Datasheet

Photocurrent
I
Ph
[µA]
Figure 18 shows the principle of how to drive the PD with the ACULED. The standard PD that is
used with the ACULED DYO is a silicon-based PIN-type photo diode and thus should be driven
at a reverse bias voltage of V
R
= -5 V. This voltage has nearly no influence on the photo current
but on the bandwidth in fast transient applications. Please note that the cathode of the PD is on
the even numbered pads whereas with most of the LED chips the cathode is on the ACULED’s
odd pads. Depending on the light captured by the PD, a photo current I
Ph
flows, caused by the
photons absorbed in the pn-junction. Even without any light, a small current called dark current
I
D
can be measured. Among others it is caused by background radiation and has to be
considered for calibration. Table 3 shows the typical characteristics of the PD used with the
ACULED. The height of the PD-Chip is similar to the height of the LED chips and thus does not
shadow the LED chips much, resulting in an almost undisturbed light emission of your ACULED
DYO. Figure 19 shows the typical photo current I
Ph
of the ACULED PD as a function of the
luminous flux Φ
V
emitted from a chip placed on the ACULED.
Parameter
Reverse dark current
Reverse breakdown
voltage
Symbol Condition
I
D
V
R
= - 5 V; E
e
= 0 W/m²
V BR I
R
= 100 µA; E
e
= 0 W/m²
Value Table 3
Electro-optical characteristics of
< 10 nA the ACULED photo diode at
T B = 25 °C. The currents are
shown in absolute (positive)
170 V values.
Open circuit voltage V
OC
Photo current I
Ph
T
CT
= 3000 K
E
e
= 50 W/m²
V
R
= - 5 V; T
CT
= 3000 K
E
e
= 50 W/m²
440 mV
7 µA
Total capacitance C
PD
V
R
= - 5 V; E
e
= 0 W/m² 10 pF
Series resistance R
PD
V R
= - 5 V
150
Ω
Turn-on/off time t
on
/ t
off
V R
= - 5 V;
R L
= 100
Ω
10 / 10 ns
Due to the small photo current in the order of magnitude of some µA usually an amplifier is used
with PDs. Figure 20 shows a typical circuit using operational amplifiers to convert the photo
current I
Ph
to an output voltage V
Out
according to the following equation:
V Out
4000
3500
3000
2500
2000
1500
1000
500
0
0
= R
N
· I
Ph
5 10 15 20 25 30
luminous flux Φ V [lm]
(13)
35 40 45 50
Figure 19
Dependency of the photo current
I Ph from the luminous flux Φ V at a
specific color at board temperature
T B of 25°C on the ACULED DYO.
I Ph is pictured positive; V R = - 5 V.
www.excelitas.com Driving the ACULED® VHL™ 18