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 7 and 8 represent
the typical ESD testing circuit used for all
three methods. The C
S
is initially charged
with the DC power supply when the first
30A
5A
0A
t=0ns
t=30ns
t
Figure 9. ESD Test Waveform for IEC1000-4-2
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.
For the Human Body Model, the current
limiting resistor (R
S
) and the source capacitor
(C
S
) are 1.5kΩ an 100pF, respectively. For
IEC-1000-4-2, the current limiting resistor
(R
S
) and the source capacitor (C
S
) are 330Ω
an 150pF, respectively.
The higher C
S
value and lower R
S
value in
the 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.
SP1481E,
SP1485E
FamiLy
HUMAn BodY
ModeL
Air discharge
iec1000-4-2
direct contact
Level
4
4
Driver Outputs ±15kV
Receiver Inputs ±15kV
±15kV
±8kV
±15kV ±8kV
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 50-668-707 • www.exar.com
SP48E-485E_00_06609
8
EXAR [ EXAR CORPORATION ]
