10

FN6046.8

February 28, 2006

In point-to-point, or point-to-multipoint (single driver on bus)

networks, the main cable should be terminated in its

characteristic impedance (typically 120

Ω) at the end farthest

from the driver. In multi-receiver applications, stubs

connecting receivers to the main cable should be kept as

short as possible. Multipoint (multi-driver) systems require

that the main cable be terminated in its characteristic

impedance at both ends. Stubs connecting a transceiver to

the main cable should be kept as short as possible.

Built-In Driver Overload Protection

As stated previously, the RS-485 spec requires that drivers

survive worst case bus contentions undamaged. The

ISL84XX devices meet this requirement via driver output

short circuit current limits, and on-chip thermal shutdown

circuitry.

The driver output stages incorporate short circuit current

limiting circuitry which ensures that the output current never

exceeds the RS-485 spec, even at the common mode

voltage range extremes. Additionally, these devices utilize a

foldback circuit which reduces the short circuit current, and

thus the power dissipation, whenever the contending voltage

exceeds either supply.

In the event of a major short circuit condition, ISL84XX

devices also include a thermal shutdown feature that

disables the drivers whenever the die temperature becomes

excessive. This eliminates the power dissipation, allowing

the die to cool. The drivers automatically reenable after the

die temperature drops about 15 degrees. If the contention

persists, the thermal shutdown/reenable cycle repeats until

the fault is cleared. Receivers stay operational during

thermal shutdown.

Low Power Shutdown Mode (ISL8483 Only)

These CMOS transceivers all use a fraction of the power

required by their bipolar counterparts, but the ISL8483

includes a shutdown feature that reduces the already low

whenever the receiver and driver are simultaneously

600ns. Disabling both the driver and the receiver for less

than 50ns guarantees that the ISL8483 will not enter

shutdown.

Note that receiver and driver enable times increase when

the ISL8483 enables from shutdown. Refer to Notes 5-8, at

the end of the Electrical Specification table, for more

information.

FIGURE 6. DRIVER OUTPUT CURRENT vs DIFFERENTIAL

OUTPUT VOLTAGE

FIGURE 7. DRIVER DIFFERENTIAL OUTPUT VOLTAGE vs

TEMPERATURE

DIFFERENTIAL OUTPUT VOLTAGE (V)

01

23

45

0

10

20

30

40

50

60

70

80

90

-40

0

50

85

2

2.2

2.4

2.6

2.8

3

3.2

3.4

3.6

TEMPERATURE (°C)

-25

25

75

ISL8483, ISL8485, ISL8488, ISL8489, ISL8490, ISL8491