SP3222EB / SP3232EB True +3.0V to +5.5V RS-232 Transceivers Theory of Operation
1/24/20 Rev 1.0.7 10
ESD Tolerance
The SP3222EB / SP3232EB Series 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 18.
This method will test the IC’s capability to withstand an
ESD transient during normal handling such as in
manufacturing areas where the IC’s tend to be handled
frequently.
Figure 18: ESD Test Circuit for Human Body Model
The IEC61000-4-2, formerly IEC801-2, is generally used
for testing ESD on equipment and systems. System
manufacturers must guarantee a certain amount of ESD
protection since the system itself is exposed to the outside
environment and human presence. 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 19. There are two
methods within IEC61000-4-2, the Air Discharge method
and the Contact Discharge method.
Figure 19: ESD Test Circuit for IEC61000-4-2
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 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 Figure 18 and Figure 19 represent the
typical ESD testing circuit used for all three methods. The
CS 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 RS,
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.
R
C
Device
Under
Test
DC Power
Source
C
S
R
S
SW1 SW2
RS and
RV add up to 330Ω for IEC61000-4-2.
R
C
Device
Under
Test
DC Power
Source
C
S
R
S
SW1 SW2
R
V
Contact-Discharge Model