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LDS8160
V F = 3.2V + [-1.5 mV/ C x (50 C - 25 C)] = 3.163V for
V F = 3.2V + [-1.3 mV/ C x (50 C - 25 C)] = 3.168V for
mA is the de-rating current for each 5 C.
de-rate to ~ 45mW at 85 C and diodes must be turned
At 85 C, the R, G , B forward voltages will be reduced
maintaining the 50 C current ratios, the B -LED current
The total power dissipated by RGB LEDs at 85 C is
I R must de-rate from 18 mA to ~ 6.19 mA from 50 C to
Table 8 of LDS8160 datasheet) from 55 C to 85 C.
LED
0 0 0
Green LED, and
0 0 0
Blue LED.
The total RGB LED power at a 50 0 C with the applied
inverse curves to equalize the luminosity vs.
temperature would be
( 2.05V x 18mA) + ( 3.16 V x 15.6 mA) + ( 3.17 V x
15mA) =~ 133.mW
The total RGB LED power for NSSM038AT-E must be
less than ~ 133 mW up to the de-rating point at 50 0 C
(see Figure 2) complies with our result.
Also from the curve in Figure 2, the total power must
0
off at higher temperatures.
0
to ~ 1.98 V , 3.11 V , and 3.12 V respectively.
The de-rating is achieved by decreasing LED currents
in constant steps (i.e. linear rate) from 50 0 C to 85 0 C to
meet the final 45 mW power dissipation.
To maintain the luminosity equalization during the de-
rating, the 50 0 C current ratios between Red, Green,
and Blue LED currents (i.e. 1.2:1.04:1) should be
preserved.
With nominal forward current, I = 15mA, and
0
at the end of de-rating (before shutdown) is calculated
as follows;
0
P 85C = (1.2 x I x V R_85C ) + (1.04 x I x V G_85C ) + (1 x I x
V B_85C ) = (1.2 x I x 1.98V ) + (1.04 x I x 3.11 V ) + (1 x I
x 3.12 V ) = 45 mW ,
where I is the Blue LED current at 85 0 C.
Solving for I gives us I = ~ 45 mW / 8.73 V = 5.16mA
for Blue, 5.37 mA for Green, and 6.19 mA for Red
LED.
0
85 0 C (seven 5 0 C steps).
For linear de-rate, each step is 18mA - 6.19mA / 7 = ~
1.69mA/step. Using the 2- x ΔPW M code scale, this is
met with the codes shown in the R -LUT table (See
0 0
In LOG mode the Δ PWM table entries for the de-rating
are found by first taking the current of the prior step
minus the de-rating current per step, then dividing the
result by the prior step current, and finally converting
to a number of dB step.
The following example clarifies:
Using prior data for RED LED, we will find required
de-rating Δ R in dB at 55oC
Δ R @ 55oC = 20Log [(18 mA – 1.69 mA) / 18 ma] =
-0.856 dB,
where 18 mA is the current for the 50oC point and 1.69
0
Dividing this value by 0.34 dB/step (in 2-x scale used for
de-rating) and rounding result to the nearest integer value
give us follow Δ PWM code
Δ PWM = INT (-0.846 dB / 0.34 dB/step) = -3 = 1101 (bin)
(see Table 8 of LDS8160 datasheet).
The Δ PWM value would then be
Δ PWM @ 60oC = INT {20Log [(16.31mA – 1.69 mA) /
16.31 mA] / 0.34} = -3 =. 1101 (bin)
Δ PWM @ 65oC = INT {20Log [(14.62 mA – 1.69 ma) /
14.62 mA) = -3 =. 1101 (bin)
Δ PWM @ 70oC = INT {20Log [(12.93 mA – 1.69 ma) /
12.93 mA) = -4 =. 1100 (bin)
Δ PWM @ 75oC = INT {20Log [(11.24 mA – 1.69 ma) /
11.24 mA) = -4 =. 1100 (bin)
Δ PWM @ 80oC = INT {20Log [(9.55 mA – 1.69 ma) / 9.55
mA) = -5 =. 1011 (bin)
Δ PWM @ 85oC = INT {20Log [(7.86 mA – 1.69 ma) / 8.1
mA) = -6 =. 1010 (bin)
At temperatures higher than 85oC, the LED current should
be zero mA due to the shutdown temperature defined as
above 85 0 C. Therefore, LUT Δ PWM entries for shutdown
regions are not used and may be zero.
To set LED current in shutdown at temperature above
85 0 C, write 85 0 C T-code (11000 (bin)) with leading 1, i.e.
111000 (bin) = 38h, into register 4Ah (see Table 7 of
LDS8160 datasheet).
To maintain constant ratio between channels during the
de-rating, the Green and Blue channels can de- rated by
the same dB steps in Logarithmic mode as the Red
channel. This will maintain the same luminosity balance
as at the starting point of the de-rating.
Users can adjust luminosity balance in the de-rating
section too to further optimize balance. This requires
more customized table entries (i.e. ratios continue to
match Luminosity vs. Temperature curve even for
temperatures where de-rating is being applied). The
decided approach is user/application dependent.
Table A1.1 shows the completed table used as LDS8160
default for Nichia NSSM038AT-E RGB LED with the
assumptions overviewed in this example.
To aide users in building and loading their specific
correction tables IXYS can provide a software
development tool to map LED vendor information and
user defined operating points to final calculated LUT data
values. Please consult factory to obtain a copy.
? 2009 IXYS Corp.
Characteristics subject to change without notice
26
Doc. No. 8160_DS, Rev. N1.0
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