Tips on reducing electrostatic discharge and improving system reliability - Embedded.com

Tips on reducing electrostatic discharge and improving system reliability

Designers of handheld products are constantly challenged to addfeatures to their products while decreasing their size.

As mobile products – such as cellphones, digital cameras, MP3players and PDAs – offer more functionality, they feature more I/Oports through which ESD can enter the system and disrupt or damage theIC.

Furthermore, with the added features and integration of multiplefunctions, IC designs have become more sensitive to ESD. This has leftdesigners with the challenge of providing the most effective ESDprotection to the IC as possible while minimizing board space and costfor additional protection devices.

When choosing the appropriate protection device for a circuit, thedesigner should consider several factors. The main function ofESD-protection diodes is to add reliability to a handheld product. Thereliability that a protection part can provide is determined by howwell the part protects a given application without disrupting thedevice's functionality over a long period of time.

The most recognized waveform for defining a typical ESD event atthe system level is the IEC61000-4-2 waveform, which is distinguishedby its fast rise time of less than 1ns (Figure1, below ).

b>Figure 1. The most recognized waveform for defining a typical ESD eventat the system level is the IEC61000-4-2 waveform, which isdistinguished by its fast rise time of less than 1 ns.

When choosing a protection part for a circuit to increasereliability, the following questions would likely be asked:

1) What are the capacitancerequirement and ESD conditions for the line being protected?

2) What voltage does theprotection device clamp the ESD pulse to?

3) How well will the partperform over a long period of time?

Device conditions
When determining an appropriate ESD-protection device, the firstquestion that must be asked is: What is the speed of the line to beprotected?

For high-speed lines such as USB 2.0, low-capacitance diodes arerequired to ensure that the integrity of the signal is not compromised.

This article focuses on common applications such as keypad, sidekey and power lines that have lower speeds and thus do not require lowcapacitance.

Many low-speed applications are exposed to the most severe ESDconditions. Applications like buttons are exposed to the highest numberof ESD strikes, because they are touched most frequently through normaluse compared to other portable devices' applications.

These applications are also most often exposed to the moststringent ESD pulses. Additionally, as portable designs get smaller,buttons get closer to the IC, which allows ESD to get coupled into thecircuit easier.

For this harsh environment, it is crucial to use an ESD-protectiondevice that offers sufficient clamping characteristics to ensure thatthe IC is not damaged.

Due to space constraints, however, many designs don't have verymuch room for ESD-protection devices. Due to their small size andESD- clamping capabilities, the two most commonly used solutions forESDprotection in low-speed lines are varistors and silicon ESD-protectiondevices. This article compares the differences between these two typesof devices.

How low?
When determining the best protection device for an application, thedesigner must consider how low the ESD-protection device clamps anincoming ESD event.

The purpose of an ESD-protection device is to reduce an 8kVIEC61000-4-2 contact input down to a safe voltage for the IC beingprotected.

A lower clamping voltage translates to less energy getting throughthe IC and less chance of damage to the device being protected. Ascreenshot of the voltage waveform over a device with an ESD pulseinput demonstrates the clamping characteristics of the device.

The clamping voltage is the voltage at which the ESD waveformlevels off after the protection device turns on. Note that for somedevices, there is no distinctive level-off voltage.

The area under the voltage waveform, however, is proportional tothe amount of energy that the IC will be exposed to during an ESDevent, so this is the most important factor when comparing twoprotection devices.

To compare the clamping characteristics of a silicon ESD protectiondevice and a varistor, one must choose two parts that are targeted forequivalent applications. They must have similar package size, workingvoltages and a capacitance range suitable for the application.

Currently, one of the most common package outline sizes and thesmallest for a single line of ESD protection is the 0402 size device,which is approximately 1mm x 0.6mm. Most low-speed lines in portableapplications are DC lines in the range of 0-5V, so they require aworking voltage between 5-6V. For low-speed lines, a capacitance over50pF is acceptable.

Two parts that meet these requirements are the ESD9X5.0ST5G, and avaristor from Amotech, the AVLC5S02100. Both come in the 0402 sizeoutline, are over 50pF capacitance, and have a working voltage between5-6V.

The best way to compare the performance of these devices is to lookat their clamping characteristics when an IEC61000-4-2 8kV contactpulse is input to the parts. Figure 2below shows the response of each part on the same graph for apositive and negative ESD pulse.

Figure2. The silicon solution (blue waveform) offers much lowerclamping voltage for ESD pulses compared with the varistor solution(black waveform).

From the screenshot, it is evident that the silicon solution (bluewaveform) offers much lower clamping voltage for ESD pulses comparedwith the varistor solution (black waveform).

The silicon device clamps the ESD pulse just above and below the DClevels (0-5V) for the line at 6.8V for the positive pulse, and 1.6V forthe negative pulse. The varistor device does not have a true clampingmechanism though.

This technology has more of an absorbing effect, which is evidentfrom the ESD-pulse response having a gradual decline to a safe levelvs. the clamping effect of the silicon device.

This slow decline allows for a greater area under the curve of thepulse, translating to more energy that the IC will see. This additionalenergy allowed by the varistor will pose a larger risk of damaging theIC than the silicon device.

Broader view
Most applications using lowspeed lines are exposed to many ESD pulseson a daily basis during normal use. Because of this, it is important toselect a protection device that will stand up to the many pulseswithout compromising the performance of the system.

To avoid disrupting a system's functionality, an ESD device mustnot turn on during normal operation, but turn on very quickly when adestructive ESD pulse is introduced.

To determine if a part is disrupting a system during normaloperation, the device's leakage over multiple ESD pulses should bemeasured. To get a broader view of the varistors' performance, a secondvaristor company should be examined.

Again, the comparison must include parts designed for equivalentapplications with 0402 size outline, over 50pF capacitance and have aworking voltage between 5-6V.

Two parts that meet these requirements are the ESD9X5.0ST5G, and anInnochip varistor, the ICVN0505X150.

Figure 3 below shows theresults 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 leakagemeasured at Vr = 5V was recorded over 1,000 ESD pulses per IEC61000-4-28kV contact.

Both the varistor and the transient voltage suppression solutionstart with low leakage (less than0.1micro-amperes ) before being exposed to any ESD pulses.

Within the first 10 pulses, there is a large spike in the leakagefor the varistor going over 100 micro-amps and then increasing slowlyfor every pulse thereafter. This is because the varistor technology isabsorbing more of the ESD pulse that causes it to degrade with everystrike.

As the leakage increases, there is also an increase in the risk ofreliability failures in a system due to normal functionality beingdisturbed or battery life suffering. The silicon part, however, clampsthe pulse without absorption and thus does not degrade up to 1,000pulses, maintaining the low leakage under 0.1 microamps. This lowleakage over numerous strikes offers less chances of a quality issueover the lifetime of a product.

Both varistors and silicon ESD protection devices reduce the amountof energy an IC will see from an ESD pulse. As designs become moresensitive, designers must add protection devices that clamp the ESDpulses to safer levels.

Silicon ESD-protection devices offer the lowest clamping voltagecompared with varistors and maintain the lowest leakage over many ESDstrikes. For designs with high-reliability requirements, a siliconESDprotection device offers the most effective solution.

Lon Robinson is MarketingEngineer for ESD Protection Diodes at ONSemiconductor Corp.

To read a PDF version of this story, go to “ESDprotection tipsto improve reliability.”

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