Datasheet
show that driving the ACULED by changing the current is not a suitable solution. A much better way
is to use pulse width modulation which will be explained in the next chapter.
Figure 11
Typical wavelength vs. current
characteristic of an LED shown by
the example of the ACULED VHL
RGYB at 25°C board temperature.
The color of the curves responds to
the emitted color of the chips.
Pulse Width Modulation
We have learned from the previous chapters that a stable LED in terms of life time, color or
intensity needs a stable forward current as provided by constant current sources. But how can
the intensity being dimmed without changing the current? The most common way is to use pulse
width modulation (PWM). With PWM the LEDs are not driven in continuous but pulsed mode at
a high repetition rate / frequency f
PWM
. Since this is in the region of some kilohertz and thus
much above the flicker fusion threshold of the human eye, it will be seen as a continuous light.
The trick is that PWM pulses switch from off (0 mA) to on (e.g. 350 mA) only and the duty cycle, i.e.
the ratio of the on-time t
P
(pulse width) to the full time of a pulse period, gives the intensity of
the
LED. No currents in between these extreme values are used and therefore all current
dependencies are almost obsolete. Figure 12 shows the principle of PWM. The “area” of a pulse
given by its width t
P
at a fixed height corresponds to the intensity of the driven LED. By changing
the
width of the pulses (modulation) the LED brightness can be changed easily without the
impacts
of a changing current. The basic relation between the flux
Φ
, duty cycle d
PWM
and pulse width is
proportional in first approximation:
Φ
~ d
PWM
~ t
P
(7)
Nevertheless temperature effects also occur with PWM since the heating of an LED chip is
caused by the power it consumes. This is given by equation 4 which must be modified when
using PWM in the following way:
P
tot
= d
PWM
· I
F PWM
· V
F
(4a)
I
F PWM
represents the forward current given by the PWM source which must not exceed the
maximum forward current I
F max
of the LED, whereas d
PWM
represents the duty cycle given by
d
PWM
= t
P
·f
PWM
www.excelitas.com
(8)
Driving the ACULED® VHL™
11