Datasheet
peak
[nm]
Peak wavelength λ
peak
= f(T
B
)
875,0
870,0
865,0
860,0
855,0
850,0
845,0
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
T
B
[°C]
Figure 16
Change of peak
wavelength λ
peak
vs.
substrate temperature T
B
for the IR ACULED VHL.
Using an NTC with the ACULED DYO
The safest way of controlling the heat is to measure it. As shown in the previous chapter, we
can do this by controlling T
B
since we can calculate the corresponding junction temperature T
J
of
the used chips. The board temperature is measured by thermistors, as negative temperature
coefficient resistors (NTCs), close to the ACULED’s board. The ACULED DYO allows you to
even put an NTC chip on one pad of the ACULED to easily control the temperature close to the
chips. The temperature T
NTC
inside the NTC is nearly the same as T
B
. Please refer to the
application note “Thermal Management of the ACULED VHL” to see how to calculate the
temperatures with NTCs. The height of the NTC-Chip used with the ACULED is very low and
thus does not shadow the LED chips in any way, resulting in an undisturbed light emission of
your ACULED DYO.
The principle of an NTC is the change of its internal ohmic resistance R
NTC
with the temperature
T. As the name indicates, R
NTC
(T) is a typically negative function, as shown in Figure 17. Its
nominal resistance is 10 k
Ω
at 25 °C with 5% tolerance. By measuring the ohmic resistance we can
now easily find the according board temperature by the following equation:
T
B
= - c
T
·
R
NTC
ln (12)
c
R
C
T
and C
R
are coefficients, whose values depend on the current I
F
used with the ACULED as
shown in figure 17. The reason for the change of these coefficients with the current and thus
with the temperature of the chips is the temperature drift from the chip pads where the NTC is
placed to the ACULED back side. Please refer to the application note “Thermal Management of
the ACULED VHL” for more information. In table 2 you can find typical values of the coefficient
for different currents. If you want to be on the safe side, you should use the coefficients given for
three chips at the maximum current you want to use. However, it is highly recommended that
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Driving the ACULED® VHL™ 16