Designers of handheld products are constantly challenged to add
features to their products while decreasing their size.
As mobile products - such as cellphones, digital cameras, MP3
players and PDAs - offer more functionality, they feature more I/O
ports through which ESD can enter the system and disrupt or damage the
IC.
Furthermore, with the added features and integration of multiple
functions, IC designs have become more sensitive to ESD. This has left
designers with the challenge of providing the most effective ESD
protection to the IC as possible while minimizing board space and cost
for additional protection devices.
When choosing the appropriate protection device for a circuit, the
designer should consider several factors. The main function of
ESD-protection diodes is to add reliability to a handheld product. The
reliability that a protection part can provide is determined by how
well the part protects a given application without disrupting the
device's functionality over a long period of time.
The most recognized waveform for defining a typical ESD event at
the system level is the IEC61000-4-2 waveform, which is distinguished
by its fast rise time of less than 1ns (Figure
1, below).
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| b>
Figure 1. The most recognized waveform for defining a typical ESD event
at the system level is the IEC61000-4-2 waveform, which is
distinguished by its fast rise time of less than 1 ns. |
When choosing a protection part for a circuit to increase
reliability, the following questions would likely be asked:
1) What are the capacitance
requirement and ESD conditions for the line being protected?
2) What voltage does the
protection device clamp the ESD pulse to?
3) How well will the part
perform over a long period of time?
Device conditions
When determining an appropriate ESD-protection device, the first
question that must be asked is: What is the speed of the line to be
protected?
For high-speed lines such as USB 2.0, low-capacitance diodes are
required to ensure that the integrity of the signal is not compromised.
This article focuses on common applications such as keypad, side
key and power lines that have lower speeds and thus do not require low
capacitance.
Many low-speed applications are exposed to the most severe ESD
conditions. Applications like buttons are exposed to the highest number
of ESD strikes, because they are touched most frequently through normal
use compared to other portable devices' applications.
These applications are also most often exposed to the most
stringent ESD pulses. Additionally, as portable designs get smaller,
buttons get closer to the IC, which allows ESD to get coupled into the
circuit easier.
For this harsh environment, it is crucial to use an ESD-protection
device that offers sufficient clamping characteristics to ensure that
the IC is not damaged.
Due to space constraints, however, many designs don't have very
much room for ESD-protection devices. Due to their small size and
ESD- clamping capabilities, the two most commonly used solutions for
ESD
protection in low-speed lines are varistors and silicon ESD-protection
devices. This article compares the differences between these two types
of devices.
How low?
When determining the best protection device for an application, the
designer must consider how low the ESD-protection device clamps an
incoming ESD event.
The purpose of an ESD-protection device is to reduce an 8kV
IEC61000-4-2 contact input down to a safe voltage for the IC being
protected.
A lower clamping voltage translates to less energy getting through
the IC and less chance of damage to the device being protected. A
screenshot of the voltage waveform over a device with an ESD pulse
input demonstrates the clamping characteristics of the device.
The clamping voltage is the voltage at which the ESD waveform
levels off after the protection device turns on. Note that for some
devices, there is no distinctive level-off voltage.
The area under the voltage waveform, however, is proportional to
the amount of energy that the IC will be exposed to during an ESD
event, so this is the most important factor when comparing two
protection devices.
To compare the clamping characteristics of a silicon ESD protection
device and a varistor, one must choose two parts that are targeted for
equivalent applications. They must have similar package size, working
voltages and a capacitance range suitable for the application.
Currently, one of the most common package outline sizes and the
smallest for a single line of ESD protection is the 0402 size device,
which is approximately 1mm x 0.6mm. Most low-speed lines in portable
applications are DC lines in the range of 0-5V, so they require a
working voltage between 5-6V. For low-speed lines, a capacitance over
50pF is acceptable.
Two parts that meet these requirements are the ESD9X5.0ST5G, and a
varistor from Amotech, the AVLC5S02100. Both come in the 0402 size
outline, are over 50pF capacitance, and have a working voltage between
5-6V.
The best way to compare the performance of these devices is to look
at their clamping characteristics when an IEC61000-4-2 8kV contact
pulse is input to the parts. Figure 2
below shows the response of each part on the same graph for a
positive and negative ESD pulse.
 |
| Figure
2. The silicon solution (blue waveform) offers much lower
clamping voltage for ESD pulses compared with the varistor solution
(black waveform). |
From the screenshot, it is evident that the silicon solution (blue
waveform) offers much lower clamping voltage for ESD pulses compared
with the varistor solution (black waveform).
The silicon device clamps the ESD pulse just above and below the DC
levels (0-5V) for the line at 6.8V for the positive pulse, and 1.6V for
the negative pulse. The varistor device does not have a true clamping
mechanism though.
This technology has more of an absorbing effect, which is evident
from the ESD-pulse response having a gradual decline to a safe level
vs. the clamping effect of the silicon device.
This slow decline allows for a greater area under the curve of the
pulse, translating to more energy that the IC will see. This additional
energy allowed by the varistor will pose a larger risk of damaging the
IC than the silicon device.
Broader view
Most applications using lowspeed lines are exposed to many ESD pulses
on a daily basis during normal use. Because of this, it is important to
select a protection device that will stand up to the many pulses
without compromising the performance of the system.
To avoid disrupting a system's functionality, an ESD device must
not turn on during normal operation, but turn on very quickly when a
destructive ESD pulse is introduced.
To determine if a part is disrupting a system during normal
operation, the device's leakage over multiple ESD pulses should be
measured. To get a broader view of the varistors' performance, a second
varistor company should be examined.
Again, the comparison must include parts designed for equivalent
applications with 0402 size outline, over 50pF capacitance and have a
working voltage between 5-6V.
Two parts that meet these requirements are the ESD9X5.0ST5G, and an
Innochip varistor, the ICVN0505X150.
Figure 3 below shows the
results of a lifetime test, where the leakage measured at Vr
= 5V was recorded over 1,000 ESD pulses per IEC61000-4-2 8kV contact.
 |
|
Figure 3 below shows the results of a lifetime test, where the leakage
measured at Vr = 5V was recorded over 1,000 ESD pulses per IEC61000-4-2
8kV contact. |
Both the varistor and the transient voltage suppression solution
start with low leakage (less than
0.1micro-amperes) before being exposed to any ESD pulses.
Within the first 10 pulses, there is a large spike in the leakage
for the varistor going over 100 micro-amps and then increasing slowly
for every pulse thereafter. This is because the varistor technology is
absorbing more of the ESD pulse that causes it to degrade with every
strike.
As the leakage increases, there is also an increase in the risk of
reliability failures in a system due to normal functionality being
disturbed or battery life suffering. The silicon part, however, clamps
the pulse without absorption and thus does not degrade up to 1,000
pulses, maintaining the low leakage under 0.1 microamps. This low
leakage over numerous strikes offers less chances of a quality issue
over the lifetime of a product.
Both varistors and silicon ESD protection devices reduce the amount
of energy an IC will see from an ESD pulse. As designs become more
sensitive, designers must add protection devices that clamp the ESD
pulses to safer levels.
Silicon ESD-protection devices offer the lowest clamping voltage
compared with varistors and maintain the lowest leakage over many ESD
strikes. For designs with high-reliability requirements, a silicon
ESDprotection device offers the most effective solution.
Lon Robinson is Marketing
Engineer for ESD Protection Diodes at ON
Semiconductor Corp.
To read a PDF version of this story, go to "ESD
protection tips
to improve reliability."
