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LTC491CN Datasheet(PDF) 10 Page - Linear Technology |
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LTC491CN Datasheet(HTML) 10 Page - Linear Technology |
10 / 12 page LTC491 10 Fault Protection All of LTC’s RS485 products are protected against ESD transients up to 2kV using the human body model (100pF, 1.5k Ω). However, some applications need more protection. The best protection method is to connect a bidirectional TransZorb from each line side pin to ground (Figure 16). LTC491 • TA17 120 Ω DRIVER Z Y Figure 16. ESD Protection with TransZorbs A TransZorb is a silicon transient voltage suppressor that has exceptional surge handling capabilities, fast response time, and low series resistance. They are available from General Semiconductor Industries and come in a variety of breakdown voltages and prices. Be sure to pick a break- down voltage higher than the common mode voltage required for your application (typically 12V). Also, don’t forget to check how much the added parasitic capacitance will load down the bus. Receiver Open-Circuit Fail-Safe Some data encoding schemes require that the output of the receiver maintains a known state (usually a logic 1) when the data is finished transmitting and all drivers on the line are forced into three-state. The receiver of the LTC491 has a fail-safe feature which guarantees the output to be in a logic 1 state when the receiver inputs are left floating (open-circuit). However, when the cable is terminated with 120 Ω, the differential inputs to the receiver are shorted together, not left floating. Because the receiver has about 70mV of hysteresis, the receiver output will maintain the last data bit received. 140 Ω RECEIVER RX +5V 1.5k Ω RECEIVER RX +5V 110 Ω 130 Ω 110 Ω 130 Ω LTC491 • TA16 120 Ω RECEIVER RX C +5V 100k Ω 1.5k Ω Figure 15. Forcing “O” When All Drivers are Off The termination resistors are used to generate a DC bias which forces the receiver output to a known state, in this case a logic 0. The first method consumes about 208mW and the second about 8mW. The lowest power solution is to use an AC termination with a pull-up resistor. Simply swap the receiver inputs for data protocols ending in logic 1. S APPLICATI I FOR ATIO |
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