May11-2007

SP481E Low Power Half-Duplex RS485 Transceivers

© 2007 Sipex Corporation

8

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 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 7 and 8 represent

the typical ESD testing circuit used for all three

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

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

1.5k

Ω an 100pF, respectively. For IEC-1000-4-

2, the current limiting resistor (R

capacitor (C

The higher C

lower R

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.

Figure 9. ESD Test Waveform for IEC1000-4-2

t=0ns

t=30ns

0A

15A

30A

t

➙

HUMAN BODY

IEC1000-4-2

MODEL

Air Discharge

Direct Contact

Level

Driver Outputs

±15kV

±15kV

±8kV

4

Receiver Inputs

±15kV

±15kV

±8kV

4

SP481E, SP485E

FAMILY