IL300 www.vishay.com Vishay Semiconductors OPERATION DESCRIPTION K3-TRANSFER FAIN LINEARITY A typical application circuit (figure 1) uses an operational The percent deviation of the transfer gain, as a function of amplifier at the circuit input to drive the LED. The feedback LED or temperature from a specific transfer gain at a fixed photodiode sources current to R1 connected to the inverting LED current and temperature. input of U1. The photocurrent, IP1, will be of a magnitude to satisfy the relationship of (IP1 = VIN/R1). PHOTODIODE The magnitude of this current is directly proportional to the A silicon diode operating as a current source. The output feedback transfer gain (K1) times the LED drive current current is proportional to the incident optical flux supplied (VIN/R1 = K1 x IF). The op-amp will supply LED current to by the LED emitter. The diode is operated in the photovoltaic force sufficient photocurrent to keep the node voltage (Vb) or photoconductive mode. In the photovoltaic mode the equal to Va. diode functions as a current source in parallel with a forward The output photodiode is connected to a non-inverting biased silicon diode. voltage follower amplifier. The photodiode load resistor, R2, The magnitude of the output current and voltage is performs the current to voltage conversion. The output dependent upon the load resistor and the incident LED amplifier voltage is the product of the output forward gain optical flux. When operated in the photoconductive mode (K2) times the LED current and photodiode load, the diode is connected to a bias supply which reverse R2 (VO = IF x K2 x R2). biases the silicon diode. The magnitude of the output Therefore, the overall transfer gain (V current is directly proportional to the LED incident optical O/VIN) becomes the ratio of the product of the output forward gain (K2) times the flux. photodiode load resistor (R2) to the product of the feedback transfer gain (K1) times the input resistor (R1). This reduces LED (LIGHT EMITTING DIODE) to An infrared emitter constructed of AlGaAs that emits at V 890 nm operates efficiently with drive current from 500 μA to O/VIN = (K2 x R2)/(K1 x R1). 40 mA. Best linearity can be obtained at drive currents The overall transfer gain is completely independent of the between 5 mA to 20 mA. Its output flux typically changes by LED forward current. The IL300 transfer gain (K3) is -0.5 %/°C over the above operational current range. expressed as the ratio of the output gain (K2) to the feedback gain (K1). This shows that the circuit gain APPLICATION CIRCUIT becomes the product of the IL300 transfer gain times the ratio of the output to input resistors VO/VIN = K3 (R2/R1). VCC 1 IL300 8 K1-SERVO GAIN Va + + R3 The ratio of the input photodiode current (IP1) to the LED Vin U1 2 7 current (I K2 V F) i.e., K1 = IP1/IF. Vb K1 CC - IF - V K2-FORWARD GAIN CC 3 6 VCC U2 Vout The ratio of the output photodiode current (I V P2) to the LED C 4 5 + current (IF), i.e., K2 = IP2/IF. lp1 R2 R1 lp2 K3-TRANSFER GAIN The transfer gain is the ratio of the forward gain to the servo gain, i.e., K3 = K2/K1. Fig. 1 - Typical Application Circuit 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