Login
Register

ENGLISH简体中文
(Chinese)日本語
(Japanese)РУССКИЙ
(Russian)

Products

Solutions

Design

Buy

Support

About Us

My Maxim

Maxim > Design
support > App notes
> A/D and
D/A Conversion/Sampling Circuits > APP
4991
Maxim > Design support >
App notes
> Amplifier
and Comparator Circuits > APP 4991
Maxim > Design support >
App notes
> Circuit
Protection > APP 4991

Mar 02, 2011

Keywords: iInitial accuracy, digital pots,
potentiometer, resistance, voltage dividers, ratiometric,
ratio steps, LSB, taps error, reference decoupling, Earth
Ground, high push-button interface, common space shuttle,
local potential
Related
parts

Download,
PDF format (42kB)

[?]

APPLICATION NOTE 4991

Oops...Practical ESD Protection vs. Foolhardy
Placebos

By:

Bill
Laumeister, Strategic Applications Engineer

Abstract: Electrical
overstress (EOS) and electrostatic discharge (ESD) are
dangerous to integrated circuits. Foolish humans make excuses,
employ placebos, and hope that the laws of physics will not
apply to them. This article exposes the folly of that thinking
and suggests practical ways to protect electronic systems.

Introduction
Electrical overstress (EOS) and electrostatic
discharge (ESD) present real dangers to electronic
circuits. In fact, EOS and ESD are the top causes of
preventable IC failures. Some people discount the seriousness
of the damage that EOS and ESD can cause. We have heard many
excuses about why it is not necessary to take proper
precautions:

"I don't believe in all that hocus pocus (i.e., sleight
of hand)."

"I am an engineer, have been doing this years without
problems."

"The humidity is high here."

"We already built ESD protection into the printed
circuit board (PCB)."

"All ICs have built-in ESD protection."

"Statistically the odds are that nothing will happen."

This application note exposes the folly of this
misguided thinking and suggests practical ways to protect
electronic systems.

The Laws of Physics and EOS/ESD
We begin by revisiting
these familiar excuses for the absence of proper EOS and ESD
protection.

Unfortunately, a belief—even a firm belief—usually has
little or nothing to do with the laws of physics. Placebos
can work in medicine because the human brain has more
control than we expect. All the believing that gravity would
stop, does not keep us from stumbling and falling. However
one thinks about "hocus pocus," when it comes to
engineering, the laws of physics will always apply.

The laws of physics exist regardless of a person's
occupation. Consequently, if a part does not instantly burn
up in a cloud of smoke, it does not mean that there was no
EOS or ESD present. Nor does apparent functionality mean
that the parts were not damaged and will not die
later.

High humidity does, indeed, reduce the ESD voltage.
However, remember that the building's air-conditioning
system is removing humidity to make us comfortable. It is
simply quite true that humans are not very good at guessing
relative humidity levels.

Board ESD protection is like air bags in our cars—they
are there for accidents not for daily use. Built-in ESD
protection reduces the chances of damage but does not
totally remove the possibility for injury.

While most ICs integrate ESD protection, the ESD
structures are made of tiny components. Again they are for
accidents not daily use.

"Statistically, it will not happen to me." If it is true
that statistically most automobile accidents occur within 25
miles of home, will there be no accidents farther away?
Remember that numbers do not lie and will not subvert the
laws of physics

Credible ESD Protection Obeys the Laws of Physics
A
normal ESD wrist strap is a conductive band
with a series (current-limiting) resistor connected through a
wire and alligator clip to a ground or reference point. Most
companies test the effectiveness of this ESD protection
several times a day. The person wearing the wrist band goes to
the ESD test station and attaches the alligator clip to the
ground terminal. While the person touches a finger to another
test metal plate, the machine adds an electrostatic potential
to the person's body. If the voltage rises, the wrist strap
has failed; if the ground effectively keeps the voltage down,
the strap is good for continued use.

The proven process
follows the laws of physics and is reliable. But still there
are sources that subvert the process and convert the effective
ESD wrist strap into an ESD placebo. This is the ESD hocus
pocus in item A above. For example, take the same wrist strap
above and do not connect the alligator clip to the ESD tester
ground. Now when the metal plate is touched with a finger, the
voltage rises and the wrist strap fails. For many years there
have been companies that sell wrist bands without ground wires
or claim that the ground wire is optional. In 2005 the ESD Journal challenged the wireless
wristband manufacturers to performance tests: send in a
wireless wrist band for evaluation. If it works, the ESD
Journal will apologize and give the manufacturer a free
advertisement for a year. They had no response to the
challenge. The journal continues to claim, "Sorry! Something
that sounds too good to be true probably is." Meanwhile, ESD
Systems was asked if wireless ESD straps work? Their short
answer is "no." (www.esdsystems.com/QuestionsAndAnswers/ShowQuestion.aspx?i=17)

This
wireless wrist strap sham sounds foolish, even like a joke,
but it is not. Several years ago a friend at another semiconductor
company related a story. They had been buying product from a
large offshore manufacturer for years, but suddenly something
changed. They were finding ESD damage in the imported parts.
When questioned, the offshore source
said that nothing had changed and that they had stringent ESD
safety precautions in place. Finally the semiconductor company
flew an ESD expert to the manufacturer's location. Immediately
upon walking into the plant, the expert saw wireless wrist
straps everywhere. The ESD problem was resolved by replacing
the grounding wires and the number of ESD-damaged parts
plummeted.

EOS/ESD Protection in an IC
How do we protect
electronics from ESD and EOS?

The ESD structures inside
an IC are designed to protect the part before the customer
mounts it on his products PC
board. Because the ICs are limited in size, they cannot
withstand system ESD events. At the board level, however,
inputs and outputs can have more robust ESD structures. These
external discrete components tend to be physically larger and
carry larger currents than the ICs' internal ESD
protection.

PC boards are protected from ESD by a
combination of discrete silicon (small signal or reference),
Schottky diodes, avalanche (zener) diodes, transient voltage
suppression (TVS)
diodes, gas-tube discharge devices, resistors, inductors, and
metal oxide varistors (MOVs). Series resistors and inductors
along with capacitors to ground can act as a lowpass filter.
The rise time of a spark is very fast, so any way that we can
slow it down will reduce the peak voltage.

Figure
1. Typical internal IC ESD structures.

The ESD
structures of Figure 1 are internal to the ICs. Figure
1C is the most common design with two diodes used on input and
output signal pins. Most IC data sheets will specify that a
pin like this will have voltage maximums from -0.3V, if ground
is the lowest voltage, to VCC (power supply) plus
0.3V. These are silicon diodes with a forward voltage drop of
0.6V, so why does the data sheet say 0.3V? The answer is
because at high temperatures the diode
starts leaking significant current.
The operation of diode to ground is generally obvious. The
diode to VCC requires that the power be low impedance
to be effective. Unless the power supply is very close, the
low impedance is handled by the decoupling or bypass
capacitors.

In Figure 1B the power pin does not have a
higher voltage to connect, so a zener diode will protect this
pin. The zener on Figure 1A is seen on pins with a hidden use.
For example, a part, especially the digital part of the IC,
may have special test or trim modes for automatic factory
test. To enter this mode the pin can be pulled a diode drop
higher than VCC. After the test, power is removed
from the IC to exit the test mode.

In addition to the
ESD protection build into most Maxim products, Maxim makes
specialized ESD-protection and other fault-protected devices
(www.maxim-ic.com/products/protection/) which
include ESD protection diodes, analog switches and
multiplexers, switch
debouncer/protectors, overvoltage protection (OVP)
controllers, signal-line protectors, lithium-ion
(Li+) battery protectors, and current-limit and USB
power switches.

Reference
Information
Maxim application notes discuss ESD, EOS,
and other protection methods:
Application note 639, "Maxim Leads the Way in
ESD Protection"
Application note 651, "ESD Protection for I/O
Ports"
Application note 4032, "HFTA-16.0: ESD
Protection for Bipolar Integrated Circuits"
Application
note 1167, "Practical
aspects of EMI protection"
Application note 4035, "Overvoltage Protection
(OVP) for Sensitive Amplifier
Applications"

Application note 4353, "Robust Contact Monitor
Simplifies Design of Body Control Computers," discusses
the MAX13362, a
24-channel automotive switch monitor. The device interfaces to
remote switches in a vehicle and is directly subjected to all
the transients observed in the wiring harness of an
automobile. The MAX13362 can protect against lost ground/power
connections, reverse-battery condition, and ground and
power-supply shifts.

Application note 4240, "Active High-Voltage
Transient Protectors Trump Conventional Approaches in
Automotive Electronics," contrasts active and passive
protection methods.

Unfortunately, human nature and
common sense do not always coincide. But ESD and EOS always
obey the rules of physics, no matter how hard we wish it were
otherwise sometimes.

Links for More ESD
Information
ESD Journal has many fine articles
at: www.esdjournal.com/

The ESD
Association is a professional voluntary association dedicated
to advancing the theory and practice of electrostatic
discharge. (www.esda.org/index.htm and www.esda.org/esd_fundamentals.html).

The
Silicon Valley EOS/ESD Society discusses electrical overstress
(EOS) and electrostatic discharge (ESD) at: www.esdiscovery.org/

Related parts

MAX13362

24-Channel Automotive
Switch Monitor

Free
samples

MAX1480E

±15kV ESD-Protected,
Isolated RS-485/RS-422 Data Interfaces

Free
samples

MAX1488E

±15kV ESD-Protected,
Quad, Low-Power RS-232 Line Driver

Free
samples

MAX1490E

±15kV ESD-Protected,
Isolated RS-485/RS-422 Data Interfaces

Free
samples

MAX16013

Ultra-Small,
Overvoltage Protection/Detection Circuits

Free
samples

MAX16014

Ultra-Small,
Overvoltage Protection/Detection Circuits

Free
samples

MAX253

1W Primary-Side
Transformer H-Bridge Driver for Isolated
Supplies

Free
samples

MAX3080

Fail-Safe, High-Speed
(10Mbps), Slew-Rate-Limited RS-485/RS-422
Transceivers

Free
samples

MAX3083

Fail-Safe, High-Speed
(10Mbps), Slew-Rate-Limited RS-485/RS-422
Transceivers

Free
samples

MAX3088

Fail-Safe, High-Speed
(10Mbps), Slew-Rate-Limited RS-485/RS-422
Transceivers

Free
samples

MAX3188

1Mbps, 1µA RS-232
Transmitters in SOT23-6

MAX3190E

±15kV ESD-Protected,
460kbps, RS-232 Transmitters in SOT23-6

Free
samples

MAX321

Precision, Dual Supply,
SPST, Analog CMOS Switches

Free
samples

MAX3223E

±15kV ESD-Protected,
1µA, 3.0V to 5.5V, 250kbps, RS-232 Transceivers
with AutoShutdown

Free
samples

MAX3225E

±15kV ESD-Protected,
1µA, 1Mbps, 3.0V to 5.5V, RS-232 Transceivers with
AutoShutdown Plus

Free
samples

MAX3226

1µA Supply Current,
1Mbps, 3.0V to 5.5V, RS-232 Transceivers with
AutoShutdown Plus

Free
samples

MAX3228

+2.5V to +5.5V RS-232
Tranceivers in UCSP

MAX3244E

±15kV ESD-Protected,
1µA, 1Mbps, 3.0V to 5.5V, RS-232 Transceivers with
AutoShutdown Plus

Free
samples

MAX3245E

±15kV ESD-Protected,
1µA, 1Mbps, 3.0V to 5.5V, RS-232 Transceivers with
AutoShutdown Plus

Free
samples

MAX3311E

±15kV ESD-Protected,
460kbps, 1µA, RS-232-Compatible Transceivers in
µMAX

Free
samples

MAX3313E

±15kV ESD-Protected,
460kbps, 1µA, RS-232-Compatible Transceivers in
µMAX

Free
samples

MAX3387E

3V, ±15kV
ESD-Protected, AutoShutdown Plus RS-232
Transceiver for PDAs and Cell Phones

Free
samples

MAX3388E

2.5V, ±15kV
ESD-Protected RS-232 Transceivers for PDAs and
Cell Phones

Free
samples

MAX3443E

±15kV ESD-Protected,
±60V Fault-Protected, 10Mbps, Fail-Safe
RS-485/J1708 Transceivers

Free
samples

MAX3483E

3.3V Powered, ±15kV
ESD-Protected, 12Mbps, Slew-Rate-Limited True
RS-485/RS-422 Transceivers

Free
samples

MAX3485

3.3V Powered, 10Mbps
and Slew-Rate Limited, True RS-485/RS-422
Transceivers

Free
samples

MAX3490

3.3V Powered, 10Mbps
and Slew-Rate Limited, True RS-485/RS-422
Transceivers

Free
samples

MAX4506

Fault-Protected,
High-Voltage Signal-Line Protectors

Free
samples

MAX4551

±15kV ESD-Protected,
Quad, Low-Voltage, SPST Analog Switches

Free
samples

MAX4558

±15kV ESD-Protected,
Low-Voltage, CMOS Analog Multiplexers Switches

Free
samples

MAX4568

±15kV ESD-Protected,
Low-Voltage, SPDT/SPST, CMOS Analog Switches

Free
samples

MAX4575

±15kV ESD-Protected,
Low-Voltage, Dual, SPST, CMOS Analog Switches

Free
samples

MAX4630

±15kV ESD-Protected,
Low-Voltage, Quad, SPST, CMOS Analog Switches

Free
samples

MAX4640

±15kV ESD-Protected,
Low-Voltage, Quad, SPST, CMOS Analog Switches

Free
samples

MAX483E

±15kV ESD-Protected,
Slew-Rate-Limited, Low-Power, RS-485/RS-422
Transceivers

Free
samples

MAX485E

±15kV ESD-Protected,
Slew-Rate-Limited, Low-Power, RS-485/RS-422
Transceivers

Free
samples

MAX487E

±15kV ESD-Protected,
Slew-Rate-Limited, Low-Power, RS-485/RS-422
Transceivers

Free
samples

MAX488

Low-Power,
Slew-Rate-Limited RS-485/RS-422 Transceivers

Free
samples

MAX491E

±15kV ESD-Protected,
Slew-Rate-Limited, Low-Power, RS-485/RS-422
Transceivers

Free
samples

MAX6397

Overvoltage Protection
Switch/Limiter Controllers Operate Up to 72V

Free
samples

MAX6398

Overvoltage Protection
Switch/Limiter Controllers Operate Up to 72V

Free
samples

MAX6399

High-Voltage,
Overvoltage/Undervoltage, Protection Switch
Controller

Free
samples

MAX6495

72V,
Overvoltage-Protection Switches/Limiter
Controllers with an External MOSFET

Free
samples

MAX6496

72V,
Overvoltage-Protection Switches/Limiter
Controllers with an External MOSFET

Free
samples

MAX6497

72V,
Overvoltage-Protection Switches/Limiter
Controllers with an External MOSFET

Free
samples

MAX6498

72V,
Overvoltage-Protection Switches/Limiter
Controllers with an External MOSFET

MAX6499

72V,
Overvoltage-Protection Switches/Limiter
Controllers with an External MOSFET

Free
samples

Automatic updates
Would you like to be
automatically notified when new application notes are
published in your areas of interest? Sign
up for EE-Mail™.

Download,
PDF format (42kB)
APP 4991: Mar 02, 2011

AN4991, AN 4991, APP4991,
Appnote4991, Appnote 4991

|

|

|     Privacy Policy
|     Legal Notices

Copyright © 2011 by Maxim Integrated
Products