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 circuit models in Figures 9 and 10 represent
the typical ESD testing circuits used for all three
methods. TheCS isinitiallychargedwiththeDC
power supply when the first switch (SW1) is on.
Now that the capacitor is charged, the second
switch(SW2)isonwhileSW1switchesoff. The
voltage stored in the capacitor is then applied
throughRS, thecurrentlimitingresistor, ontothe
device under test (DUT). In ESD tests, the SW2
switch is pulsed so that the device under test
receives a duration of voltage.
Forthe HumanBodyModel, the current limiting
resistor (RS) and the source capacitor (CS) are
1.5kΩ an 100pF, respectively. For IEC-1000-4-
2,thecurrentlimitingresistor(RS)andthesource
capacitor (CS) are 330Ω an 150pF, respectively.
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
handheldsystems,theESDchargecanbedirectly
dischargedtotheequipmentfromapersonalready
holdingtheequipment. Thecurrentistransferred
ontothekeypadortheserialportoftheequipment
directly and then travels through the PCB and
finally to the IC.
The higher CS value and lower RS value in the
IEC1000-4-2 model are more stringent than the
HumanBodyModel. Thelargerstoragecapacitor
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.
SP490E/SP491E HUMAN BODY
IEC1000-4-2
Family
MODEL
Air Discharge
Direct Contact
Level
Driver Outputs
Receiver Inputs
±15kV
±15kV
±15kV
±15kV
±8kV
±8kV
4
4
Table 1. Transceiver ESD Tolerance Levels
SP490EDS/14
SP490E Enhanced Full Duplex RS-485 Transceivers
© Copyright 2000 Sipex Corporation
9
SIPEX [ SIPEX CORPORATION ]
