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LDS8160
Appendix 2
ADJUSTMENTS FOR RGB WHITE BALANCE
The LDS8160 allows two ways for LED current
setting. One of them is using registers 00h – 02h
(static mode) and other one by using PWM signal to
decrease average LED current value set by these
registers (dynamic mode). For dynamic mode, the
LDS8160 integrates 3 digital PWM generators that
operate at a frequency of ~ 285 Hz. In Logarithmic
Mode (which is required for RGB applications), the
PWM generators are 12-bit resolution and can be
programmed with an 8-bit code to provide 256
internally mapped 12-bit logarithmic duty cycle
steps to adjust the dimming level. In Linear Mode,
the PWM generates 256 linear duty cycle steps to
adjust the dimming levels from the user
programmed 8-bit code. The Linear Mode is not
recommended for RGB LED applications that
require color mixing, and is useful for WLED or
other single color LED applications).
The advantage of PWM dimming is stable LED
color temperature / wavelength that are determined
by the maximum LED current value set by registers
00h – 02h.
To use the dynamic PWM mode for LED current
dB per step with 0dB dimming at the 256 step.
setting, the maximum I LED value should be set by
registers 00h – 02h as described above in static
mode and desired dimming should be set by
registers 05h – 07h. In Logarithmic Mode, set by
default, dimming resolution is approximately -0.17
th
In this example based on data from Appendix 1, it is
chosen that all 3 channels (RGB) operate at 15 mA
at 25oC temperature and do not exceed 133 mW of
total power dissipation prior to temperature de-
rating at more than 50oC. Since the maximum
current for Red channel is 18mA @ 50oC, we
assume that all static LED currents could be set to
18mA and the average 15 mA current achieved by
applying Dynamic Dimming with PWM Duty Cycle
15 mA / 18 mA = 83.3%. This allows sufficient
range for temperature compensation with Δ PWM
adjustments steps.
However, this equal current setting at 25oC may not
meet requirements for R G B white balance color
mixing. A typical color balance ratio for RGB diodes
is given in the Nichia Application Note “Balancing
White Color.” Here for white light at x = 0.33 and y =
0.33 on the (x, y) chromaticity curve (see Figure
A2.1), the luminous intensity ratios for R:G:B =
3:7:1.
Figure A2.1: Chromaticity Curve
Nichia specifies the NSSM038AT-E RGB diode
luminous intensities of 550 mcd for Red , 1100 mcd for
Green , and 240 mcd for Blue, all at 20mA of current.
Also per the NSSM038AT-E datasheet, relative
luminosity vs. forward current is ~ 1:1:1 for current
below 25mA.
Therefore for current of 15mA, luminous intensity
levels are 15 mA/20 mA = 0.75 of the 20mA specified
level, i.e. luminous intensity is 412 mcd for Red , 825
mcd for Green , and 180 mcd for Blue , that gives us
intensity ratio 2.3:4.6:1.
To achieve the desired white balance at intensity ratio
3:7:1, forward current levels for each color channel
should be adjusted. If the maximum intensity for Green
LED is 825 mcd at 15mA current, the intensity of other
LEDs should be 825 x 3/7 = 354 mcd for Red and 825
x 1/7 = 118 mcd for Blue . That responds to the
following LED currents: 354/412 x 15 mA = 12.9 mA for
Red LED, and 118/180 * 15mA = 9.8 mA for Blue LED.
This could be achieved via adjustment to the user-set
Dynamic Dimming levels for each channel.
Since Green LED has the highest intensity, all static
LED currents should be set equal to the Green LED
maximum forward current at 15.6 mA instead of 18 mA
as we assume previously. Then to insure 15 mA
? 2009 IXYS Corp.
Characteristics subject to change without notice
29
Doc. No. 8160_DS, Rev. N1.0
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