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HFBR-2416TCZ Datasheet(PDF) 11 Page - AVAGO TECHNOLOGIES LIMITED |
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HFBR-2416TCZ Datasheet(HTML) 11 Page - AVAGO TECHNOLOGIES LIMITED |
11 / 24 page 11 5 MBd Logic Link Design The resistor R1 is the only significant element in the drive circuit (see Figure 2) that limits the current through the LED, apart from the gate´s output port. Depending on the actual gate used, the voltage drop on the output port Vport could be neglected. The forward voltage val- ue, VF, of the LED depends on the desired LED current and on the temperature (see Figure 9). Make sure you take this behavior into account for the calculations. The curves in Figure 3, Figure 4, and Figure 5 are con- structed assuming no inline splice or any additional system loss. Besides fiber attenuation, for correct power budget calculation, make sure you take into account the effect of bending, humidity, ambient temperature, aging and other relevant influences. All these additional losses reduce the achievable link distance accordingly. For calculating the LED´s aging effect, an additional loss of about 1.5 dB is recognized. The following example will illustrate the technique for selecting the appropriate value of IF and R1: Figure 2. Typical Circuit Configuration The following diagrams (Figure 3 to Figure 5) serve as an aid in Link Design and are based on theoretical calcula- tions. For broad use, no additional effects such as aging were taken into account. The additional losses and the individual safety buffer values should be added sepa- rately. These diagrams reflect the pure viewing of power budget and do not allows conclusions about the actual link quality. Overdrive: Maximum optical output power of Tx com- bined with receiver sensitivity of -10 dBm over the entire temperature range. Typical 25 °C: Typical optical output power of Tx com- bined with receiver sensitivity of -25.4 dBm at TA = 25 °C. Worst Case: Minimum optical output power of Tx com- bined with receiver sensitivity of -24 dBm over the entire temperature range. Maximum distance required = 2000 meters by using HFBR-14x4Z/24x2Z logic link with 62.5/125 µm fiber. Figure 4 shows the“worst-case”drive current of about 43 mA for reaching a distance of about 2000 meters. Figure 9 shows the transmitter forward voltage of about VF = 1.62 V. If the typical circuit configuration (Figure 2) is used at Vcc = 5.0 V, the resistor value “R1” should be choosen to 78.6 Ω (3.38 V/43 mA) for reaching driver current of about 43 mA. Page 16 shows the guaranteed HFBR-14x4Z´s optical output power limit of -16.0 dBm (for driver current of 60 mA) over the entire temperature range. Figure 10 shows the normalized typical output power. When the transmitter will be driven with 43 mA the opti- cal output power is about 0.70 or -1.55 dB lower than at 60 mA. With an assumed fiber attenuation of 3.2 dB/km and the reduced driver current of 43 mA, the minimum optical output power at fiber end is about -24 dBm, which is equal to the receiver sensitivity over the entire tempera- ture range. For balancing the individual additional system losses, the driver current must be increased accordingly. - = I V V R F F CC 1 +5 V SELECT R 1 TO SET IF R 1 I F 1 K DATA IN ½ 75451 2 6 7 3 T HFBR-14xxZ TRANSMITTER TRANSMISSION DISTANCE = HFBR-24x2Z RECEIVER R TTL DATA OUT 2 6 7 & 3 R L V CC 0.1 µF Note: A bypass capacitor (0.01 µF to 0.1 µF ceramic) must be connected from pin 2 to pin 7 of the receiver. Total lead length between both ends of the capacitor and the pins should not exceed 20 mm. |
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