Led Driver Applications For Portable Products

 Light Emitting Diodes (i.e., LEDs) have
played a pivotal role in electronics for many years and
have found their way into a broad range of
applications.  Recent advances in LED technology
have provided designers with small form factor and
cost effective blue and white LEDs that support
various lighting applications (e.g., Display lighting). 
Liquid Crystal Displays (LCD) have emerged as the
primary technology used in a variety of devices such
as cellular telephones, personal digital assistants
(PDA), and computer monitors.  LCD technology
comes in monochrome and color topologies using
either passive or active matrix formats depending
upon speed and resolution requirements.  These
displays require some form of external lighting source
to allow the user to see the information on the Display  
Medium to large sized Displays have relied on the
benefits of fluorescent lighting to produce a bright
white light that supports viewing the Display in bright
ambient lighting conditions.  A clean white-light
source is also necessary for viewing color LCD   The
PDA uses a smaller LCD and a fluorescent lamp for
panel illumination.  However, many PDA
manufacturers have moved to using white LEDs to
back or side-light the color LCD   The reasons for
migration from a fluorescent lamp to LED technology
include cost reduction, enhanced reliability, and
Electromagnetic Interference (EMI) reduction. 
Cellular telephones have long used LEDs for both
display and keypad lighting. The market trend toward
cell phones that provide a host of features (i.e.,
information management, games, and digital
photography) now require a larger color LCD thus
necessitating the use of white LEDs for effective
illumination.  Based upon the recent trend towards
implementing white LEDs in LCD Display
applications, several semiconductor manufacturers
have created a multitude of products designed to
effectively illuminate LEDs in portable applications. 
 This method improves upon the basic LED
drive scheme previously mentioned by replacing the
resistor with a controlled current source.  EMI
concerns associated with switched-mode boost
converter methods are eliminated.  However, one
disadvantage of this method is the necessity of two
available voltage sources dependent upon the
application.  Figure 2 shows the  LX1990 driving 4
LEDs.  An external voltage source (up to 12VDC
maximum) is required at  the LED’s anode depending
upon the application.  The LED cathodes are
connected to the sink inputs (i.e., IOUTA & IOUTB).  The
internal current mirrors precisely control the LED
current in each of the two sink inputs (i.e., up to
30mA per input) and the forward current magnitude is
easily controlled using either a DC voltage or PWM
signal at pin 1 (ISET). 

 The LX1991 builds upon the LX1990 s topology by
offering 6 sink inputs and allowing the anode voltage
to be as high as 36V DC (see Figure 3).  The forward
current magnitude in each string is maintained to
within +3% to any other string.  This capability
significantly reduces brightness variation between
LEDs in multi-lamp applications.  The large anode
voltage rating coupled with six sink inputs allows over
50 white LEDs to be driven from one controller (i.e.,
up to 1.5W output power).  Further, a designer may
implement any combination of colored LEDs to
achieve unique color schemes in applications that
require additional lighting capability (e.g., key pad,
multiple Display and sign illumination).  The LX1991
provides for LED dimming by accepting either a DC
voltage or PWM signal at pin 1.  However, the
LX1991 reduces EMI by precisely controlling the rise
and fall-time of the LED sink currents. Further, the 
 internal control circuitry maintains a symmetrical rise
and fall time thus preserving  the LX1991 s ability to
accurately control the output current response versus a
narrow dimming control PWM signal. The LX1990
and LX1991 are available in MSOP-8 pin and MLP-16
pin packages respectively and rated over the extended
temperature range of –40o
C to +85o
C.   Microsemi offers the  LX1882 as a Charge Pump
solution for low-power boost applications such as
driving LEDs   The LX1882 is a Charge Pump with a
built-in low drop-out Regulator (LDO) and operates
from an input voltage source up to 5VDC.  The
internal LDO  reduces switching noise at the output
making the LX1882 attractive for noise sensitive
applications.  Output voltage is adjustable from  2.5 to
5.5VDC and the maximum output current is 50mA;
thus 3 LEDs could be driven with a forward current of
15mA each.  Figure 4 shows the LX1882 in a typical
LED driver application. 
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Typical LX1991 Application Circuit
Typical LX1991 Application Circuit

Typical LX1990 Application Circuit
Typical LX1990 Application Circuit

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