Protect Circuits From ESD
ESD discharges are one of those nasty things that can wreak havoc on portable and other electronic products. Manufacturers must build in protection against current from these high-voltage pulse discharges. This is especially important for I/O and power connectors.
There are a number of methods for either blocking the ESD current pulse or diverting it to earth through the product's grounding system. Series-connected devices such as ferrite beads, common-mode chokes, and small-value series resistors can block or reduce the current pulse. Parallel-connected devices like capacitors; back-biased (or back-to-back, depending on the application) diodes; spark gaps; or gas-discharge devises (neon bulbs, for instance) may be connected across data lines and will divert ESD currents to ground. All the above have issues, primarily in the parasitic capacitance they also add to your circuit.
Several manufacturers now make low-capacitance shunt devices that will divert much of the ESD current to ground, without affecting the signal integrity of the data line(s). These include Murata, Littlefuse, TE Connectivity, and many others.
Two of the best-performing technologies for high-speed data lines include ceramic ESD devices and silicon ESD devices. Ceramic protection offers extremely low capacitance (about 0.05pF) and is very robust with a long lifetime. Peak voltage for a 8kV ESD pulse might be 300V, with a clamping voltage of 40V. Silicon ESD devices have slightly higher capacitance (0.25pF). The advantage of the silicon devices is their very fast turn-on time, which limits the peak voltage to 50V or so, with a clamp voltage of just 8V to 10V. One other big advantage of silicon protection devices is they can be made in multi-channel packages, which is ideal for the newer USB 3.0 connectors that include six data lines.
For example, TE Connectivity surface mount SESD0402X1UN-0020-090 silicon ESD protection device (data sheet) can protect USB 3.0 or HDMI data signals from ESD. There are two connections for the USB 1.0 and 2.0 data lines, a larger ground connection, and four connections for the USB 3.0 data lines. The parallel capacitance to ground is just 0.2pF, and the clamping voltage is 9.2V. The figure shows surface-mount ESD protection devices on a USB 3.0 connector.