sp488ect Exar Corporation, sp488ect Datasheet - Page 7

• Manufacturer Part Number: sp488ect
• Description: Enhanced Quad Rs-485/rs-422 Line Receivers
• Manufacturer: Exar Corporation

SP488E/489EDS/07
environment and human presence. The premise
with IEC1000-4-2 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
IEC1000-4-2 is shown on Figure 6. There are
two methods within IEC1000-4-2, the Air
Discharge method and the Contact Discharge
method.
Figure 7. ESD Test Waveform for IEC1000-4-2
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
Table 3. Transceiver ESD Tolerance Levels
15A
30A
0A
Driver Outputs
Receiver Inputs
DEVICE PIN
TESTED
t=0ns
t
HUMAN BODY
SP488E/489E Enhanced Quad RS-485/RS-422 Line Receivers
+15kV
+15kV
MODEL
t=30ns
Air Discharge
7
+15kV
+15kV
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 situations 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 5 and 6 represent
the typical ESD testing circuit used for all three
methods. The C
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
device under test (DUT). In ESD tests, the SW2
switch is pulsed so that the device under test
receives a duration of voltage.
For the Human Body Model, the current limiting
resistor (R
1.5kW an 100pF, respectively. For IEC-1000-4-2,
the current limiting resistor (R
capacitor (C
The higher C
IEC1000-4-2 model are more stringent than the
Human Body Model. The larger storage capacitor
injects a higher voltage to the test point when
SW2 is switched on. The lower current limiting
resistor increases the current charge onto the test
point.
S
, the current limiting resistor, onto the
S
Direct Contact
) and the source capacitor (C
S
IEC1000-4-2
) are 330W an 150pF, respectively.
S
value and lower R
S
+8kV
+8kV
is initially charged with the DC
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S
) and the source
S
value in the
Level
4
4
S
) are