11

FN6886.0

April 3, 2009

ESD Protection

All pins on this device includes class 3 (>7kV) Human Body

Model (HBM) ESD protection structures, but the RS-485

pins (driver outputs and receiver inputs) incorporate

advanced structures allowing them to survive ESD events

in excess of ±16.5kV HBM and ±16.5kV (1/2 duplex)

IEC61000-4-2. The RS-485 pins are particularly vulnerable

to ESD strikes because they typically connect to an

exposed port on the exterior of the finished product. Simply

touching the port pins, or connecting a cable, can cause an

ESD event that might destroy unprotected ICs. These new

ESD structures protect the device whether or not it is

powered up, and without degrading the RS-485 common

mode range of -7V to +12V. This built-in ESD protection

eliminates the need for board level protection structures

(e.g., transient suppression diodes), and the associated,

undesirable capacitive load they present.

IEC61000-4-2 Testing

The IEC61000 test method applies to finished equipment,

rather than to an individual IC. Therefore, the pins most likely

to suffer an ESD event are those that are exposed to the

outside world (the RS-485 pins in this case), and the IC is

tested in its typical application configuration (power applied)

rather than testing each pin-to-pin combination. The

IEC61000 standard’s lower current limiting resistor coupled

with the larger charge storage capacitor yields a test that is

much more severe than the HBM test. The extra ESD

protection built into this device’s RS-485 pins allows the

design of equipment meeting level 4 criteria without the need

for additional board level protection on the RS-485 port.

AIR-GAP DISCHARGE TEST METHOD

For this test method, a charged probe tip moves toward the

IC pin until the voltage arcs to it. The current waveform

delivered to the IC pin depends on approach speed,

humidity, temperature, etc., so it is difficult to obtain

repeatable results. The ISL3158AE 1/2 duplex RS-485 pins

withstand ±16.5kV air-gap discharges.

CONTACT DISCHARGE TEST METHOD

During the contact discharge test, the probe contacts the

tested pin before the probe tip is energized, thereby

eliminating the variables associated with the air-gap

discharge. The result is a more repeatable and predictable

test, but equipment limits prevent testing devices at voltages

higher than ±9kV. The RS-485 pins of all the ISL3158AE

versions survive ±9kV contact discharges.

Data Rate, Cables, and Terminations

RS-485/RS-422 are intended for network lengths up to

4000’, but the maximum system data rate decreases as the

transmission length increases. Devices operating at 10Mbps

are limited to lengths of less than 100’.

Twisted pair is the cable of choice for RS-485/RS-422

networks. Twisted pair cables tend to pick up noise and

other electromagnetically induced voltages as common

mode signals, which are effectively rejected by the

differential receivers in these ICs.

Proper termination is imperative, when using the 10Mbps

devices, to minimize reflections. Terminations are

recommended unless power dissipation is an overriding

concern.

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 specification requires that

drivers survive worst case bus contentions undamaged.

These devices meet this requirement via driver output short

circuit current limit circuitry.

The driver output stages incorporate short circuit current

limiting circuitry which ensures that the output current never

exceeds the RS-485 specification, even at the common

mode voltage range extremes.

Low Power Shutdown Mode

This CMOS transceiver uses a fraction of the power required

by it’s bipolar counterparts, but it also includes a shutdown

trickle. This device enters shutdown whenever the receiver

DE = GND) for a period of at least 600ns. Disabling both the

driver and the receiver for less than 60ns guarantees that

the transceiver will not enter shutdown.

Note that receiver and driver enable times increase when

the transceiver enables from shutdown. Refer to Notes 5, 6,

7, 8 and 9, at the end of the “Electrical Specification” table on

page 6, for more information.

ISL3158AE