IL300
www.vishay.com
Vishay Semiconductors
Rev. 1.8, 02-Jun-14
2
Document Number: 83622
For technical questions, contact: optocoupleranswers@vishay.com
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
OPERATION DESCRIPTION
A typical application circuit (figure 1) uses an operational
amplifier at the circuit input to drive the LED. The feedback
photodiode sources current to R1 connected to the inverting
input of U1. The photocurrent, I
P1
, will be of a magnitude to
satisfy the relationship of (I
P1
= V
IN
/R1).
The magnitude of this current is directly proportional to the
feedback transfer gain (K1) times the LED drive current
(V
IN
/R1 = K1 x I
F
). The op-amp will supply LED current to
force sufficient photocurrent to keep the node voltage (Vb)
equal to Va.
The output photodiode is connected to a non-inverting
voltage follower amplifier. The photodiode load resistor, R2,
performs the current to voltage conversion. The output
amplifier voltage is the product of the output forward gain
(K2) times the LED current and photodiode load,
R2 (V
O
= I
F
x K2 x R2).
Therefore, the overall transfer gain (V
O
/V
IN
) becomes the
ratio of the product of the output forward gain (K2) times the
photodiode load resistor (R2) to the product of the feedback
transfer gain (K1) times the input resistor (R1). This reduces
to
V
O
/V
IN
= (K2 x R2)/(K1 x R1).
The overall transfer gain is completely independent of the
LED forward current. The IL300 transfer gain (K3) is
expressed as the ratio of the output gain (K2) to the
feedback gain (K1). This shows that the circuit gain
becomes the product of the IL300 transfer gain times the
ratio of the output to input resistors
V
O
/V
IN
= K3 (R2/R1).
K1-SERVO GAIN
The ratio of the input photodiode current (I
P1
) to the LED
current (I
F
) i.e., K1 = I
P1
/I
F
.
K2-FORWARD GAIN
The ratio of the output photodiode current (I
P2
) to the LED
current (I
F
), i.e., K2 = I
P2
/I
F
.
K3-TRANSFER GAIN
The transfer gain is the ratio of the forward gain to the servo
gain, i.e., K3 = K2/K1.
K3-TRANSFER FAIN LINEARITY
The percent deviation of the transfer gain, as a function of
LED or temperature from a specific transfer gain at a fixed
LED current and temperature.
PHOTODIODE
A silicon diode operating as a current source. The output
current is proportional to the incident optical flux supplied
by the LED emitter. The diode is operated in the photovoltaic
or photoconductive mode. In the photovoltaic mode the
diode functions as a current source in parallel with a forward
biased silicon diode.
The magnitude of the output current and voltage is
dependent upon the load resistor and the incident LED
optical flux. When operated in the photoconductive mode
the diode is connected to a bias supply which reverse
biases the silicon diode. The magnitude of the output
current is directly proportional to the LED incident optical
flux.
LED (LIGHT EMITTING DIODE)
An infrared emitter constructed of AlGaAs that emits at
890 nm operates efficiently with drive current from 500 μA to
40 mA. Best linearity can be obtained at drive currents
between 5 mA to 20 mA. Its output flux typically changes by
-0.5 %/°C over the above operational current range.
APPLICATION CIRCUIT
Fig. 1 - Typical Application Circuit
8
7
6
5
K1
1
2
3
4
K2
R1
R2
IL300
Vb
Va
+
-
U1
V
in
lp1
-
U2
+
lp2
V
out
V
CC
V
CC
V
CC
V
CC
I
F
V
C
+
R3
