LTC487ISW(RevA) 데이터 시트보기 (PDF) - Linear Technology

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LTC487ISW
(Rev.:RevA)

Quad Low Power RS485 Driver

Linear Technology 

LTC487

APPLICATI S 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 120Ω 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.

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.

5V

110Ω 130Ω 130Ω 110Ω

RECEIVER

RX

120Ω

C

RECEIVER

RX

5V

1.5k

140Ω

RECEIVER

RX

C = LINE LENGTH (FT) x 16.3pF

LTC487 • TA11

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 120Ω 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/(120Ω × 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 120Ω, the differential inputs to the receiver are

shorted together, not left floating. Because the receiver

100k

5V

1.5k

C

120Ω

RECEIVER

RX

LTC487 • TA12

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, 1.5kΩ). 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).

TransZorb is a registered trademark of General Instruments, GSI

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.

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