Electrostatic discharge (ESD) looms as one of the biggest threats to a mobile phone’s sensitive components. ESD is a sudden high voltage spike caused by charged objects touching, or in close proximity to, each other. Since these voltage spikes typically produce thousands of volts, they may damage sensitive components (e.g., ICs) in the system.
ESD can occur when an electronic device comes near the human body or when it’s around another device (machine interface contact). A prime example involves connecting together two devices, such as plugging a mobile phone into a laptop. The person’s hand may touch the connecting pins, or if the device is charged up, ESD may occur when mating the connectors.
Today’s electronics systems are more susceptible to ESD than ever before. With ICs being built at smaller and smaller process nodes, the transistors physically shrink, too. Silicon layers are more likely to rupture, and metal traces are vulnerable to opening or bridging at the smaller process nodes. In addition, the arrival of high-speed communications standards, such as USB 3.0 and HDMI, introduces even more stringent requirements for signal integrity.
As a result, protecting sensitive electronics from permanent damage requires the incorporation of ESD devices. These devices, which come in many different types, divert the charge away from sensitive parts of the system. After an ESD event, the ESD device “clamps” the voltage at a certain level, which leads to shunting of the current to ground.
Various factors are at play when deciding what type of ESD device fits a certain application (Fig. 1). For example, high-speed data transfer requires an ESD protection device with less than 1pF capacitance to prevent high capacitance affecting signal integrity.
Traditional ESD Devices
Varistors are essentially nonlinear variable resistors, and suppressors are low-capacitance varistors. Though relatively inexpensive, these devices present several disadvantages. One concerns low performance—typical clamping voltage for a suppressor might be 150V to 500V, still way above the limit for most ICs. Another is their finite lifespan of only 10 to 20 ESD events.
Transient voltage suppressors, or Zener diodes, provide a fast response when faced with an ESD event. The devices’ low current capability restricts their use in circuits with low current spikes. However, they have good lifetimes, and can be used in series to lower capacitance in high-speed data lines.
Ceramic ESD Devices
Ceramic ESD devices feature ultra-low capacitance (0.05pF), are extremely robust, and have a long lifetime. Thus, they’re strong candidates for use in high-speed data lines. A graph of voltage versus time for an ESD event of 8kV as per IEC61000-4-2 level 4 helps illustrate the ceramic device’s effectiveness (Fig. 2). The red line shows the response of Murata’s 0.05pF ceramic device. The peak voltage rises to 300V, but clamping voltage holds at 40V. The device is compared with a 1pF varistor (green line) and 3pF varistor (blue line), which feature clamping voltages of 200V and 100V, respectively.
Ceramic devices also feature extremely low insertion loss (-0.004dB at 2.4GHz), which is another advantage over varistors. Size also is important—a Murata ceramic ESD protection device, for instance, typically measures 1.0 x 1.5 x 0.33mm.
Silicon ESD Devices
Silicon ESD-protection devices offer excellent ESD suppression performance as well. However, their capacitance—in the region of 0.25pF—usually isn’t as low as ceramic devices.
Advantages of silicon devices include very fast turn-on time, minimising peak voltage. Comparing a part from Murata’s silicon ESD family and a transient-voltage-suppression (TVS) diode when subjected to 8kV (as per IEC61000-4-2 level 4) helps illustrate this point (Fig. 3). In further tests, the silicon device also proved to have a very small on-resistance, just 0.3Ω compared to the TVS diode’s 0.8Ω. Small on-resistance means clamping voltages can be kept low. In this example, it’s kept to just 8V, compared to 35V for the TVS diode.
One key advantage of silicon versus ceramic is the potential to save board space, because silicon comes in multichannel versions. A comparison shows the difference between using discrete devices and a 10-channel silicon ESD device (this device from Murata also incorporates LC filters to simultaneously protect against EMI) (Fig. 4). The multichannel device measures 2mm x 2mm, whereas the discrete solution, which features varistors and discrete LC filters, occupies more than 10X the board space.
A large range of silicon parts is available with different capacitances, case sizes, and number of channels for different applications.
To illustrate how ESD devices are used, let’s consider a typical clamshell-style mobile phone (Fig. 5). ESD protection is required for the antenna ports, data lines, power amplifier, USB interface, keypad, SIM slot, and speaker/microphone.
The mobile phone’s antenna is a key interface between the system and the outside world, and of course is susceptible to ESD. For ESD protection, system designers may choose a low-cost ceramic ESD device with ultra-low capacitance (e.g., Murata’s LXES15AAA1-100), or a single-channel silicon device for high performance with low capacitance (e.g., the LXES1UBBB1-008). An alternative solution is a two-channel silicon ESD device (e.g., the LXES1TBAA2-013). The two channels connected across an inductor, creating an additional EMI filter.
Multichannel ESD devices are particularly useful when protecting data lines, such as on a USB2.0 interface. Single-channel devices could be used, but it would be more efficient to use an array solution, especially to support dual ports. A suitable four-channel device could be the LXES2TBCC4-028, which measures 2.5mm x 1.0mm x 0.6mm and has a capacitance of 0.5pF.
Array solutions are even more beneficial for USB3.0-related applications, because more data lines need protection. HDMI, Firewire, and DVI ports, with their many data lines, can also make use of silicon multichannel array solutions.
A mobile phone’s keypad is another place where multichannel arrays come in handy, particularly occur. Two four-channel devices (e.g., the LXES1WCAA4-038) with integrated EMI filters could be used, or perhaps an eight-channel variety (e.g., the LXES3YCAA8-039).
For ESD protection at the speaker and microphone, a pair of single-channel devices is sufficient. A ceramic device such as the LXES15AAA1-100, which measures 1.0mm x 0.5mm x 0.33mm, or a silicon device like the LXES1UBAA1-096, which measures 1.0mm x 0.6mm x 0.6mm, can help reduce cost and board area consumption.
Many types of ESD protection devices are available, ranging from TVS/Zener diodes and varistors to tiny ceramic and silicon devices. Selecting the right component for the application at hand will require an analysis of the performance level, available board space, and cost.
Two of the best options are ceramic and silicon protection devices. Ceramic devices come with ultra-low capacitance, excellent lifetime, and low cost. Silicon devices feature low capacitance, fast response times and the possibility of an array solution in one package—even incorporating EMI protection, which can save board space.