The slope of the line is key to ESD protection
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Electrostatic discharge, more commonly known as an ESD event, represents a common, over-voltage transient. And, in fact, modern ICs are generally equipped with a minimum level of ESD protection to safely survive the manufacturing process. However, the level is typically very low (i.e. 500V) and the ESD test model used is nothing like what can be seen in the field while the application is in the hands of the user. As a design engineer, it is important to note that significantly higher ESD levels can be generated during day-to-day use of their product. For instance, humans can generate ESD levels in excess of 15kV simply by walking across a carpet. If an ESD event of this severity is discharged into a USB port, for example, it can severely damage on-chip protection circuits and the circuitry they are meant to protect. This would then render the USB useless.
Fig. 1: ESD current paths for a generic I/O port.
Protection for sensitive ICs needed
A case in point illustrates how closer collaboration between global electronics manufacturers and their component supplier can be the key to developing solutions to these problems: Last year, a market-leading manufacturer of consumer electronics, mobile communications and home appliances contacted Littelfuse to help them solve a potentially serious problem. They determined that a key IC was experiencing frequent damage via the USB port. The IC in question was used to make an LCD TV “smart” by providing WiFi connectivity. The company had already tested many different devices and only been able to obtain a maximum of 3.5kV with respect to the system’s ESD immunity. None of the ESD solutions from other companies had been able to meet their requirements for protection. After investigating the design and understanding the target ESD immunity levels, Littelfuse confirmed that it could in fact provide a device capable of meeting their needs.
Fig. 2: Graphical representation of dynamic resistance.
The two companies met at Littelfuse’s manufacturing and testing center in Wuxi, China, so they could determine an appropriate solution. During the application-level testing, Littelfuse demonstrated the importance of dynamic resistance (of the ESD suppressor) on system level performance. Dynamic resistance is the effective resistance of the suppressor while clamping an ESD pulse. This is the key factor in determining how well a device will shunt the ESD pulse which ultimately determines if the IC will be protected or not. Another way to look at this is that the V-I curve of the protection device will have a gentler slope with lower dynamic resistance. By testing a multitude of off the shelf devices with varying dynamic resistances, it was determined that a device with a dynamic resistance of approximately 0.25Ω (measured via TLP or Transmission Line Pulsing) would protect the IC sufficiently. Ultimately Littelfuse confirmed it could design a new device in the desired form factor with a dynamic resistance very close to what was needed, and still keep the capacitance sufficiently low to preserve signal integrity of the USB port.
The SP3031 was born
Four months later, we had successfully provided the SP3031-01ETG, a component that includes two low capacitance steering diodes with an additional low clamping TVS diode all within a 0402 package measuring only 1.0×0.6mm.
In general, capacitance and dynamic resistance are at odds with each other. The larger the diodes, the more capacitance they have, but of course the additional area can provide a low dynamic resistance. So, many times, customers have to trade off signal integrity (i.e. higher capacitance) for a lower dynamic resistance (i.e. better clamping). In this particular case, the High-Speed USB could function with approximately 1-2 picofarads, but the customer’s target was less than 1pF to ensure maximum signal integrity. On top of that, this target came along with a request for a low dynamic resistance value more customarily found in products with 15 to 20pF of capacitance. The new solution required the use of proprietary silicon processing and silicon design techniques.
It proved to be advantageous that the customer was able to meet with its component supplier at its manufacturing site in China to see how they perform testing and how their components interact with the overall system. This provided them the opportunity to gain a better understanding of the relationship between and the performance of an individual ESD component and overall system ESD immunity.
The SP3031 is now in mass production, shipping since the end of 2013. Knowing that 2015 model televisions will require a higher level of ESD immunity (possibly 6kV), Littelfuse is already working on designing a new protection component to meet this requirement by further lowering the dynamic resistance to obtain a lower clamping voltage and at the same time maintaining low capacitance and signal integrity in the system. Looking at this case study, we could conclude that preserving a smart 60-inch screen from ESD depends on 0.6mm2 of silicon.
About the author
Jim Colby is Manager of Business and Technology Development at Littelfuse – www.littelfuse.com – He can be reached at jcolby@littelfuse.com
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