LTC491IS#TRPBF【PDF 12/12 页】IC TXRX RS485 LOW POWER 14-SOIC

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LTC491

491fa

PPLICATI

I FOR ATIO

When the reflected signal returns to the driver, the ampli-

tude will be lowered. The width of the pedestal is equal to

twice the electrical length of the cable (about 1.5ns/foot).

If the cable is lightly loaded (470

), the signal reflects in

phase and increases the amplitude at the driver output. An

input frequency of 30kHz is adequate for tests out to 4000

feet of cable.

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

resistors, causing

33mA of DC current to flow in the cable when no data is

being sent. This DC current is about 60 times greater than

the supply current of the LTC491. One way to eliminate the

unwanted current is by AC coupling the termination resis-

tors as shown in Figure 14.

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

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

× 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. 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

, the differential inputs to the receiver are shorted

together, not left floating. Because the receiver has about

70mV of hysteresis, the receiver output will tend to main-

tain the last data bit received, but this is not guaranteed.

The termination resistors are used to generate a DC bias

which forces the receiver output to a known state; in the

case of Figure 15, 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 proto-

cols ending in logic 1.

LTC491 F14

120

RECEIVER

C = LINE LENGTH (ft) x 16.3pF

Figure 14. AC Coupled Termination

Figure 15. Forcing “O” When All Drivers are Off

140

RECEIVER

1.5k

RECEIVER

110

130

110

130

LTC491 F15

120

RECEIVER

100k

1.5k

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