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AN3010 Design Guideline for a Renesas/CEL Optocoupler …

A p p l i c at i o n N o t e AN3010 . Design Guideline for a Renesas/CEL Optocoupler with Transistor Output Authors: Van N. Tran CEL Staff Application Engineer, Opto Semiconductors Larry Sisken CEL Product Marketing Manager, Opto Semiconductors Wei Z. Jiang, Graduate Intern (MSEE), SJSU. Introduction Parameter Definition Current Transfer Ratio (CTR) is the gain of the Optocoupler . A standard Optocoupler provides signal transfer between It is the ratio of the phototransistor collector current to the an isolated input and output via an infrared Emitting Diode IRED forward current. (IRED) and a silicon phototransistor. Electrical isolation is achieved by sending a beam of infrared energy to an CTR = (IC / IF) * 100 optical receiver in a single package with a light-conducting It is expressed as a percentage (%). medium between the emitter and detector.

ApplicAtion note Page 1 Introduction A standard optocoupler provides signal transfer between an isolated input and output via an infrared Emitting Diode (IRED) and a silicon phototransistor.

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Transcription of AN3010 Design Guideline for a Renesas/CEL Optocoupler …

1 A p p l i c at i o n N o t e AN3010 . Design Guideline for a Renesas/CEL Optocoupler with Transistor Output Authors: Van N. Tran CEL Staff Application Engineer, Opto Semiconductors Larry Sisken CEL Product Marketing Manager, Opto Semiconductors Wei Z. Jiang, Graduate Intern (MSEE), SJSU. Introduction Parameter Definition Current Transfer Ratio (CTR) is the gain of the Optocoupler . A standard Optocoupler provides signal transfer between It is the ratio of the phototransistor collector current to the an isolated input and output via an infrared Emitting Diode IRED forward current. (IRED) and a silicon phototransistor. Electrical isolation is achieved by sending a beam of infrared energy to an CTR = (IC / IF) * 100 optical receiver in a single package with a light-conducting It is expressed as a percentage (%). medium between the emitter and detector.

2 This mechanism provides complete electrical isolation of electronic circuits The CTR depends on the current gain (hfe) of the transistor, from input to output while transmitting information from the supply voltage to the phototransistor, the forward current one side to the other, and from one voltage potential to through the IRED and operating temperature. another. Below is the CTR vs. Forward Current, I F graph of the This application note addresses the two common modes PS2501A-1-A. of operation: Linear mode and digital logic mode. What is linear mode? Current Transfer Ratio vs. Forward Current In linear mode, the Optocoupler output produces a signal, 300. VCE = 5V. which is a copy of the input signal; its amplitude is a n=3. product of the input signal and the Current Transfer Ratio 250. Current Transfer Ratio CTR (%). Sample A.

3 (CTR). In an Optocoupler the CTR is specified as a ratio of B. 200 C. the collector current IC at the output to the forward cur- rent, IF, applied to the Infrared Emitting Diode (IRED) on 150. the input side. A key condition for the Linear Mode is the collector - emitter voltage, VCE is not in saturation. 100. 50. What is digital logic mode? In digital logic mode, the output signal is either logic high 0. 1 10 100. (~Vcc) or low (ground level, for example), and ideally the Forward Current IF (mA). phototransistor on the receiver side goes into saturation when the transistor output switches to logic high so that Fig 1: CTR vs. Forward Current, IF. the power consumption would be at the very low level. Typically, for a silicon-based phototransistor, the satura- This application note is written around renesas tion voltage across the collector and emitter, VCE would phototransistor Optocoupler PS2501A-1-A in DIP 4 package, be at V or less.

4 The measurements were done at nominal room temperature, the common collector circuit configuration was used and was operating at 5 Volts, a common operating voltage for use with digital circuits, and in other situations where data or pulse-edge events communicate between units. Since the emitter current is approximately equal to collector current, IE =~ IC, they will be used interchangeably in this application note. Page 1. AN3010 . Linear Mode Operation In the common collector configuration (see figure ), the collector is the common point of this transistor circuit, and VCE vs. Forward Current, IF. 6. the output is taken at the emitter. The output transitions 5. from a low state to a high state when the Optocoupler input 4 RL = 100 OHM. transitions high. VCE (V). 3 RL = 320 OHM. IF 2 RL = 470 OHM. VCC. 1. VIN. IC 0. 0 5 10 15 20 25 30.

5 Forward Current, IF (mA). VOUT. Rin RL. Fig : Collector-Emitter Voltage vs. Forward current at different load resistors Common Collector Amplifier Fig : Common Collector Amplifier Emitter Current, IE, vs. Forward current IF. 25. The output voltage is at the load resistor between the emitter Emitter Current, IE (mA). RL = 100 OHM. 20. pin and ground. 15 RL = 320 OHM. In general, choose the load resistor RL, to make sure that 10. Vcc (IE *RL) > 5. RL = 470 OHM. Where, IE is the emitter current through load resistor RL. 0. 0 5 10 15 20 25 30. To work effectively in the linear mode, RL should be around Forward Current, IF (mA). 470 or less, depending on the drive current IF and the Design objective. Fig : Emitter Current vs. Forward Current at different load resistors To show how the forward current, I F, and load resistor, RL, affect the collector-emitter voltage VCE, and emitter current IE, data was collected and is shown in figures and for Digital Logic Mode reference.

6 Option without using CTR. From these graphs, we can see that the load resistor RL at To operate in the digital logic mode, we plan for the 470 would limit the linear operating region of the forward Optocoupler output transistor to operate in saturation mode. current, IF to around 6mA or less. First, review the renesas Optocoupler datasheet for values In comparison, the load resistor value of 100 would extend useful in calculating circuit values at the input and output. the linear operating region up to 12mA. Note that the Our example uses graphs and information from the PS2501A- values for VCE do not go below about A resistor value 1-A data sheet as shown in figure somewhere between 100 and 320 would provide a linear range of VCE values with different slope. Figure shows the Collector Current vs. Collector Saturation Voltage.

7 The collector current shown is developed from Figure shows a fairly linear range of collector current IC. the input forward current IF and CTR value for the selected with the 100 resistor value. Depending on the desired goal, Optocoupler . various resistor values provide the desired results. Page 2. AN3010 . Collector Current vs. Collector Saturation Voltage Figureapplications Some : Optocouplers mayused to drive discrete be required components to tolerate higherforforward motor drive application. 10 IF= 10 mA current, IF, such as, 10 mA, for example. IF= 5 mA. The option is either to keep the same load resistor, RL, at Collector Current IC (mA). IF= 2 mA 13k or choose the Ic = mA as shown in figure , then IC = mA select the RL, so the product of (RL * IC) > 5V. In this example, IF= 1 mA. 1 select any resistance value of * or greater.

8 In this IC = mA. example, would be sufficient. IC = mA. Note that a load resistor change from RL = 13k to . would improve the switching time, such as, rise and fall time 0 as shown in table below. Collector Saturation Voltage VCE(sat) (V). CTR = 140% @ VCC = 5V, IF = 5mA. IF (mA) RL (K ) tr (us) tf (us). 1 Fig : Collector Current vs. Collector Saturation Voltage 10 The simple approach to operating the phototransistor in sat- However, there is a trade-off between high input current and uration mode is by selecting the constant collector current the degradation of an Optocoupler over the lifetime. at as shown in figure , so that VCE is at or less. Note that one could select Ic = and VCE = Option with CTR. with an input current of IF = 1mA. However, in the example, Ic = is selected as a demonstration. In some instances, when the designers prefer to use opto device with a specific rank for the digital mode to switch it Calculate the RL : ON or OFF.

9 RL=(Vcc - VCE ) = ( ) = . There are two things must be observed: In practice, RL should be chosen with additional 30%. tolerance or more, so by selecting RL = as a standard Design based on the worst-case scenario value, it would be sufficient. In other words, Design it based on the low end of the CTR. Next, what is the forward current needed to achieve the rank. For example, if the PS2501A-1-Q-A is used. From the desired result? data sheet, the Q rank shows that the CTR = 100% to 200%. Key goals of the Design are to achieve a long service lifetime at LED forward current, IF = 5mA, then use the CTR = 100% at for the device and to operate at low power consumption. IF = 5mA as the reference point for the Design and the same forward current, IF = must be used. Other forward Choose the forward current IF between to mA as currents that are different from IF = 5mA would produce shown in figure since further increase of IF will not make different sets of CTR graphs as shown in figure any significant difference in the output, Vout, and higher input drive does not drive the saturation voltage lower.

10 Data is collected in table for reference. Calculate Load Resistance, RL. RL = K . By using the same configuration as shown in figure as an Forward Current, IF ( mA) VOUT (V) VCE (V) IE (mA). example, calculate RL as follows: 1 RL_calculated = (Vcc ) / (5mA * Lower limit of CTR rank). 2 And select RL_actual > = 2 * RL_calculated 3 For example, assume Vcc=5V, IF = 5mA and PS2501A-1-Q is 4 used, then 5 6 RL_calculated = (5V - )/5mA)=960 . And RL_actual = . Table : Forward Current, IF vs. Vout Page 3. AN3010 . To set the forward current, IF of the IRED. The input resistor to set the forward current, IF is calculated: Rin = (Vin VF)/ IF (Refer to Figure ). Find the value for VF from the graph in figure This will not be exact, and a nominal value will work. For the IF = , we can select VF = and Vin = 5V at a temperature of 25 C from figure as a reference: Rin = ( ) / 5mA = 770 , so choose the standard value of 750 or less.


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