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Texas Instruments Incorporated
Power Management
TLC5940 dot correction compensates
for variations in LED brightness
By Michael Day, Applications Manager, Portable Power Products (Email: m-day@ti.com),
and Tarek Saab, Product Marketing Engineer, Portable Power Products (Email: tareksaab@ti.com)
The abundance of light-emitting diodes (LEDs) in
various types of end equipment has surpassed
even the most aggressive expectations. The
reduction in LED prices, coupled with increased
LED efficiency (lumens per watt), has fueled the
redesign of many common devices. LEDs are
entering new markets such as architectural light-
ing, LCD TV backlighting, car headlamps, and
traffic lights. At the same time, they continue to
dominate other markets such as high-quality,
large form-factor video displays and alphanumeric
displays. As the efficiency and brightness of LEDs
improve and the cost decreases, it is anticipated
that LED usage will eventually replace conven-
tional lighting methods in consumer applications.
Some of these markets, such as LCD TV back-
lighting and large form-factor video displays,
require a much higher degree of LED brightness
uniformity than is possible from the LEDs alone.
This article shows how the dot correction func-
tion in the Texas Instruments (TI) TLC5940 and
in other similar LED drivers generates uniform
LED brightness across thousands of pixels in
these displays.
A stadium or advertising display like that shown
in Figure 1 integrates dozens of display panels and
thousands of LEDs. The individual LEDs inside
each array vary significantly in brightness, with
the delta in lumens between the brightest and
dimmest LED regularly approaching 15 to 20%, if
not more. The design engineer must ensure that
each LED is calibrated to provide the same amount of
brightness so that when the entire screen is turned on, it
appears uniform. Without this calibration, the screen will
have a blotchy, uneven look. Even after the display is
properly calibrated and deployed to the field, variations in
LED aging will generate changes in brightness. As a result,
companies must continually solve difficult quality and
maintenance issues. To compensate for variations in LED
brightness and aging, manufacturers often employ two
techniques: First, they purchase matched LEDs from a
supplier (also known as “binning”); and second, they utilize
a high-quality LED driver with dot correction functionality.
Figure 1. Large form-factor video display
LED suppliers offer the benefit of matched LEDs for an
incremental increase in price. They measure and bundle
these red, green, and blue (RGB) diodes together with
LEDs that generate similar lumens at a specified current.
Using this method can provide the desired uniformity with
minimal design considerations for low-end lighting systems.
However, the variance in decay rate, or degradation in
brightness, per pixel over time makes this method a short-
lived solution. In other words, in a year or two the picture
will become blotchy. Furthermore, should a defective
panel need replacement, the lumenal output of the new
panel will be visually dissimilar to the others.
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Analog Applications Journal
4Q 2005
Analog and Mixed-Signal Products
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Power Management
Texas Instruments Incorporated
High-end display systems require brightness-matching
levels that are unattainable by simply binning the LEDs.
To achieve pixel and panel uniformity over the life span
of a display unit, manufacturers use advanced LED drivers
with dot correction capability. Dot correction is a method
for managing pixel brightness by adjusting the current
supplied through each individual LED in the array. The
dot correction feature enables the processor to control full
current to a panel of LEDs while the LED driver scales the
current to each LED and creates uniform brightness. This
frees the processor for other functions, since it no longer
has to check a look-up table or perform complex multipli-
cation tasks for each LED in every refresh cycle.
To implement dot correction, manufacturers measure
the brightness of individual LEDs through photo capture.
The dimmest LED in the system is designated as the “base”
LED to which every other pixel is matched. To accomplish
this calibration, the current supplied to each pixel is multi-
plied by a fractional value proportional to the LED’s lumenal
output. In a device like TI’s TLC5940, the dot correction
value for each LED can be dynamically changed every
refresh cycle or stored inside an integrated EEPROM. This
dual dot correction method offers the flexibility to update
overall panel brightness as external lighting conditions
change, and provides long-term, nonvolatile dot correction
information that ensures panel uniformity. The EEPROM
data can be rewritten as lumenal measurements vary over
time or as panels fail, requiring correction or replacement,
respectively. The following example shows how dot correc-
tion is used to match LED brightness at production.
A typical display panel has anywhere from dozens to
thousands of LED drivers and from hundreds to hundreds
of thousands of individual LEDs. For simplicity, this example
considers only the 16 LEDs connected to a single driver.
Figure 2 shows the typical schematic of a single driver. An
external power supply, V LED , provides the power to the
LEDs. R EXT sets the absolute maximum current through
any LED. An external processor programs the TLC5940 to
turn on or off each individual LED and to set its current to
a percentage of the maximum programmed current.
The first step in calibrating a panel’s brightness is to set
the maxium current. This example requires a green LED
to have a luminous intensity of 80 millicandela (mcd). The
LED (Osram LP E675) is available in 4 different luminosity
bins: 45–56, 56–71, 71–90, and 90–112 mcd, each measured
at a normalized current of 50 mA. Selecting the highest bin
guarantees at least 80 mcd per LED. R EXT must set the
current high enough to allow even the dimmest LED to
produce 80 mcd. According to the datasheet for the
LP E675, setting the LED current to 43 mA guarantees
Figure 2. Typical TLC5940 implementation
V LED
TLC5940
OUT8
OUT9
OUT10
OUT11
OUT12
OUT13
OUT14
OUT15
XERR
SOUT
GSCLK
DCPRG
IREF
VCC
OUT7
OUT6
OUT5
OUT4
OUT3
OUT2
OUT1
OUT0
VPRG
SIN
SCLK
XLAT
BLNK
GND
Microprocessor
R EXT
V CC
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Analog and Mixed-Signal Products
4Q 2005
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Texas Instruments Incorporated
Power Management
80 mcd. At production, the brightness of all LEDs is
measured at full current (43 mA). This might produce an
LED histogram of luminous intensity resembling Figure 3.
As shown, the brightness variation measured between each
LED in the panel may vary as much as ±10% without dot
correction. A brightness deviation this large is unacceptable
in higher-end displays. The TLC5940 dot correction feature
can now be used to calibrate the LED brightness. When
programmed to full brightness, the IC must dot correct
the luminous intensity of LED1 from 83 mcd to 80 mcd.
The TLC5940 has 6-bit dot correction (64 steps), which
corresponds to a full-scale resolution of 1.56% per step.
The following formula calculates the correct dot correc-
tion level for each LED:
By rounding the calculated dot correction value to the
closest fractional number and then multiplying the original
luminosity by the new dot correction ratio, one can produce
the updated LED brightness.
DC
Pr
oduction
L
=
×
L
=
80.4 mcd
Pr
oduction
Initial
64
After the dot correction values are calculated and stored,
the TLC5940 is capable of automatically generating a uni-
form brightness in all LEDs. When the processor programs
the TLC5940 to drive full current, the TLC5940 automati-
cally adjusts the actual current in each channel to properly
calibrate the LED brightness. The current in LED1 is
calculated as
DC
L
L
LED
1
Baseline
Initial
I
=
×
I
=
41.66 mA,
DC
=
×
64
=
61 7
.,
LED
1
max
64
Pr
oduction
where I LED1 is the actual LED1 current, DC LED1 is the dot
correction value for LED1 (62), and I max is the maximum
LED current programmed by R EXT (43 mA). Applying these
where DC Production is the required dot correction value at
production, L Baseline is the desired brightness level, and
L Initial is the measured brightness at maximum current.
Figure 3. LED brightness and forward current histogram
before dot correction
100
80
60
40
20
0
12
3
4
5
6
7
8
9
10
11
12
13
14
15
16
LED/Dot Number
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Power Management
Texas Instruments Incorporated
formulas to the remaining LEDs
produces the histogram in
Figure 4. If programmed into
the TLC5940’s nonvolatile
EEPROM, the dot correction
data is available each time the
panel is turned on and remains
constant until the next time
the panel is recalibrated.
For indoor/outdoor industrial
applications such as billboards
and large form-factor video
displays, “static” adjustment
(calibration that remains fixed
until manually adjusted) is
sufficient. dot correction values
don’t change until the next
routine maintenance cycle. The
newer market applications such
as LCD TV backlighting require
a dynamic dot correction
scheme. Products such as the
Sony 40
Figure 4. LED brightness and forward current histogram
after dot correction
100
80
60
40
20
0
123456789 0 1 2 3 4 5 6
LED/Dot Number
Qualia 005 and the
Samsung 46″ LNR460D have each introduced LCD TVs
that incorporate LED-based backlighting. Contrary to
popular belief, the diodes in these TV displays are not
white. RGB LEDs are controlled and mixed to create
“tunable” white light.
The advantages of LED backlighting over conventional
lamps are numerous: enhanced power efficiency, reduced
motion artifacts, broader color spectrum (>105% NTSC in
some cases), longer life span, tunable color temperature,
etc. The picture quality is incomparable. However, LCD TV
engineers encounter not only the same lumenal variance
challenges as conventional panel makers but also tempera-
ture concerns. TV backlighting applications are sensitive
to changes in LED brightness as a function of temperature.
In addition, a TV set achieves optimum display quality only
when its backlighting properties are adjusted to meet the
constantly changing ambient lighting conditions for each
consumer’s living room. These considerations, coupled
with the fact that this is a consumer application, create a
need for dynamic brightness adjustment.
To create this dynamic control loop, internal sensors
that measure LED temperature and brightness fluctuations,
as well as external sensors that measure ambient-light con-
ditions, are required. The control loop, in its most basic
form, begins with the sensors gathering data and feeding
these measurements into a processor. The processor eval-
uates this data and provides the “intelligence” to an LED
driver such as the TLC5940. The processor combines the
original factory-calibrated dot correction data with the new,
dynamic data and generates updated dot correction values.
In the previous example, if the ambient-light meter detects
low ambient-light conditions that require only 70% of full
″
brightness, or 56 mcd, the processor calculates a new
ambient-light dot correction value of 44.8. If, simultane-
ously, the LED light output drops 10% due to an increase
in temperature, the processor calculates a temperature
dot correction value of 71.1. Combining all three dot
correction values generates the new dot correction data,
compensating for all three brightness variations.
80
0 7
80
×
.
DC
=
×=
64
44 8
.
Ambient
80
DC
=
×=
64
71 1
.
Temp
80
×
0 9
.
DC
D
C
DC
Temp
Ambient
Pr
oduction
DC
=
64
=
48 0
.
Total
64
64
64
As shown in the following equation, the combined dot
correction value of 48 yields the desired brightness of
56 mcd. Note that the initial current in this calculation is
set to 90% of the initial production current due to the
brightness drop caused by the temperature.
=
DC
48
64
Total
L
(
83
×=× =
0 9
. )
74 7
.
56 mcd
Final
64
Advanced LED drivers such as the TLC5940 are capable
of providing a dynamic dot correction value to optimize the
lighting solution for a consumer’s specific viewing conditions.
Related Web sites
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