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AN-3002 Datasheet(PDF) 2 Page - Fairchild Semiconductor |
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AN-3002 Datasheet(HTML) 2 Page - Fairchild Semiconductor |
2 / 4 page AN-3002 APPLICATION NOTE 2 REV. 4.00 4/30/02 The resulting LED forward current that will keep the output circuitry conducting is shown as the result of proper design. Figure 3. Pulse Train Waveforms A desired sine wave to be present is shown in Figure 4. The resulting LED forward current that will keep the output circuitry conducting is shown as the result of proper design. Figure 4. Sine Wave Waveforms Matrices Opto-Coupling With the low input LED current advantage of the 6N139, the ability to drive matrices with but one TTL output is now pos- sible as shown in Figure 5. Figure 5. Opto-Coupling out of Matrices Non-Loading Line Receiver For virtual non-loading, the 6N139 is compatible with the differential amplifier circuit of Figure 6. Figure 6. Differential Amplifier Drive For a virtual no-load optoisolator circuit use: X=Non-latching output circuitry to follow LED=Input diode of 6N139 Current requirement at “in” will be less than 20 micro Amperes. Example: If “VREF” is made to be +1.4 Volts and the RE is 1.2 KΩ, the circuit will respond nicely to TTL “0” and “1” levels. That is, a “0” at “In” will cause LED current resulting in the conduc- tion of the output circuitry. Conversely, a “1” at “In” will result in no LED current. Notice that depending upon which collector the LED is in series with it will give the option of LED current flowing with a “0” or a “1” at “In”. 6N139 Output Circuitries The following are two examples of 6N139 output circuitry. One latching (Figure 7); the other non-latching (Figure 8), but both capable of driving a TTL gate directly. Referring to Figure 7 and assuming that the “RESET” has been actuated by a momentary ground and no input signal is being received, all transistors shown are non-conducting (Output high, “1”). The arrival of an input signal will cause all transistors to turn on. (Output low, “0”). The PNP transis- tor, being turned on by the output transistor, will in turn latch that same output transistor or until the “RESET” is again initiated. In Figure 8, where no signal is being received, the input transistor is not conducting. The output transistor is very slightly conducting. The 4.7M Ω resistor causing this slight conduction will not bring the “Output” to a “0” level. The purpose of this slight conduction is to reduce the turn-on delay time. When a signal is received, both input INPUT (Volts) + t=0 OUTPUT (Volts) 5.0 0.4 LED (mA) 1.0 0.5 0 INPUT (Volts) + 0 t=0 OUTPUT (Volts) 5.0 0.4 LED (mA) 1.0 0.5 0 1 of 16 ACTIVE LOW SCAN CONTROL 16 X 16 MATRIX 5V 5.5K Ω RESISTORS ALL ARE INPUT DIODES OF 6N139 IN 10K Ω or here +V 6N139 AS EXAMPLE VREF VREF RE LED 0.5 mA X 1mA +V |
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