LP5520TL NSC [National Semiconductor], LP5520TL Datasheet - Page 9

• Manufacturer Part Number: LP5520TL
• Description: RGB Backlight LED Driver
• Manufacturer: NSC [National Semiconductor]

RGB Driver Functionality
WHITE BALANCE CONTROL
LP5520 is designed to provide spectrally rich white light using
a three color RGB LED. White light is obtained when the Red,
Green and Blue LED intensities are in proper balance. The
LED intensities change independently with temperature. For
maintaining the purity of the white color and the targeted total
intensity, precise temperature dependent intensity control for
each LED is required. The color coordinates in this document
refer to the CIE 1931 color graph (x,y system).
The Intensity vs. Temperature graph shows a typical RGB
LED intensity behavior on a 12-bit scale (0 to 4095) at con-
stant 20mA LED currents. The next graph shows the typical
color coordinate change for an uncompensated RGB LED.
The Compensation PWM Values graph shows the corre-
sponding PWM values for achieving constant intensity white
light across the temperature range. The PWM values have
been saturated at 104°C to avoid overheating the LED and to
better utilize the PWM range. The white balance is not main-
tained above 104°C in this case.
LED Intensity vs. Temperature
CIE 1931 Color Graph
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The compensation values for the measured temperatures can
be easily calculated when the intensity vs. temperature infor-
mation is available. For the best accuracy the iterative cali-
bration approach should be used. The calibration procedure
is described in the Application Note AN1459, LP5520 RGB
Backlight Driver Calibration.
The compensation values need to be converted to 16°C in-
tervals when they are programmed to the calibration EEP-
ROM. The evaluation software has import function, which can
be used to convert the measured compensation data to the
16°C interval format. The measured data can have any tem-
perature points and the software will fit a curve through the
measured points and calculate new PWM values in fixed tem-
peratures using the curves. The procedure is explained in the
Application Note AN1462, LP5520 RGB Backlight Driver
Evaluation Kit. By using the evaluation software to convert the
measurement results to EEPROM memory map the user
does not need to care about the details of the EEPROM struc-
ture.
Typical color coordinate and intensity stability over tempera-
ture are shown in the following two graphs.
Typical Color Coordinates vs. Temperature for
Compensation PWM Values
uncompensated RGB LED
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