AN-3001

www.fairchildsemi.com Application Note AN-3001 Optocoupler Input Drive Circuits An optocoupler is a combination of a light source and a photosensitive detector. In the optocoupler, or photon coupled pair, the coupling is achieved by light being generated on one side of a transparent insulating gap and being detected on the other side of the gap without an electrical connection between the two sides (except for a minor amount of coupling capacitance). In the Fairchild Semiconductor optocouplers, the light is generated by an infrared light emitting diode, and the photo-detector is a silicon diode which drives an amplifier, e.g., transistor. The sensitivity of the silicon material peaks at the wavelength emitted by the LED, giving maximum signal coupling. Where the input to the optocoupler is a LED, the input characteristics will be the same, independent of the type of detector employed. The LED diode characteristics are shown in Figure 1. The forward bias current threshold is shown at approximately 1 volt, and the current increases exponentially, the useful range of IF between 1 mA and 100 mA being delivered at a VF between 1.2 and 1.3 volts. The dynamic values of the forward bias impedance are current dependent and are shown on the insert graph for RDF and ∆R as defined in the figure. Reverse leakage is in the nanoampere range before avalanche breakdown. The LED equivalent circuit is represented in Figure 2, along with typical values of the components. The diode equations are provided if needed for computer modeling and the constants of the equations are given for the IR LED’s. Note that the junction capacitance is large and increases with applied forward voltage. An actual plot of this capacitance variation with applied voltage is shown on the graph of Figure 3. It is this large capacitance controlled by the driver impedance which influences the pulse response of the LED. The capacitance must be charged before there is junction current to create light emission. This effect causes an inherent delay of 10-20 nanoseconds or more between applied current and light emission in fast pulse conditions. The LED is used in the forward biased mode. Since the current increases very rapidly above threshold, the device should always be driven in a current mode, not voltage driven. The simplest method of achieving the current drive is to provide a series current-limiting resistor, as shown in Figure 4, such that the difference between VAPP and VF is dropped across the resistor at the desired IF, determined from other criteria. A silicon diode is shown installed inversely parallel to the LED. This diode is used to protect the reverse breakdown of the LED and is the simplest method of achieving this protection. The LED must be protected from excessive power dissipation in the reverse avalanche region. A small amount of reverse current will not harm the LED, but it must be guarded against unexpected current surges. The forward voltage of the LED has a negative temperature coefficient of 1.05 mV/°C and the variation is shown in Figure 5. The brightness of the IR LED slowly decreases in an exponential fashion as a function of forward current (IF) and time. The amount of light degradation is graphed in Figure 6 which is based on experimental data out to 20,000 hours. A 50% degradation is considered to be the failure point. This degradation must be considered in the initial design of optoisolator circuits to allow for the decrease and still remain within design specifications on the current-transfer-ratio (CTR) over the design lifetime of the equipment. Also, a limitation on IF drive is shown to extend useful lifetime of the device. In some circumstances it is desirable to have a definite threshold for the LED above the normal 1.1 volts of the diode VF. This threshold adjustment can be obtained by shunting the LED by a resistor, the value of which is determined by a ratio between the applied voltage, the series resistor, and the desired threshold. The circuit of Figure 7 shows the relationship between these values. The calculations will determine the resistor values required for a given IFT and VA. It is also quite proper to connect several LED’s in series to share the same IF. The VF of the series is the sum of the individual VF’s. Zener diodes may also be used in series. Where the input applied voltage is reversible or alternating and it is desired to detect the phase or polarity of the input, the bipolar input circuit of Figure 8 can be employed. The individual optocouplers could control different functions or be paralleled to become polarity independent. Note that in this connection, the LED’s protect each other in reverse bias. REV. 4.00 4/30/02 AN-3001 APPLICATION NOTE VF - FORWARD VOLTAGE (VOLTS) ∆R = 300Ω 1.5 1.4 1.3 1.2 30Ω 3Ω 0.3Ω IF RS RDF= 13Ω 120Ω 1KΩ 10KΩ LED EQUIVALENT CIRCUIT FORWARD BIAS IF mA 100 80 60 SLOPE SLOPE V = RDF = I VF RP Cj D Vj D - IDEAL DIODE TA = 25˚C 1.1 1.0 0.9 0.8 0.1 1.0 10 VF IF Cj -5 55 0 - 1 10 - V 100 IF - FORWARD CURRENT (mA) AVALANCHE 20 VR 18 16 14 12 10 8 6 4 2 0 40 20 0.5 0.01 1.0 ∆V = ∆R = ∆I 100 mA 1.3 0.3 pF V nA Ω Ω 100 300 500 1.0 1.1 30 1.2 3 VF - VFT k IF IFT 1.5 VF VOLTS Vj IR RS RP >109 10K VA MCT2 β>200 0.5V LED 30Ω 110 IL2W ITH LED R = 200Ω I - mA 220Ω 115 CNY17-4 IL SANS LED 1 R = 1.6K 105 3.0 3.1 3.2 3.4 3.6 3.8 4.0 TERMINAL VOLTAGE - VA Figure 17. Constant-Current Shunt Impedance Figure 18. Shunt Impedance Performance 6 REV. 4.00 4/30/02 AN-3001 APPLICATION NOTE DISCLAIMER FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS. LIFE SUPPORT POLICY FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, or (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. www.fairchildsemi.com 4/30/02 0.0m 001 Stock#AN300000xx  2002 Fairchild Semiconductor Corporation