Datasheet

current
Radiant flux
Φ
e
[mW]
Excelitas ’ LED drivers utilize PWM for a safe and stable operation of the ACULED VHL
and DYO products. In the next chapter we will learn about the influence of the temperature. Due
to the temperature effects on the one hand and non-perfect slope of the pulses on the other, the
dependency of the flux according to relation 7 differs in practice as shown in figure 13.
Figure 12
Principle of PWM: Square wave
pulses are modulated in their duty
cycle. The “wider” the pulses and
thus the longer the on-time t P , the
higher the intensity of the driven
LED.
time
low intensity mid intensity high intensity
240
220
200
180
160
140
120
100
80
60
40
20
1 2 3
Max. junction temp: 125°C
typical green LED (520nm)
Duty cycle D=t
P
/T:
0,5
0,25
0,1
0,05
4 5 6 7 8 9 10
Pulse width t
p
[ms]
Figure 13
Dependency of radiant flux Φ e from
pulse width t P at a given duty cycle
with the ACULED green chip.
Influence of Temperature
With the ACULED and other high power LEDs, thermal management is a main issue in any
application since 60 - 90% of the power consumed will still be converted into heat rather than light.
When driving the ACULED, this heating must be taken into account to avoid damage to the LED. For
detailed information concerning this, please refer to the application note called “Thermal
Management of the ACULED VHL”.
Beside the necessity to avoid the chip from being damaged by overheating, the knowledge of
the temperature impacts on the LED chips helps us to get a grip on these parameters when
www.excelitas.com Driving the ACULED® VHL™ 12