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LTC487CN Datasheet(PDF) 7 Page - Linear Technology |
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LTC487CN Datasheet(HTML) 7 Page - Linear Technology |
7 / 8 page 7 LTC487 S APPLICATI I FOR ATIO AC Cable Termination Cable termination resistors are necessary to prevent un- wanted reflections, but they consume power. The typical differential output voltage of the driver is 2V when the cable is terminated with two 120 Ω resistors, causing 33mA of DC current to flow in the cable when no data is being sent. This DC current is about 220 times greater than the supply current of the LTC487. One way to eliminate the unwanted current is by AC coupling the termination resis- tors as shown in Figure 10. LTC487 • TA11 C = LINE LENGTH (FT) x 16.3pF 120 Ω RECEIVER RX C Figure 10. AC Coupled Termination The coupling capacitor must allow high-frequency energy to flow to the termination, but block DC and low frequen- cies. The dividing line between high and low frequency depends on the length of the cable. The coupling capacitor must pass frequencies above the point where the line represents an electrical one-tenth wavelength. The value of the coupling capacitor should therefore be set at 16.3pF per foot of cable length for 120 Ω cables. With the coupling capacitors in place, power is consumed only on the signal edges, and not when the driver output is idling at a 1 or 0 state. A 100nF capacitor is adequate for lines up to 4000 feet in length. Be aware that the power savings start to decrease once the data rate surpasses 1/(120 Ω × C). 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. All LTC RS485 receivers have 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. If the receiver output must be forced to a known state, the circuits of Figure 11 can be used. LTC487 • TA12 140 Ω RECEIVER RX 5V 1.5k RECEIVER RX 5V 110 Ω 130 Ω 110 Ω 130 Ω 120 Ω RECEIVER RX C 5V 100k 1.5k Figure 11. Forcing ‘0’ 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. 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 12). Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights. TransZorb is a registered trademark of General Instruments, GSI |
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