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ICs & Semiconductors


Device insurance


Investing in protection ICs may add to the bill of materials cost at the beginning of a design project but the advantages can be significant, as Vishal Goyal explains


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magine you bought the latest electronic device and it got damaged just by the touch of your finger to its USB port or your laptop getting damaged by an electrical surge in the LAN cable. That’s not unusual especially when devices that getting faster and smaller are becoming more susceptible to electrical surges. This can be avoided, however, if circuits are properly protected against these surges by using specific silicon devices called protection ICs. These devices can absorb these surges and not let them reach the functional ICs.


The range of applications requiring this type of protection are extensive. Take for example a mobile phone. In one device you need to be able to protect: audio lines, USB interfaces, bottom connectors and keypads, touch screens, Video Serial Interface, HDMI, MIPI (Mobile Industry Processor Interface), memory cards, SIM cards, battery and charger ports. A similar problem is confronted by manufacturers of consumer goods, industrial applications and suppliers of networking and telecom infrastructure. The types of protection available


include:


Electro Static Discharge protection The human body can pick up an electrical charge from the environment or from interaction with another charged object. Sometimes you can experience an electrical shock by touching a non- electrical device. Any of these sudden discharges of current can generate enough of an ESD pulse to damage the silicon chip of an electronic device. There are various standards that relate to IC ESD such as:


MIL STD 883E-Method 3015-7 (human body model, HBM): This simulates an ESD event when a person is charged either to a positive or negative potential and touches an IC at another potential. JESD22-A115-A (machine model, MM): This simulates an ESD event that occurs when part of the equipment or tool comes into contact with a device at a different potential.


ESD STM5.1- 2007 (charged device model, CDM): This simulates an ESD event when a device discharges from a certain potential to equipment at another potential.


Most silicon chips have one of the above built-in ESD protection standards to


prevent the chip getting damaged in the production line. But this protection is not enough to protect the IC when the device is actually used by the end user. When we touch a port of a device, the induction of the PCB trace can amplify the surge discharge. So the surge which reaches the IC is much higher. As a result the ESD standard needed to protect the device at the consumer’s end should be more stringent than HBM. IEC 61000-4-2 (electromagnetic compatibility) simulates the whole environment where equipment may be subjected to ESD. This standard is considered as a reference in the wireless environment because this simulates more of a real-case stress. IEC61000-4-2 ensures protection at the system level whereas HBM defines protection at IC level. It is difficult to ensure that IEC61000-4-


2 protection is embedded in the functional IC. So specialised protection ICs should be used to comply with this standard. In fact as the ICs become more complex, faster, and smaller, the gate oxide thickness is also being reduced. Hence, implementing protection is becoming increasingly difficult as protection is occupying a major portion of the silicon landscape. Moreover, better control of ESD risks in the factories (ionizers, grounded straps, grounded boots on conductive floors) help to ensure that the ICs don’t get damaged due to ESD failure. The Industry Council on ESD target levels propose a reduction for HBM (Human Body Model) levels from 2 kV to 1 kV. But the user environment remains the same, so the need for external ESD protection is even more necessary.


Electrical Over Stress protection Lightening is a major concern when it comes to telecommunications lines. Direct lightning can strike equipment it can induce radiation in the air and cause surges through coupling with the line and it can induce a potential change in the ground which disturbs the equipment. Lightning can also damage central


office equipment, gateways, phones, and 28 November 2011 Components in Electronics


other electronic devices. In addition to these surges, a telecommunication line may also be disrupted by the power line. Two scenarios are possible: power induction, where a current is induced in the telecommunication line - this is generally a low current surge - and power contact, involving a direct contact between the telecommunication line and the power line. This is generally quite a high current over several minutes. Depending on the country, these surges have been modelled in standards: the Telcordia GR-1089 core and TIA-968-A (formerly FCC part 68) for America and the ITU-T K series for the rest of the world.


Basics of protection ICs Protection basically uses the diode technology for its functioning. A rectifier uses the 1st quadrant and ensures that no negative pulse passes through it. A protection IC works in 3rd quadrant and does not allow voltage beyond a specified value to pass through it. A rectifier is put in series with the circuit whereas the protection is put in parallel to the circuit. Protection ICs are classified in broadly two categories – transils and trisils. Transils - A Transil diode is a solid state, monolithic PN junction device. Transils are used in sensitive semiconductors for parallel protection against EOS or ESD. A Transil clamps any over-voltage above its Breakdown Voltage (VBR). Transils can be unidirectional or bidirectional. Trisils – In Transils the Ipp is proportional to Vpp. In


telecommunication systems, surges are high in terms of energy, so a Transil will blow up. For this reason, Trisils based on Thyristor/Triac technology have been developed: the electrical characteristics of a Trisil are similar to those of a Triac, except that the Trisil has only two terminals.


Why protection ICs? The two main alternatives of silicon-based protection ICs are gas tubes and Metal


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