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LTC487ISW Datasheet(PDF) 5 Page - Linear Technology |
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LTC487ISW Datasheet(HTML) 5 Page - Linear Technology |
5 / 8 page 5 LTC487 TEST CIRCUITS LTC487 • TA04 OUTPUT UNDER TEST CL S1 500 CC V Ω S2 Figure 5. Driver Timing Test Load #2 DRIVER 1 LTC487 • TA03 DI A B EN12 RDIFF CL1 CL2 Figure 4. Driver Timing Test Circuit LTC487 • TA02 A B R R OD V OC V Figure 3. Driver DC Test Load S APPLICATI I FOR ATIO Typical Application A typical connection of the LTC487 is shown in Figure 6. A twisted pair of wires connect up to 32 drivers and receivers for half 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 impedance, typically 120 Ω. The optional shields around the twisted pair help reduce unwanted noise, and are connected to GND at one end. Thermal Shutdown The LTC487 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 °C and turns them back on when the temperature cools to 130 °C. If the outputs of two or more LTC487 drivers are shorted directly, the driver outputs can not supply enough current to activate the thermal shutdown. Thus, the thermal shutdown circuit will not prevent con- tention faults when two drivers are active on the bus at the same time. Cable and Data Rate The transmission line of choice for RS485 applications is a twisted pair. There are coaxial cables (twinaxial) made for this purpose that contain straight pairs, but these are less flexible, more bulky, and more costly than twisted pairs. Many cable manufacturers offer a broad range of 120 Ω cables designed for RS485 applications. Figure 6. Typical Connection EN12 4 LTC487 • TA07 120 Ω DX 1 2 3 SHIELD 120 Ω RX RX SHIELD 3 DX EN12 4 2 EN12 4 1 2 3 RX RX 3 DX EN12 4 1 DX 1/4 LTC489 1/4 LTC487 1/4 LTC489 1/4 LTC487 2 1 |
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