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'''Protecting Portable and Wireless Device Equipment from Failure Caused by ESD

ESD Vulnerability in Portable and Wireless Devices
As portable and wireless devices continue to shrink, the components used to assemble them are increasingly susceptible to failure caused by electrostatic discharge (ESD).

An ESD is a sudden and momentary electric current  that flows between two objects at different electrical potentials. It can be caused directly by friction between dissimilar materials (walking across a carpet, for example) or induced by nearby lightning or other high-voltage or high-current events.

ESDs generally involve high voltages that can break down delicate semiconductor devices, damaging or destroying them. They can also cause latent defects in which a component is damaged but does not fail immediately. Even if they do not cause actual physical damage, ESDs can cause soft failures, in which data is corrupted or equipment must be rebooted.

Circuit Protection
The engineers who design such electronic add circuit protection devices to protect their designs. The selection of the proper device depends on the type of circuit being protected. In typical electronic devices, network connections, the user interface, memory cards, antennas, and even the built in speaker and microphone are all vulnerable to the damage from ESD. These areas provide an electrical path for high potential currents generated during a typical day’s activities.

Portable and wireless devices present an even greater challenge because of the wide range of environments in which they are used.

When performing an ESD survival analysis on a typical electronic device, numerous opportunities for ESD to enter the device become apparent. The goal of any ESD mitigation effort is to limit the damage to the device resulting from ESD. Technologies such as silicon transient voltage suppression (TVS) diodes, ceramic multilayer varistors (MLV), and composite polymer suppressors are all options.

ESD is characterized by very fast rise times and high peak voltages, so protective devices must have a similarly fast response time and high voltage withstand rating. Depending on the circuit and conditions, a single-channel ESD suppressor might be used, or in the case of a communications interface, a multichannel silicon protection array may be needed.

Earphone/microphone
The earphone and microphone ports on portable electronic devices are particularly vulnerable. An ESD incursion at this point can enter into the interior of the phone and arc via the speaker circuit. One option for protecting against this threat is a multilayer varistor with a capacitance value higher than 100 pF.

This provides ESD suppression by clamping the transient to a safe level and provides EMI/RFI filtering of noise from the ambient environment.

If the earphone and microphone circuits are adjacent to each other on the circuit board, then another appropriate strategy would be a multi-line diode array. This approach offers the additional advantage of using a single device on the circuit board to provide protection. If the speaker and microphone lines can be protected by a three-diode array, the design will lower both manufacturing costs and required board space.

Keypad/push buttons/switches
Keypads and other switches allow an ESD event direct access to the circuit board. Since these switches are essentially DC lines, a high capacitance suppressor is sufficient to provide protection; however, multilayer varistors and diode arrays may be appropriate as well.

If a small number of lines are the problem, discrete MLVs may provide an attractive alternative. On the other hand, a multi-line diode array, which can protect multiple lines in a single package, is a good alternative if there are a large number of lines in close proximity.

Power port
Potable devices need to charge a battery. This is accomplished via a low voltage input that charges the battery and provides direct power to the circuits. Since this is a true DC circuit, a high capacitance suppressor is recommended.

This circuit is also exposed to higher-energy transients, which necessitates a multilayer varistor that has capabilities over and above ESD protection.

In the event of a sustained overcurrent event, such as a battery malfunction or circuit failure, a surface mount fuse can be used to disrupt the overcurrent condition and protect the system.

I/O port – edge connector
The I/O port is another vulnerable component. The capacitance of the chosen suppressor device is a critical element in light of the faster data rates typical in portable electronics because the design must not introduce any signal integrity issues. Circuits in the I/O port, running at low speeds, for example analog audio, or RS232, need to be protected with higher capacitance multilayer varistors or TVS diode arrays.

Higher data rate protocols, such as USB 2.0 – 480 Mbps, typically require use suppressors with minimal capacitance so the system is able to transmit and receive the data with no loss of signal quality.

Antenna
An ESD suppressor device added to the circuit must present as much impedance as possible so as to not attenuate the communication signal. This is particularly important when operating frequencies between 800 and 1,900 MHz, which is typical for the antennae on a portable electronics device.

A polymer suppressor device with capacitance values below 1.0 pF would be ideal, because most antennas are encased in plastic or embedded inside the device. If the antennae elements are exposed the probability of user introduced ESD increases.

An Integrated Future
Circuit protection for portable and wireless devices can typically be met with discrete devices. However, space and higher volume manufacturing dictates the use of Integrated Passive Devices (IPDs). There are products available that combine ESD protection, EMI filtering, line resistance and termination resistance in one package. These devices significantly reduce component part counts and provide more circuit protection functionality.