Datasheet
driving the ACULED. The parameters influenced by temperature are quite similar to those
impacted by current, particularly
•
lifetime [t
Life
]
•
forward voltage [V
F
]
•
maximum forward current [I
F max
]
•
flux [
Φ
e
and
Φ
V
]
•
wavelength [
λ
] resp. color [x
2°
/ y
2°
] and color temperature (T
CT
).
A big advantage of the ACULED VHL and DYO versus other similar products on the market is
minimized thermal crosstalk between the pads. For more information please refer to the
application note called “Thermal Management of the ACULED VHL”. As a result of this low
thermal crosstalk, the warming of one LED has minimal affect on neighboring chips, resulting in
excellent constancy in the parameters described above.
Influence on lifetime
Overheating an LED chip, i.e. exceeding its junction temperature T
J
over the allowable
maximum, will damage the chips within a short time. But long-term temperature effects also
influence a decreased lifetime. During operation, the lower the temperature, the longer the
lifetime for the chip and the whole ACULED product. Some degeneration processes require a
minimum temperature to get started. Thus, a low T
J
will dramatically increase the product’s
lifetime. Since these processes are very complex and not fully understood today, it is virtually
impossible to get reliable curves of t
Life
versus T
J
today.
Influence on forward voltage
The forward voltage V
F
usually decreases in the range of several mV per Kelvin with increases in
temperature. Since this change in approximation is linear over the typical small temperature
changes, it is provided in the temperature coefficient of forward voltage TC
VF
found in the
ACULED VHL datasheet or the chip datasheets of the ACULED DYO chips. Figure 14 shows
typical curves for red, green, and blue chips.
The change
∆
V
F
of the forward voltage is calculated by the following equation:
∆
V
F
= TC
VF
·
∆
T
J
(9)
V
F1
= TC
VF
· (T
J1
- T
J0
) + V
F0
(9a)
V
F1
is the forward voltage that we want to calculate at a temperature T
J1
, whereas V
F0
is a known
forward voltage at a known temperature T
J0
, e.g. values given by the datasheets. In a steady
state, the junction temperature T
J
and the substrate temperature T
B
are interchangeable in the
equation:
V
F1
= TC
VF
· (T
B1
- T
B0
) + V
F0
(9b)
With a decrease of the forward voltage, power consumption drops as well at a given current. But
due to the small change, it is of no practical significance.
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Driving the ACULED® VHL™ 13