SP3080E datasheet(18/21 Pages) SIPEX

18 / 21 page

Rev N 9/15/2006

SP3080E Advanced RS485 Transceivers

DESCRIPTION

ESD TEST CONDITIONS

ESD performance depends on a variety of con-

ditions. Contact Sipex for a reliability report

that documents test setup, methodology and

results.

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 SP3080E 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 us-

ing 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

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

sured using the human body model.

The air-gap test involves approaching the

device 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 RS-485 receiver input impedance

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

drive up to 32 unit loads. The SP3080E fam-

ily of transceivers has only a 1/8th unit load re-

ceiver input impedance (96kΩ), thereby allow-

ing eight times as many, up to 256, transceivers

to be connected in parallel on a communication

line. Any combination of these devices and oth-

er RS-485 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

200nA of supply current. DE and RE may be

tied together and driven by a single control sig-

nal. Devices are guaranteed not to enter shut-

down 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 shutdown.

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 tZL(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.

LINE LENGTH, EMI, AND REFLECTIONS

SP3080E feature controlled slew-rate driv-

ers that minimize EMI and reduce reﬂections

caused by improperly terminated cables.

SP3080E driver rise and fall times are limited

to no faster than 667ns, allowing error-free data

transmission up to 115kbps.

The RS-485/RS-422 standard covers line

lengths up to 4,000ft. Maximum achievable line

length is a function of signal attenuation and

noise. Use of slew-controlled drivers such as

the SP3080E may help to reduce crosstalk in-

terference and permit communication over lon-

ger transmission lines.

Termination prevents reﬂections by eliminating

the impedance mismatches on a transmission

line. Line termination is typically used if rise

and fall times are shorter than the round-trip

signal propagation time. Slew-limited drivers

may reduce or eliminate the need for cable ter-

mination in many applications.

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