ESD and EMI interference in mobile phones and their solutions

This article briefly explores the causes and consequences of ESD and EMI in mobile audio systems. It then examines the use of ESD suppressors and EMI filters to avoid these threats. Finally, it compares three current solutions.

Modern materials and technologies cause electrostatic discharge (ESD) and electromagnetic interference (EMI) to become an ever-present danger. What we wear and what we touch can cause electrostatic discharge. Digital technology already has EMI. Electrostatic discharge can damage electronic components in mobile phones. Mobile phones are easily replaced, but the harm to users is great. Mobile phone circuit designers must ensure that necessary measures are taken to eliminate the damage caused by ESD.

If there is electromagnetic interference (EMI) in the audio circuit, there will be sounds such as hissing, crackling, and buzzing, and the sound quality is very poor. Mobile phone users cannot tolerate such interference. Therefore, it is necessary to find a way to filter the electromagnetic interference of the audio circuit.

Electrostatic Discharge—Causes, Effects, and Suppression

cause

Almost everyone has experienced the effects of static electricity. We saw it in lightning when we were prehistoric Stone Age cavemen. Of course, it is still a significant threat today and is everywhere. Use a plastic comb to comb your hair and you can see the generation of static electricity. Hold your arm close to a television screen and you will see the hair on your arm stand up. This is also the effect of static electricity.

When you open the door and step out of your car, you may feel a shock of electricity from a discharge of static electricity. With more and more electrical devices in our homes and workplaces, static electricity is a constant danger. People who make or repair electrical equipment protect themselves and the equipment they work with by connecting themselves to the equipment to avoid harm caused by static discharge from the electrical equipment.

in conclusion

We can see lightning strike buildings and trees, it has destructive power. If the ESD protection of electronic circuits is not optimal, even a small discharge can damage sensitive electronic circuits, which has been detected. Mobile phones have some ESD protection. The external connections of audio circuits are the most common source of ESD. Simply plugging in headphones and speakers may mean that the mobile phone will be affected by ESD.

As shown in Figure 1, what happens when an electronic component is affected by ESD? A tiny hole is created and oxide invades the component.

inhibition

Like all commodities, mobile phones must identify their ESD according to the IEC61000-4-2 regulations. The regulations stipulate that mobile phones can withstand 15 kV air discharge (through 330Ω/150 pF), which means that a 45 A current is passed for approximately not less than 1 nanosecond. In this case, the mobile phone should continue to work without being damaged. The above is a comparison between a high-energy pulse and an ESD human body model experiment. In order to protect the main chip, additional ESD protection must be added at each potential ESD entry point. Generally speaking, devices that suppress ESD generate a controlled output, called a clamping voltage.

Figure 2 shows the output (clamping voltage) of an ESD protection device during an ESD event.

Electromagnetic Interference EMI - Causes, Effects and Filters

cause

When an electric current flows, it creates a magnetic field around a conductor. When the current changes, the magnetic field changes. So simply switching the current on and off creates a change in the magnetic field. This change in the magnetic field can cause other nearby conductors to generate signals. That's the basic principle of electricity.

Domestic and industrial electricity uses 50Hz or 60Hz alternating current. This is the audible frequency range. The current is constantly changing, and nearby conductors of the same frequency will generate a signal. If you have used Hi-Fi, using separate players and amplifiers, and at the same time they are not connected together in the chassis, you will hear a humming sound.

Consider today's electronic world, where signals are constantly changing:

Audio input/output can generate radiated and conducted EMI, which then emits higher frequency RF lines, causing signal distortion.

The mobile phone antenna (TDMA pulses) emits RF signals that can be picked up by long-wired headphones, causing EMI noise in the audio signal path.

The GSM (Global System for Mobile Communications) mobile phone standard uses frequency division multiplexing and time division multiplexing to transmit a large number of calls simultaneously, as shown in Figure 3.

A specific mobile phone transmits only in its own time slot. The fundamental frequency of the envelope signal is 1/4.615 ms = 217 Hz. The harmonic frequencies are 434 Hz, 651 Hz, etc. Such frequencies are audible. Figure 4 shows the envelope signal of a mobile phone.

result

When a mobile phone communicates with a base station, or two mobile phones are close to each other, the transmit pulse is transmitted into the audio path through the microphone, speaker, or headphone line. See Figure 5.

The result is a significant reduction in audio quality.

filter

The EMI filter is placed as close as possible to the entry point of EMI interference to ensure the audio quality as much as possible. See Figure 6.

The filter should be selected based on its bandwidth, cutoff frequency and stopband rejection characteristics. Another factor in creating high-quality sound is total harmonic distortion (THD). Poor THD can ruin the sound quality of an otherwise excellent audio system. Ideally, the THD value of the EMI filter is better than the weakest signal chain.

Representative features:

The stopband attenuation in the 800-2480 MHz frequency band is not less than -25 dB

The stopband attenuation of the 10-800 MHz frequency band is not less than -20dB

MIC line is not less than -70 dB(A)THD+N(0.03%), providing high quality audio.

Consider Board Space

Mobile phones are integrating more and more multimedia functions, such as GPS, MP3, FM, Bluetooth, and DVB-H. These functions require additional board space. Designers must squeeze out space for ESD and EMI solutions.

Comparison of three solutions

Some solutions in the market do not provide a perfect method. There are three possible solutions in Figure 7.

Discrete Solutions

This solution uses 24 discrete components to form the ESD suppressor and EMI filter. This solution is not optimized. The cost and reliability of its operation are limited by the 24 discrete components.

Low Temperature Co-fired Ceramic (LTCC) and Varistor Solutions

Low temperature co-fired ceramic (LTCC) EMI filters can accomplish filtering requirements well. However, varistors have high clamping voltages (maximum VCL>100V), and thus do not provide optimal sensitive sub-microchip ESD protection.

Integrate passive and active devices

This technology combines protection diodes with passive components such as resistors and high-density capacitors in integrated circuit silicon chips. Compared with the previous two solutions, the advantages of the IPAD solution are as follows:

Can fulfill all ESD suppression and EMI filter requirements.

· Saves a lot of board space (about 78%)

·Using natural silicon devices, it can provide significantly higher reliability and lower operating costs.

in conclusion

This article describes the causes and potential consequences of ESD and EMI in mobile audio interfaces and outlines the need for ESD suppression and EMI filters.

Comparing the available integrated ESD protection and EMI filter solutions, the one that provides the best ESD protection (lowest VCL) and the best stop-band attenuation can also offer other benefits such as better reliability and lower operating costs.