Choosing the right integrated EMI filtering and ESD protection solution for portable applications

Today's mobile phones and other portable devices are becoming smaller and thinner, while integrating more and more new functions or features, such as large-size displays, high-resolution camera modules, high-speed data interfaces, Internet access, TV reception, etc., making the data rate and clock frequency of portable devices higher and higher. In this way, portable devices face the risks of many potential electromagnetic interference (EMI)/radio frequency interference (RFI) sources, such as switching loads, power supply voltage fluctuations, short circuits, inductive switching, lightning, switching power supplies, RF amplifiers and power amplifiers, ribbon cables and video display interconnections, and high-frequency noise of clock signals. Therefore, designers need to select suitable EMI/RFI filtering solutions for portable devices for multiple locations such as audio jacks/headphones, USB ports, speakers, keyboards, microphones, cameras, and display interconnections.

Common EMI/RFI filter types and filtering requirements

For EMI/RFI filters, the most common architecture is the "Pi" filter, which, as the name implies, is similar to the Greek letter "π". There are two common π-type filters, CRC (capacitor-resistor-capacitor) filters and CLC (capacitor-inductor-capacitor) filters. Among them, the CRC filter (see Figure 1a) is also called the RC-π filter or π-type RC filter, which is used for audio and low-speed data filtering applications; the CLC filter (see Figure 1b) is also called the LC-π filter or π-type LC filter, which is used for audio, low-speed and high-speed data filtering applications.

There is also an extended type of π-type filter, namely the ladder-shaped filter that is shaped like a ladder. The most common of these is the LC (inductor-capacitor) ladder filter (see Figure 1c). This filter can withstand higher data rates, but when the number of filter elements (inductors or capacitors) increases, size and cost become problems, resulting in higher material costs and larger packages.

Figure 1: Schematic diagram of the structures of common EMI/RFI filter types: a) π-type RC; b) π-type LC; c) LC ladder.

Among these types of EMI/RFI filters, from the comparison of frequency response, the π-type RC filter has the widest transition band and the lowest rolloff; the π-type LC filter has a lower rolloff but a moderate transition band; the ladder filter can achieve an extremely high rolloff and a narrow transition bandwidth.

In terms of EMI/RFI filtering, taking mobile phone applications as an example, a traditional reference frequency is 800 MHz, because 800 MHz is close to the starting frequency of the frequency band used by mobile phones. In most cases, mobile phone design engineers require filtering at frequencies above 800 MHz, which generally means a minimum signal attenuation of 30 dB. With the increase in functions in mobile phones and the classification of clock and data signals, the reference frequency is decreasing. Many portable electronic product manufacturers require EMI/RFI filtering at 400 MHz, and may require EMI/RFI filtering at lower frequencies in the future.

Integrated EMI filtering and ESD protection

In portable products, filters are often located near connectors, microphones, and speakers. These are also the places that can be exposed to electrostatic discharge (ESD) events. For example, if discrete components are used for EMI filtering and ESD protection, the problem is the number of components required to perform these two functions. For the EMI filter, if a discrete π-type LC filter is used, two surface mount capacitors and one surface mount inductor are required. For ESD protection, some type of surface mount transient voltage suppressor (TVS) diode is also required. Thus, EMI filtering and ESD protection for one audio line require four separate components, not to mention the precious space these components take up in the portable device. If there is more than one audio line, the discrete component solution is even more impractical.

Therefore, the simplest solution is to integrate EMI filtering and ESD protection functions in the same component. First, the integrated TVS diode in the integrated solution also provides the capacitance required for EMI filtering; second, improved process technology can also greatly improve the quality of integrated inductors. By integrating these integrated components onto silicon wafers, solutions that originally required multiple independent components can now use an integrated EMI filtering + ESD protection solution (see Figure 2).

Figure 2: Integrated EMI filtering + ESD protection solution: a) π-type RC; b) π-type LC; c) LC ladder

Among them, in terms of ESD protection level, the IEC61000-4-2 standard specifies system-level test conditions and protection levels in detail. These levels are divided into four types. Portable applications usually require level 4 protection, that is, under the 8 kV contact discharge or 15 kV air discharge IEC61000-4-2 test conditions, portable devices need to be able to withstand the impact of ESD events.

Overall, by cost-effectively integrating EMI filtering and ESD protection, portable application designers can reduce costs, bill of materials (BOM) component count, and board space.