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LTC1484IS8 数据表(PDF) 12 Page - Linear Technology |
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LTC1484IS8 数据表(HTML) 12 Page - Linear Technology |
12 / 16 page 12 LTC1484 APPLICATIONS INFORMATION Shutdown Mode The receiver output (RO) and the driver outputs (A, B) can be three-stated by taking the RE and DE pins high and low respectively. Taking RE high and DE low at the same time puts the LTC1484 into shutdown mode and ICC drops to 20 µA max. In some applications (see CDMA), the A and B lines are pulled to VCC or GND through external resistors to force the line to a high or low state when all connected drivers are disabled. In shutdown, the supply current will be higher than 20 µA due to the additional current drawn through the external pull-up and the 22k input resistance of the LTC1484. ESD Protection The ESD performance of the LTC1484 A and B pins is characterized to meet ±15kV using the Human Body Model (100pF, 1.5k Ω),IEC-1000-4-2level(±8kV)contact mode and IEC-1000-4-2 level 3 ( ±8kV) air discharge mode. This means that external voltage suppressors are not required in many applications when compared with parts that are only protected to ±2kV. Pins other than the A and B pins are protected to ±4.5kV typical per the Human Body Model. When powered up, the LTC1484 does not latch up or sustain damage when the A and B pins are tested using any of the three conditions listed. The data during the ESD event may be corrupted, but after the event the LTC1484 continues to operate normally. The additional ESD protec- tion at the A and B pins is important in applications where these pins are exposed to the external world via connec- tions to sockets. Fault Protection When shorted to –7V or 10V at room temperature, the short-circuit current in the driver pins is limited by internal resistance or protection circuitry to 250mA. Over the industrial temperature range, the absolute maximum positive voltage at any driver pin should be limited to 10V to avoid damage to the driver pins. At higher ambient temperatures, the rise in die temperature due to the short-circuit current may trip the thermal shutdown circuit. When the driver is disabled, the receiver inputs can withstand the entire – 7V to 12V RS485 common mode range without damage. The LTC1484 includes a thermal shutdown circuit which protects the part against prolonged shorts at the driver outputs. If a driver output is shorted to another output or to VCC, the current will be limited to 250mA. If the die temperature rises above 150 °C, the thermal shutdown circuit three-states the driver outputs to open the current path. When the die cools down to about 130 °C, the driver outputs are taken out of three-state. If the short persists, the part will heat again and the cycle will repeat. This thermal oscillation occurs at about 10Hz and protects the part from excessive power dissipation. The average fault current drops as the driver cycles between active and three-state. When the short is removed, the part will return to normal operation. Carrier Detect Multiple Access (CDMA) Application In normal half-duplex RS485 systems, only one node can transmit at a time. If an idle node suddenly needs to gain access to the twisted pair while other communications are in progress, it must wait its turn. This delay is unaccept- able in safety-related applications. A scheme known as Carrier Detect Multiple Access (CDMA) solves this prob- lem by allowing any node to interrupt on-going communi- cations. Figure 11 shows four nodes in a typical CDMA communi- cations system. In the absence of any active drivers, bias resistors (1.2k) force a “1” across the twisted pair. All drivers in the system are connected so that when enabled, they transmit a “0”. This is accomplished by tying DI low and using DE as the driver data input. A “1” is transmitted by disabling the driver’s “0” output and allowing the bias resistors to reestablish a “1” on the twisted pair. Control over communications is achieved by asserting a “0” during the time an active transmitter is sending a “1”. Any node that is transmitting data watches its own |
类似零件编号 - LTC1484IS8 |
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类似说明 - LTC1484IS8 |
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