SP3022: Lower capacitance to avoid interfering with high-speed data transfer
By James Colby, Littelfuse, Inc.
A high-performance processor, FPGA or System-on-Chip IC could be the most valuable component on any given board. Adequate protection is vital to prevent events such as Electrostatic Discharge (ESD) destroying its internal circuitry.
This Design Note from Littelfuse discusses the ESD threat facing the latest generations of ICs, explains key parameters defining the performance of ESD-protection devices, and shows how protection can be improved without impacting on IC performance.
Each new generation of integrated circuits delivers enhanced capabilities, such as faster data rates, smaller size, or lower power consumption. These improvements enable innovative and higher performing applications that can take advantage of the new ICs’ capabilities. However, the benefits also come with a potential pitfall; successive new generations of ICs are more sensitive to ESD than ever before.
In the case of devices such as MCUs and DSPs, the ESD threat enters through the interface pins, which will have some exposure to the outside world. For low-level or entry-level units, there will be basic I/Os including UART and I2C. For higher-level units, there will be interfaces including USB, CAN and LIN. The ESD-protection solution will need to be coordinated with the specific interface with respect to operating voltage and capacitance. Capacitance is particularly important where high-speed buses are concerned, to avoid any adverse effect on signal integrity.
Fortunately for IC manufacturers and application developers, suppliers of semiconductor-based board-level ESD-protection components relentlessly improve the capabilities of these solutions. Companies like Littelfuse, Inc. continually invest in developing new processes that enhance their protection products. Some key recent developments are listed below.
During an ESD event, the primary job of the ESD protector is to divert and dissipate as much of the energy contained in the ESD transient as possible. The characteristic governing its ability to do this is typically called ‘dynamic resistance’. By reducing the dynamic resistance, the ESD protector carries substantially more of the surge current than the circuit being protected. In doing this, it reduces the electrical stress on the integrated circuit and ensures that it survives. For example, the SP3014 series TVS diode array from Littelfuse has dynamic resistance of less than 0.1Ω to provide best-in-class performance.
Although circuit protection is central to an ESD protection device’s purpose, it must perform this role without interfering with the day-to-day functioning of the circuit being protected. For example, on an RF interface, such as a Bluetooth® or ZigBee® interface, or wired port such as USB2.0, the ESD protector must not cause distortion or loss of strength of the data signals. To ensure signal integrity, the capacitance of the ESD protector must be minimised without compromising protection levels. The SP3022 series TVS diode from Littelfuse features a capacitance value of 0.35pF, ensuring that it will remain ‘invisible’ to high-speed signals.
No matter how well a protection device performs, it is not able to provide value if it cannot fit into the application it is meant to protect.
For example, wearable technologies such as smart watches or wristbands will continue to get thinner and smaller, and ultimately may be incorporated directly into clothing. Hence the circuit boards will have minimal space available for any ESDprotection circuitry. Discrete diodes are ideal for providing designers with exceptional flexibility when determining the layout of a new board. The 30pF SP1020 and 6pF SP1021 series from Littelfuse are packaged in the 01005 package outline to minimise the amount of real-estate they take up.
So, although upcoming MCUs and DSPs will continue to improve their capabilities as well as those of new applications, they will challenge the designers who have to make sure that any new ICs can perform their required tasks and also provide long-lasting reliability. Manufacturers of ESD-protection devices continue to work hard to provide protection for these devices while not interfering with their core functionality.