Datasheet
/
25°C
in %
Relative Luminous Flux = f(T
B
)
140,00
130,00
120,00
110,00
100,00
90,00
80,00
70,00
60,00
50,00
40,00
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
T
B
[°C]
Figure 15
Change of relative
luminous flux Φ
V
vs. board
temperature T
B
for the
RGYB chips of the
ACULED VHL.
Besides a changing mixed color ratio due to the different intensity changes, each chip also
changes color as a result of wavelength drift caused by temperature.
Influence on wavelength and color
The change of the wavelength (
λ
peak
resp.
λ
dom
) and, therefore, the color versus the temperature,
is linear in first approximation at small temperature changes and can be determined by the
temperature coefficients TC
λ
peak
and / or TC
λ
dom
. These are found in the ACULED VHL data-
sheet or the specific chip datasheets of the ACULED DYO. Figure 16 shows a typical curve of
the IR ACULED VHL. This curve is shown in each individual ACULED VHL datasheet.
The change
∆λ
of the peak or dominant wavelength can be calculated analogous to the
calculation of the forward voltage drift over temperature by the following equation:
∆λ
= TC
λ
·
∆
T
J
(11)
λ
1
= TC
λ
· (T
J1
- T
J0
) +
λ
0
(11a)
λ
0
is the known wavelength at a known temperature T
J0
given by the datasheets. In a steady
state, the junction temperature T
J
is interchangeable with the substrate temperature T
B
in the
equation:
λ
1
= TC
λ
· (T
B1
- T
B0
) +
λ
0
(11b)
We learned that all these temperature effects are linear in approximation for smaller
temperature changes. So with the equations 9 to 11, temperature drifts can easily be balanced
with the ACULED driver. But how do we know about the temperature drift? A sufficient way is to
measure it, for example with an NTC close to the ACULED. With the ACULED DYO you can
even have the NTC inside the ring on a chip pad which is the closest placement to the LED
chips that one can get. The influence of the temperature on the NTC resistance itself (R vs. T
characteristic) and the effect on photo diodes used with the ACULED DYO are explained in the
following chapters.
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Driving the ACULED® VHL™ 15