February 2014 / Volume 4, Issue 2

In this issue:Markets & Trends:

Planning ahead•

Mixed Signal & Analogue:Mixing it up•

EnergyCharging made easy•Wireless charging is on the rise•Securing smart energy•

LEDsChanging the rules for LEDs•Caught in the headlights•It’s a function of the junction•Driving automotive LED forward lighting•systemsPut the investment in to get the heat out•Lighting up the streets•

Consumer:Paying for the right protection•

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Win a PIC MicrostickII development

platform

Securing Smart Energy

MMany consumers are oblivious to thefact that they, themselves, pose thesingle biggest risk to the electronic

devices they use everyday. Circuit protection isa feature of all electronic devices — whether incars, at home, or at work — because wheneverhumans touch any device containing sensitiveelectronic semiconductors, they can give rise toan ESD (electrostatic discharge) event.

If the surrounding air is particularly dry, becauseit’s a really hot or freezing cold day for example,just sliding from behind the steering wheel of a carcan deliver a shocking jolt when touching themetal of the car door. While the jolt will havebeen no more than a nuisance to the driver, the

effect may be much greater for sensitive electronicequipment. Imagine picking up a smartphone ortablet PC, only to find that some of the buttons ordata ports are no longer working properly; theseand other real-life headaches could be the directresult of that same jolt being injected into thedevice rather than the metal body of the car.

Even though ESD may not cause catastrophicevents such as cell phones ‘exploding’, in theabsence of ESD protection it might well prevent acell phone from responding to any keypad orbutton inputs. Similarly, ESD damage can causean interface port such as USB or Ethernet to nolonger function properly when it is connected toother devices.

Consumer

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Paying for the rightprotection

Once aware of the danger, design engineers can easily help reduce the risk ofchip damage caused by user-induced overvoltage transients, such as ESD.

By Phillip Havens & Chad Marak

Figure 1: ESD testing

Consumer

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The physics behind ESDThese ESD events can all be traced back to aphenomenon known as triboelectric charging,which occurs when two materials make contactand then are quickly separated. As electrons aretransferred between the two materials, one ofthem builds up positive charge and the othernegative charge; the charge generates dependson several factors, including contact area, speedof separation, relative humidity and the chemistryof the materials. Although the process occursthousands of times every day, it generally goesunnoticed except when the discharge is enough tocause someone mild, brief discomfort, such aswalking across a carpet and grasping a doorhandle. The charge generated can range fromhundreds of volts to tens of thousands of volts.Some examples of electrostatic generation areshown in Table 1.

Given the ongoing miniaturisation of silicongeometries via wafer processing, exposure to ESDevents has become a real problem for today’ssophisticated yet cost-sensitive consumerelectronics. Structures that offer protection againstESD are now too large and costly relative to thesilicon integrated circuits (ICs) themselves to beincluded in the IC package. The result is that ICsuppliers have removed or greatly reduced built-inESD protection; the problem is that once these ICsare installed in consumer products and leave thecontrolled environment in which they weremanufactured, they may well be subjected touncontrolled ESD events.

What’s more, while IC manufacturers havehistorically used an ESD test model (MIL-STD-

883, Method 3015: Human Body Model) thatrelates specifically to a manufacturingenvironment, equipment manufacturers — whoare concerned with ESD events in the field —have used a harsher model as defined by theIEC (International Electrotechnical Commission),namely the IEC 61000-4-2 standard. As of thiswriting, most IC suppliers test their products at500V with the Human Body Model (HBM),while end-user equipment manufacturers willtest at 8000V (and beyond) using the IEC61000-4-2 standard.

Table 1 compares the HBM (Human BodyModel) ESD currents used by many chipsetproviders against the environmental ESDevents outlined in IEC 61000-4-2 that manyconsumers will unknowingly inject into theirconsumer devices:

It is clear that the worst-case HBM ESD peakcurrent is much lower than the worst-case ESDpeak current under IEC 61000-4-2 (see thefigures highlighted in red in Table 2).Compared to the 8kV event described in theHBM, the 8kV event under IEC 61000-4-2implies a current that is 5.6 times greater. Achipset that survives HBM testing — asemployed in the manufacturing environment — isnot guaranteed to survive in the field, wherethe ESD exposure will be much more severe.Finally, as mentioned earlier, the majority of ICsuppliers test only to 500V using the HumanBody Model. If exposed to an 8kV ESDtransient voltage in the field, a chipset willexperience a near 100-fold increase in current;unless the IC design includes ESD protection,this level of current is more than enough to sealthe chipset’s fate.

In recent years, application testingrequirements have been tightened again andagain, such that an ESD event of 8kV istypically the lowest level now used. Testinglevels are trending towards 20kV and even30kV, but at the same time IC suppliers havecontinued to remove protection from chipsetdesigns to free up silicon area for morefunctionality. The following figure shows thegap between chipsets’ built-in ESD robustnessand likely ESD exposure levels in fieldapplications, highlighting the growing need forsupplementary ESD protection.

Figure 2: A protected IC

Protection is crucial If consumer electronics are to survive an ESDevent and continue to function as originallydesigned, it is absolutely crucial to select thecorrect ESD protection device — typically referredto as a transient voltage suppressor (TVS) diodearray. Dynamic resistance is one of the key ESDprotection parameters to consider, perhaps eventhe most important when selecting a protectioncomponent. Any protection solution has anintrinsic resistance value associated with itsclamping voltage characteristic. Ideally, intrinsic

resistance is minimised to ensure that theprotection solution has the lowest impedance pathto ground during a surge event.

The schematic diagram in Figure 2 describes thisissue. During an ESD event, the clamping devicewill turn on, going from its nominal state of highimpedance to one of low impedance. If its seriesresistance is high, a high voltage (V = IR) willdevelop across the component, which means itoffers the IC less effective protection. If its seriesresistance is low, the voltage that develops acrossthe protection component will also be lower, in

Figure 3: Silicon vsVaristors

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Electronic Specifier DesignElectronic Specifier Design | February 2014 || February 2014 | 4949

Consumer

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turn lowering the IC’s exposure level. This lowerdynamic resistance (the protection component’sresistance value during clamping mode) allowsmore of the surge current to be routed away fromthe IC and to ground, as depicted in Figure 2.Littelfuse TVS diode arrays are designed to attainthe lowest dynamic resistance value, thusminimising the overall voltage drop across theprotection component and maximising the currentthat passes through it instead of through theprotected IC.

In general, silicon protection devices provide thebest ESD protection thanks to their inherentlylower dynamic resistance compared to competingtechnologies such as polymers or ceramics. Thetypical dynamic resistance of silicon componentsranges from 0.2Ω to 3.0Ω, depending on thesupplier, while ceramic solutions (with an equalcapacitance) offer dynamic resistances in therange of 2Ω to 5Ω on average.

The graph in Figure 3 shows the difference in let-through energy between a silicon component andits varistor counterpart when an 8kV ESD transient

is injected into each component. It highlights thedifference in leading-edge turn-on and in finalclamping voltage. The area between the curves(the arrows) represents the difference between thetwo components in terms of how much energy theIC or chipset must be able to survive in order toprevent potential damage or premature failure.

Protection through standardsThere seems to be no end to the range andconstant evolution of consumer electronics: LCDTVs, smartphones, tablets, eReaders, set-topboxes, gaming consoles, digital cameras, audioplayers, and many more. But despite this variety,the devices commonly feature the same ports, orinterconnects. As the device’s interface to theoutside world, these interconnects requireprotection against ESD. Nearly all consumerproducts share some of the following functions:

AC input/output•DC input/output•Battery pack•Keypads/push buttons•

Table 1: ESD generation

Table 2: ESD events outlined

Video signals (HDMI, S-Video, composite•video, LCD module)Audio output•Low-speed data interface (USB1.1, IEEE 1394,•RS 232C, RS 485)High-speed data interface (USB2.0, USB3.0,•10BaseT, 100BaseT, 1000BaseT)CATV/RF input/output•

Some of these functions have to demonstratecompliance with national safety standards, whichmeans they need overcurrent and overvoltageprotection. Other functions may need protectionfrom environmental factors such as ESD, but alsonearby lightning surges or EFT (electrically fasttransients) caused when nearby equipment with ahigh inductive load cycles on and off (e.g. avacuum cleaner).

Products that are directly connected to the ACmains (120V to 250V AC rms) may be exposed tosevere surge transients (lightning, load switching,etc.) and short circuit/overload conditions, makingit necessary to fit a combination of components toprotect against overcurrent (fuses or PTCs) andovervoltage (MOVs, TVSs, or TVS diode arrays).Standards that may specifically require thisprotection are:

IE 61000-4-4 (EFT user level environment)•IEC 61000-4-5 (lightning induced surges)•IEC/EN 60950-1 (safety standard)•

Portable consumer products that contain an ACor DC adapter may have specific ESD and low-level lightning exposure threats that must beconsidered. Standards that may specificallyrequire this protection include:

IEC 61000-4-2 (ESD user level environment)•IEC 61000-4-5 (lightning induced surges)•

Keypads or other manual interface buttons maybe an entry point for the destructive energy ofESD. Audio lines may have similar ESD exposuredue to the speaker wire connections and exposureto manual handling. Connectors for S-Video,composite video and HDMI are also susceptible toESD exposure since they often undergo manualhandling. Battery pack applications will enduresimilar ESD exposure as well as potentialovercurrent run-away conditions that must beprotected against (IEC 61960 & IEC 62133specifically apply). Low-speed and high-speeddata lines, too, have an ESD exposure; dependingon their actual placement, they may also beexposed to lightning-induced surge events.International standards that typically apply forthese types of applications include:

IEC 61000-4-2 (ESD user level environment)•IEC 61000-4-5 (lightning induced surges)•

Consumer products face an ever increasing riskof being damaged by overvoltage transients suchas ESD. As IC designers continue to pack morefunctionality into their chipsets, there has been amajor decrease in built-in ESD survivability —which makes it necessary to employ externalprotection components. Equipment manufacturerstest their equipment to the international IEC61000-4-2 standard; to ensure the life expectancyof their products, TVS diode arrays arerecommended for protection. TVS diode arraysare not only able to meet ICs’ small footprintrequirements, they can also offer very lowclamping voltages compared to competingtechnologies, which means they are capable ofsafeguarding state-of-the-art ICs. Exposure to EFTs,nearby lightning strikes, or potential power faultevents in turn make it essential to put overcurrentand overvoltage protection solutions in place.

By employing the correct overcurrent andovervoltage protection components, manufacturerscan ensure their products remain an integral partof their consumer’s life. Choosing the rightprotection components also ensures thatapplications comply with regulations relating toboth safety and functional factors.

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Author Profiles: Phillip Havens is a Principal Engineer atLittelfuse and holds a BSEE and MSEE from LouisianaTech University. He represents Littelfuse at electronicssafety, circuit protection, and telecom-related industryassociations such as ITU, TIA, ATIS, IEC, IEEE, PEG, andUL497/60950-1/62368-1 STPs. He also helps define,direct, and support the company’s silicon-basedprotection product lines. His email address isphavens@littelfuse.com. Chad Marak is Director of Technical Marketing and TVSDiode Array Products in the Semiconductor Business Unitof Littelfuse, Inc. He received his BSEE from Texas A&MUniversity and MSEE from Santa Clara University. He hasbeen in the semiconductor industry for 10 years andholds four US patents. He can be reached atcmarak@littelfuse.com.

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