Datasheet

Figure 6
Left: Circuit of parallel
connection of the ACULED
VHL chips, which is highly
discouraged.
Right: Circuit with common
anode.
Common anode / cathode
If the number of connections is limited and the ACULED is not cascaded with other ACULEDs (see
next section), a common anode or cathode can be used. Figure 6, right side shows the diagram of
such a circuit. Though each channel can still be driven individually, only five instead of eight
connections have to be used.
Figure 7
Temperature profiles taken by
an IR camera for a line of
parallelly driven LED chips.
Left: After half a minute.
Right: After a few minutes.
Cascading the ACULED
When connecting more than one ACULED of a kind, usually chips with the same color should
be driven simultaneously. With the rules learned above, we now know that connecting the
corresponding chips serially as shown in figure 8 is the best solution for constant and
homogeneous color of each channel. According to equation 5, the forward voltages of the chips
have to be added, so you have to take care that the PSU can provide the voltage needed. When
cascading a larger number of ACULEDs or chips to be precise, the summary voltage V
F sum
can
easily achieve high values. Therefore it can be helpful to make a cross connection after a couple
of serially connected chips as shown in figure 9. This also adds some redundancy into the
driving circuit, helping to leave some LEDs of a channel illuminated if a single chip fails during
operation. It is recommended to have at least a minimum of four to five chips serially connected
before adding a cross section to average over the different forward voltages of the individual
LED chips.
The power consumption P
Cn
of a single chip can easily be calculated according to equation 4.
To calculate the power consumption P
tot
of a single ACULED or a bunch of connected
ACULEDs, the individually P
Cn
can be added up. This gives a hint of the minimum total power
that a PSU must supply. The minimum current and voltage that has to be provided by the PSU
www.excelitas.com Driving the ACULED® VHL™ 8