MT-092
TUTORIAL
Electrostatic Discharge (ESD)
OUT-OF-CIRCUIT OVERVOLTAGE PROTECTION FROM ESD
Linear ICs such as op amps, in-amps, and data converters be protected prior to the time that they
are mounted to a printed circuit board. That is an out-of-circuit state. In such a condition, ICs are
completely at the mercy of their environment as to what stressful voltage surges they may see.
Most often the harmful voltage surges come from electrostatic discharge, or, as more commonly
referenced, ESD. This is a single, fast, high current transfer of electrostatic charge resulting from
one of two conditions. These conditions are:
1. Direct contact transfer between two objects at different potentials (sometimes called
contact discharge)
2. A high electrostatic field between two objects when they are in close proximity
(sometimes called air discharge)
The prime sources of static electricity are mostly insulators and are typically synthetic materials,
e.g., vinyl or plastic work surfaces, insulated shoes, finished wood chairs, Scotch tape, bubble
pack, soldering irons with ungrounded tips, etc. Voltage levels generated by these sources can be
extremely high since their charge is not readily distributed over their surfaces or conducted to
other objects. The generation of static electricity caused by rubbing two substances together is
called the triboelectric effect. Some common examples of ordinary acts producing significant
ESD voltages are shown in Figure 1.
Walking Across a Carpet
1000V - 1500V
Walking Across a Vinyl Floor
150V - 250V
Handling Material Protected by Clear Plastic Covers
400V - 600V
Handling Polyethylene Bags
1000V - 2000V
Pouring Polyurethane Foam Into a Box
1200V - 1500V
Note: Above Assumes 60% RH. For Low RH (30%),
Voltages Can Be > 10 Times
Figure 1: ESD Voltages Generated By Various Ordinary Circumstances
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MT-092
ICs can be damaged by the high voltages and high peak currents generated by ESD.
Precision analog circuits, often featuring very low bias currents, are more susceptible to damage
than common digital circuits, because traditional input-protection structures which protect
against ESD damage increase input leakage—and thus can't be used.
For the design engineer or technician, the most common manifestation of ESD damage is a
catastrophic failure of the IC. However, exposure to ESD can also cause increased leakage or
degrade other parameters. If a device appears to not meet a data sheet specification during
evaluation, the possibility of ESD damage should be considered. Figure 2 outlines some relevant
points on ESD induced failures.
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ESD Failure Mechani sms:
Dielectric or junction damage
Surface charge accumulation
Conductor fusing
ESD Damage Can Cause:
Increased leakage
Degradation in performance
Functional failures of ICs
ESD Damage is often Cumulative:
For example, each ESD "zap" may increase
junction damage until, finally, the device fails.
Figure 2: Understanding ESD damage
All ESD-sensitive devices are shipped in protective packaging. ICs are usually contained in
either conductive foam or antistatic shipping tubes, and the container is then sealed in a static-
dissipative plastic bag. The sealed bag is marked with a distinctive sticker, such as in Figure 3,
which outlines the appropriate handling procedures.
The presence of outside package notices such as those shown in Figure 3 is notice to the user that
device handling procedures appropriate for ESD protection are necessary.
In addition, data sheets for ESD-sensitive ICs generally have a bold statement to that effect, as
shown in Figure 4.
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