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LT1791A Datasheet(PDF) 11 Page - Linear Technology |
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LT1791A Datasheet(HTML) 11 Page - Linear Technology |
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11 / 18 page  LT1785/LT1785A/ LT1791/LT1791A 11 178591fd For more information www.linear.com/LT1785 applicaTions inForMaTion Overvoltage Protection TheLT1785/LT1791RS485/RS422transceiversansweran applications need for overvoltage fault tolerance on data networks. Industrial installations may encounter common mode voltages between nodes far greater than the –7V to 12V range specified for compliance to RS485 standards. CMOS RS485 transceivers can be damaged by voltages above their absolute maximum ratings of typically –8V to 12.5V. Replacement of standard RS485 transceiver componentswiththeLT1785orLT1791deviceseliminates field failures due to overvoltage faults or the use of costly external protection devices. The limited overvoltage toler- ance of CMOS RS485 transceivers makes implementation of effective external protection networks difficult without interfering with proper data network performance within the –7V to 12V region of RS485 operation. The high overvoltage rating of the LT1785/LT1791 facili- tates easy extension to almost any level. Simple discrete component networks that limit the receiver input and driver output voltages to less than ±60V can be added to the device to extend protection to any desired level. Figure 11 shows a protection network against faults to the120VAC line voltage. The LT1785/LT1791 protection is achieved by using a high voltagebipolarintegratedcircuitprocessforthetransceiv- ers. The naturally high breakdown voltages of the bipolar process provides protection in powered-off and high impedance conditions. The driver outputs use a foldback current limit design to protect against overvoltage faults while still allowing high current output drive. ESD Protection The LT1785/LT1791 I/O pins have on-chip ESD protection circuitry to eliminate field failures caused by discharges to exposedportsandcablesinapplicationenvironments.The LT1785 pins A and B and the LT1791 driver output pins Y andZareprotectedtoIEC-1000-4-2level2.Thesepinswill survivemultipleESDstrikesof±15kVairdischargeor±4kV contact discharge. Due to their very high input impedance, the LT1791 receiver pins are protected to IEC-1000-4-2 level 2, or ±15kV air and ±4kV contact discharges. This level of ESD protection will guarantee immunity from field failures in all but the most severe ESD environments. The LT1791 receiver input ESD tolerance may be increased to IEC level 4 compliance by adding 2.2k resistors in series with these pins. Low Power Shutdown The LT1785/LT1791 have RE and DE logic inputs to control the receive and transmit modes of the transceivers. The RE input allows normal data reception when in the low state. The receiver output goes to a high impedance state when RE is high, allowing multiplexing the RO data line. TheDElogicinputperformsasimilarfunctiononthedriver outputs. A high state on DE activates the differential driver outputs, a low state places both driver outputs in to high impedance. Tying the RE and DE logic inputs together may be done to allow one logic signal to toggle the transceiver from receive to transmit modes. The DE input is used as the data input in CAN bus applications. Disabling both the driver and receiver places the device into a low supply current shutdown mode. An internal time delay of 3µs minimum prevents entering shutdown due to small logic skews when a toggle between receive and transmit is desired. The recovery time from shutdown mode is typically 12µs. The user must be careful to allow for this wake-up delay from shutdown mode. To allow full 250kbaud data rate transmission in CAN applications, the RE pin should be tied low to prevent entering shutdown mode. |
Similar Part No. - LT1791A_15 |
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Similar Description - LT1791A_15 |
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