SP490E Datasheet, PDF (8/15 Pages) Sipex Corporation – Enhanced Full Duplex RS-485 Transceivers

ESD Tolerance
The SP490E/SP491E devices incorporates
ruggedized ESD cells on all driver output
and receiver input pins. The ESD structure
is improved over our previous family for
more rugged applications and environments
sensitive to electro-static discharges and
associated transients. The improved ESD
tolerance is at least ±15kV without damage
nor latch-up.
There are different methods of ESD testing
applied:
a) MIL-STD-883, Method 3015.7
b) IEC61000-4-2 Air-Discharge
c) IEC61000-4-2 Direct Contact
The Human Body Model has been the
generally accepted ESD testing method
for semiconductors. This method is also
specified in MIL-STD-883, Method 3015.7
for ESD testing. The premise of this ESD test
is to simulate the human body’s potential to
store electro-static energy and discharge it
to an integrated circuit. The simulation is
performed by using a test model as shown
in Figure 9. This method will test the IC’s
capability to withstand an ESD transient
during normal handling such as in manu-
facturing areas where the ICs tend to be
handled frequently.
The IEC-61000-4-2, formerly IEC801-2, is
generally used for testing ESD on equipment
and systems. For system manufacturers,
they must guarantee a certain amount of ESD
protection since the system itself is exposed
to the outside environment and human pres-
ence. The premise with IEC61000-4-2 is that
the system is required to withstand an amount
of static electricity when ESD is applied to
points and surfaces of the equipment that
are accessible to personnel during normal
usage. The transceiver IC receives most
of the ESD current when the ESD source is
applied to the connector pins. The test circuit
for IEC61000-4-2 is shown on Figure 10.
There are two methods within IEC61000-4-2,
the Air Discharge method and the Contact
Discharge method.
With the Air Discharge Method, an ESD
voltage is applied to the equipment under
test (EUT) through air. This simulates an
electrically charged person ready to connect
a cable onto the rear of the system only to
find an unpleasant zap just before the person
touches the back panel. The high energy
potential on the person discharges through
an arcing path to the rear panel of the system
before he or she even touches the system.
This energy, whether discharged directly or
through air, is predominantly a function of the
discharge current rather than the discharge
voltage. Variables with an air discharge such
as approach speed of the object carrying the
ESD potential to the system and humidity
will tend to change the discharge current.
For example, the rise time of the discharge
current varies with the approach speed.
The Contact Discharge Method applies the
ESD current directly to the EUT. This method
was devised to reduce the unpredictability
of the ESD arc. The discharge current rise
time is constant since the energy is directly
transferred without the air-gap arc. In situ-
ations such as hand held systems, the ESD
charge can be directly discharged to the
equipment from a person already holding
the equipment. The current is transferred
on to the keypad or the serial port of the
equipment directly and then travels through
the PCB and finally to the IC.
The circuit model in Figures 9 and 10 repre-
sent the typical ESD testing circuit used for
all three methods. The C_S is initially charged
with the DC power supply when the first
switch (SW1) is on. Now that the capacitor
is charged, the second switch (SW2) is on
while SW1 switches off. The voltage stored
in the capacitor is then applied through R_S,
the current limiting resistor, onto the device
under test (DUT). In ESD tests, the SW2
switch is pulsed so that the device under
test receives a duration of voltage.
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • [phone] • www.exar.com

SP490E,491E_100_052711