Electrostatic discharge (ESD) refers to the transfer of electrostatic charge between two objects with different electrostatic potentials due to direct contact or electrostatic induction. When static electricity accumulates to a certain level, it breaks through the medium between them and discharges. Therefore, ESD protection is ubiquitous, and ESD overvoltage protection has become a basic requirement for the design of various electronic equipment.

Most of the new wireless handheld products are equipped with high-speed data interfaces, high-resolution LCD screens and camera modules, and some phones even have the function of receiving TV programs. Although many new functions have been added, the size of mobile phones continues to become thinner and smaller. Many functions are gathered in a small space, and many major integrated circuits are manufactured using more advanced processes, which makes the ESD problem in mobile phone design more serious. It may cause the phone to work abnormally, freeze, or even damage the phone. These problems must be solved at the beginning of mobile phone design.

ESD protection part in mobile phone

Here we take a mobile phone as an example. The parts of the mobile phone that need ESD protection are: 1) SIM card interface circuit; 2) keyboard circuit; 3) earphone and microphone circuit; 4) data and USB port; 5) power port; 6) color LCD drive port and LCD backlight circuit.

ESD selection in design

ESD protection should take into account both performance and cost in design. The following principles should be noted:
1) The protection circuit should respond quickly—the rising edge of the IEC61000-4-2 waveform is 1ns.
2) Able to handle large transient currents.
3) Consider that transient voltage occurs in both positive and negative polarity directions.
4) The capacitive load effect and resistive loss effect of the signal are controlled within the allowable range.
5) Consider the size factor to fit into lightweight portable devices.
6) Product cost factors.

There are basically two ways to implement ESD protection: protection inside the IC and protection outside the IC. Although the former can increase reliability and is relatively simple because it does not require external protection, it will also increase the silicon chip area. At the same time, because of the presence of parasitic capacitance and parasitic resistance in the silicon chip, it will cause mutual crosstalk between signals. Therefore, external ESD protection solutions are now more and more widely used. External ESD protection circuits include varistors, polymer resistors, TVS avalanche diodes, Schottky diodes, magnetic beads/LC filters, etc.

Among them, TVS avalanche diodes are increasingly widely used due to their good clamping function, extremely low breakdown voltage, tiny packaging, and relatively stable electrical characteristics during their life cycle. Its notable features are rapid response, which effectively curbs transient pulses before they damage the circuit or device, and low cut-off voltage, which is more suitable for low-voltage circuit environments powered by batteries. In addition, improvements to the TVS diode design have made it have lower leakage current and junction capacitance, so it has more ideal performance when dealing with electrostatic shocks in high-speed conduction circuits.