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LT1785 Datasheet(PDF) 9 Page - Linear Technology |
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LT1785 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LT1785/LT1785A LT1791/LT1791A APPLICATIO S I FOR ATIO 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 the 120VAC line voltage. The LT1785/LT1791 protection is achieved by using a high voltage bipolar integrated circuit process for the trans- ceivers. 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 overvolt- age 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 exposed ports and cables in application environments. The LT1785 pins A and B and the LT1791 driver output pins Y and Z are protected to IEC-1000-4-2 level 2. These pins will survive multiple ESD strikes of ±15kV air dis- charge or ±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 transceiv- ers. 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. The DE logic input performs a similar function on the driver outputs. A high state on DE activates the differential driver outputs, a low state places both driver outputs into high impedance. Tying the RE and DE logic inputs to- gether 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 3 µs 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 12 µs. 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. Slew Limiting for EMI Emissions Control The LT1785/LT1791 feature controlled driver output slew rates to control high frequency EMI emissions from equipment and data cables. The slew limiting limits data rate operation to 250kbaud. Slew limiting also mitigates the adverse affects of imperfect transmission line termi- nation caused by stubs or mismatched cable. In some low speed, short distance networks, cable termination may be eliminated completely with no adverse effect on data transmission. Data Network Cable Selection and Termination Long distance data networks operating at high data trans- mission rates should use high quality, low attenuation cable with well-matched cable terminations. Short dis- tance networks at low data rates may use much less expensive PVC cable. These cables have characteristic impedances as low as 72 Ω. The LT1785/LT1791 output drivers are guaranteed to drive cables as low as 72 Ω. |
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