Yoshiyuki Kobayashi, Product Manager for ESD Devices at Murata Europe, explains the need for ESD protection and the differences between the types of ESD protection device on offer.
One of the biggest threats to the sensitive components in a mobile phone is electrostatic discharge (ESD). ESD is a sudden high voltage spike caused by charged objects touching, or being in close proximity to each other. Since these voltage spikes are typically thousands of volts, it can be damaging to sensitive components such as ICs in the system. ESD may occur when an electronic device comes near the human body or when it’s near to another device (machine interface contact). A prime example is when two devices are being connected together, for example, plugging a mobile phone into a laptop. The connecting pins may be touched by the person’s hand, or if the device has been charged up, ESD may occur when the connectors are mated.
Today’s electronics systems are increas- ingly susceptible to ESD. ICs are being built at smaller and smaller process nodes, so the transistors are physically smaller. Silicon layers are more likely to rupture, and metal traces are more likely to open or bridge at the smaller process nodes. In addition, the intro- duction of high speed communications standards such as USB 3.0 and HDMI have more stringent requirements for signal integrity than ever before. Protection of sensitive electronics from permanent damage therefore requires the incorporation of ESD devices. These devices, which come in a vari- ety of different types, divert the charge away from sensitive parts of the system such as ICs. After an ESD event, the voltage is ‘clamped’ at a certain level by the ESD device and the current from the event is shunted to ground. To decide what type of ESD device is used in what application, there are vari- ous factors that must be considered. As an example, for high speed data trans- fer, an ESD protection device with ca- pacitance less than 1pF must be used to prevent high capacitance affecting sig- nal integrity. Figure 1 shows a diagram of ESD suppression voltage versus capacitance that is used by Murata to help customers select the right device for their application.
Varistors are essentially non-linear vari- able resistors, and suppressors are low capacitance varistors. These devices are relatively inexpensive but they have several disadvantages. Firstly, low per- formance – typical clamping voltage for a suppressor might be 150 to 500V, still way above the limits for most ICs. Sec- ondly, these components have a finite lifespan, typically 10-20 ESD events.
Transient voltage suppressors, or Ze- ner diodes, provide a fast response when faced with an ESD event but have a relatively low current capability so are restricted to use in circuits with low current spikes. They have good lifetimes and are typically used to pro- tect high speed data lines since they can be used in series to lower capacitance.
Murata offers two additional types of ESD devices – ceramic and silicon.
Ceramic ESD devices feature ultra-low capacitance (0.05pF), meaning they can be used for high speed data lines, and are extremely robust with a long lifetime. Figure 2 shows a graph of voltage versus time for an ESD event of 8kV as per IEC61000-4-2 level 4. The red line shows the response of the Murata 0.05pF ceramic device – the peak voltage rises to 300V but clamp- ing voltage is held at 40V. Compare this with the 1pF varistor (green line) and the 3pF varistor (blue line) which feature clamping voltages of 200 and 100V respectively. Ceramic devices also feature extremely low insertion loss (-0.004dB@2.4GHz) , giving them another advantage over varistors. A typical Murata ceramic ESD protection device measures 1.0 x 1.5 x 0.33mm.
Silicon ESD protection devices also have excellent ESD suppression per- formance, but their capacitance is not typically as low as ceramic devices – in the region of 0.25pF.
Advantages of silicon ESD protection devices include very fast turn-on time, minimising peak voltage as shown in Figure 3 which is a comparison be- tween a part from Murata’s silicon ESD range and a TVS diode when subjected to 8kV as per IEC61000-4-2 level 4. In further tests, the silicon device also proved to have a very small on-resist- ance, just 0.3 Ohms compared to the TVS’s 0.8 Ohms. 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.
One of the key advantages of using a silicon device versus a ceramic one is that multi-channel versions are avail- able to save board space in today’s compact electronics where real estate is at a premium. Figure 4 shows a com- parison between using discrete devices and one of Murata’s 10-channel silicon ESD devices, which also incorporates LC filters to protect against EMI simul- taneously. The Murata multi-channel device measures 2 x 2mm whereas the discrete solution, which features varis- tors and discrete LC filters, takes more than 10X the board space.
There is a large range of silicon parts available with different capacitances, case sizes and number of channels for different applications.
As an example of how ESD devices are used, let’s consider a typical mobile phone like the clamshell model shown in Figure 5. Places where ESD protec- tion is required include the antenna ports, data lines, power amplifier, USB interface, keypad, SIM slot and speak- er/microphone.
The mobile phone’s antenna is a key interface between the system and the outside world, and of course is suscepti- ble to ESD. For ESD protection, system designers may choose a low-cost ce- ramic ESD device with ultra-low capaci- tance such as the LXES15AAA1-100, or a single-channel silicon device for high performance with low capacitance such as the LXES1UBBB1-008. An alterna- tive solution is a 2-channel silicon ESD device such as the LXES1TBAA2-013, which with the two channels connected across an inductor, creates an additional EMI filter.
Multi-channel ESD devices are particu- larly useful when protecting data lines. For example, on a USB2.0 interface. Single channel devices could be used, but it would be more efficient to use an array solution, especially where dual ports need to be supported. A suitable 4-channel device, such as the LXES2TBCC4-028, measures just 2.5 x 1.0 x 0.6mm and has a capacitance of 0.5pF. Array solutions are even more beneficial where USB3.0 is used as there are more data lines that need protection. HDMI, Firewire and DVI ports, with their many data lines, are also able to make use of silicon multi- channel array solutions.
The keypad on a mobile phone is an- other place where multi-channel arrays are useful, especially those with built- in EMI filters. When the user touches the keypad, radiation noise and ESD occur. Two 4-channel devices such as the LXES1WCAA4-038 with integrat- ed EMI filters are a typical solution, or alternatively, the 8-channel LXES3Y- CAA8-039.
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 just 1.0 x 0.5 x 0.33 mm, or a silicon device such as the LXES1UB- AA1-096, which measures 1.0 x 0.6 x 0.6 mm, should help keep board area down and costs low.
There are many types of ESD protec- tion device available ranging from TVS/ Zener diodes and varistors to the tiny ceramic and silicon devices produced by Murata. Selecting the right component for the application at hand will require an analysis of the performance level re- quired, the board space available and the cost. Ceramic devices are available with ultra-low capacitance, excellent lifetime and low cost. Silicon devices are available with low capacitance, fast response times and the possibility of an array solution in one package, even in- corporating EMI protection, which can save board space. All of these devices come in the tiny packaging for which Murata is world-famous.