because of its size and power consumption.
The AC coupled peaking circuit requires some resistors, capacitors, and inductors and is
not offered as an IC. There are also examples of driver circuits that can be used as
templates for design, and they reach speeds of 270 Mbps [6]. The basic design consists
of an op amp that controls switching transistors that are connected with capacitors and
resistors. These circuits are not in IC packaging and can be built in a lab.
References
[1]
Advanced Analogic Technologies Incorprated (October 2007). AnalogicTech
Announces New Family of High-Current Flash LED Drivers for High-End Cell Phones
[WWW Newsroom] URL http://news.thomasnet.com/fullstory/533236 (visited 2008
January 21).
[2]
Suzuki, Tomihiro (July 1986). High-Speed 1.3-um LED Transmitter Using GaAs
Driver IC. Journal of Lightwave Technology, vol. lt-4, no. 7.
[3]
U.S. Patent and Trademark Office (1986). LED Driver Circuit (United States
Patent 4571506). Arlington, VA.
[4]
Maxim High-Frequency/Fiber Communications Group (2006). Improving Speed
Performance of Slow LEDs using the MAX3967A with Added Peaking Elements
(Application Note). Sunnyvale, CA: Maxim.
[5]
Semiconductor Components Industries, LLC (2006). MC10SX1130/D LED
Driver: Data Sheet [WWW Document] (Rev. 3). URL
http://www.onsemi.com/pub/Collateral/MC10SX1130-D (visited 2008, January 18).
[6]
Force Incorporated (2005). Light Emitting Diode [WWW Document]. URL
http://www.fiber-optics.info/articles/LEDs.htm (visited 2008, January 18).