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LTC491CN Datasheet(PDF) 7 Page - Linear Technology |
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LTC491CN Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page 7 LTC491 491fa Typical Application A typical connection of the LTC491 is shown in Figure 9. Two twisted-pair wires connect up to 32 driver/receiver pairs for full duplex data transmission. There are no restrictions on where the chips are connected to the wires, and it isn’t necessary to have the chips connected at the ends. However, the wires must be terminated only at the ends with a resistor equal to their characteristic imped- ance, typically 120 Ω. The input impedance of a receiver is typically 20k Ω to GND, or 0.6 unit RS-485 load, so in practice 50 to 60 transceivers can be connected to the same wires. The optional shields around the twisted pair help reduce unwanted noise, and are connected to GND at one end. The LTC491 can also be used as a line repeater as shown in Figure 10. If the cable length is longer than 4000 feet, the LTC491 is inserted in the middle of the cable with the receiver output connected back to the driver input. Thermal Shutdown The LTC491 has a thermal shutdown feature which pro- tects the part from excessive power dissipation. If the outputs of the driver are accidently shorted to a power supply or low impedance source, up to 250mA can flow through the part. The thermal shutdown circuit disables the driver outputs when the internal temperature reaches 150 °Candturnsthembackonwhenthetemperaturecools to 130 °C. If the outputs of two or more LTC491 drivers are shorted directly, the driver outputs can not supply enough current to activate the thermal shutdown. Thus, the ther- mal shutdown circuit will not prevent contention faults when two drivers are active on the bus at the same time. APPLICATIO S I FOR ATIO Figure 9. Typical Connection Figure 10. Line Repeater LTC491 • F09 120 Ω 120 Ω LTC491 DRIVER RECEIVER LTC491 DRIVER DX RX DX RX 2 5 9 10 11 12 RECEIVER 9 10 11 12 5 2 120 Ω 3 4 3 4 120 Ω RECEIVER LTC491 DRIVER 9 10 11 12 5 4 3 2 DX RX LTC491 • F10 120 Ω LTC491 DRIVER DX RX 2 5 9 10 11 12 RECEIVER DATA IN DATA OUT 3 4 120 Ω |
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