SP1486E datasheet(13/15 Pages) SIPEX

13 / 15 page

Rev J 3/8/2007

SP1486E PROFIBUS/RS485 Transceivers

DESCRIPTION

IEC 1000-4-2

The IEC 1000-4-2 standard covers ESD testing

and performance of finished equipment. How-

ever, it does not specifically refer to integrated

circuits. The SP1486E helps you design equip-

ment to meet IEC 1000-4-2, without sacrificing

board-space and cost for external ESD-protec-

tion components.

The major difference between tests done using

the Human Body Model and IEC 1000-4-2 is a

higher peak current in IEC 1000-4-2. Series

resistance is lower in the IEC 1000-4-2 model.

Hence, the ESD withstand voltage measured to

EC 1000-4-2 is generally lower than that mea-

sured using the human body model.

The air-gap test involves approaching the de-

vice with a charged probe. The contact dis-

charge method connects the probe to the de-

vice before the probe is energized.

MACHINE MODEL

The machine model for ESD tests all pins using

a 200pF storage capacitor and zero discharge

resistance. The objective is to emulate the

stress caused when I/O pins are contacted by

handling equipment during test and assembly.

256 TRANSCEIVERS ON THE BUS

The standard RS485 receiver input impedance

is 12kΩ (1 unit load). A standard driver can

drive up to 32 unit loads. The SP1486E has

only a 1/8th unit load receiver input impedance

(96kΩ), thereby allowing eight times as many,

up to 256, transceivers to be connected in par-

allel on a communication line. Any combination

of these devices and other RS485 transceivers

up to a total of 32 unit loads may be connected

to the line.

LOW POWER SHUTDOWN MODE

Low-power shutdown mode is initiated by bring-

ing both RE high and DE low simultaneously.

While in shutdown devices typically draw only

50nA of supply current. DE and RE may be tied

together and driven by a single control signal.

Devices are guaranteed not to enter shutdown

if RE is high and DE is low for less than 50ns. If

the inputs are in this state for at least 600ns, the

parts are shut-down.

Enable times tzh and tzl apply when the part is

not in low-power shutdown state. Enable times

tzh(shdn) and tzl(shdn) apply when the parts

are shut down. The drivers and receivers take

longer to become enabled from low-power shut-

down mode tzh(shdn) and tzl(shdn) than from

driver/receiver-disable mode (tzh, tzl).

DRIVER OUTPUT PROTECTION

Two mechanisms prevent excessive output cur-

rent and power dissipation caused by faults or

by bus contention. First, a driver-current limit

on the output stage provides immediate protec-

tion against short circuits over the whole com-

mon-mode voltage range. Second, a thermal-

shutdown circuit forces the driver outputs into

a high-impedance state if junction temperature

becomes excessive.

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