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.

As far as EMI/RFI filtering is concerned, 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. [page]

EMI filtering + ESD protection solutions for specific applications in portable devices

Mobile phones occupy an important position in the portable device market. Naturally, filters with integrated ESD protection are applied in many places in mobile phones. For mobile phone applications, in addition to LC and RC filters, common mode choke EMI filters are also commonly used. This filter helps reduce parasitic inductance and provides better common mode filtering. The bandwidth spectrum of these mobile phone filters is shown in Figure 3.

Figure 3: Bandwidth spectrum of mobile phone filter types.

Different mobile phone applications require different filters. Generally speaking, the higher the data rate of the application, the smaller the total capacitance of the filter line should be. For mobile phone applications with relatively low data rates such as audio lines, the standard capacitance of EMI filters in the range of several hundred pF is sufficient to provide excellent filtering performance and minimal signal interference.

Among them, in audio applications such as speakers, microphones, audio jacks/headphones, LC or RC type filters can be selected, such as ON Semiconductor's NUF2441FCT1G, NUF2450MUT1G, NUF2114MNT1G, NUF2116MNT1G and NUF4220MNT1G, etc. These filters all integrate ESD protection functions to protect 2 to 4 audio lines, and the diode capacitance is between tens of pF and 240 pF. Taking NUF2441FC as an example, this LC filter provides EMI filtering and IEC61000-4-2 Level 4 ESD protection for 2 audio lines. It uses a flip-chip package to replace 2 inductors, 4 capacitors and 4 TVS diodes with a single IC, helping to save costs and reduce the space occupied by the circuit board. NUF2450MUT1G is also a two-line LC type EMI filter and ESD protection device, providing greater than -30 dB attenuation in the frequency range of 800 MHz to 5.0 GHz; this device uses the space-saving uDFN 1.2×1.8×0.5 mm ultra-small package, providing a cutoff frequency of 20 MHz and extremely small line impedance, which is very suitable for audio applications that require low passband attenuation. NUF2114MNT1G is a two-line RC type EMI filter and ESD protection device, using DFN8 package, providing greater than -30 dB attenuation in the frequency range of 900 MHz to 3.0 GHz, and also replacing up to 10 discrete components with a single IC, helping to reduce costs and save space.

In mobile phone applications, data lines such as displays and cameras have higher bandwidth than audio, and RC filters can be used, such as ON Semiconductor's NUF40xx, NUF60xx and NUF80xx series devices in uDFN packages. In addition, ON Semiconductor has also launched the X3 series RC filters in extremely small WDFN/uDFN packages for data line filtering applications, including NUF4310MN and NUF4110MN. This new series of 4-channel RC filters has a line capacitance of only 17 pF at 2.5 V and integrates 40 discrete components to provide an extremely small footprint (1.0×1.4×0.75 mm/1.0×1.6×0.75 mm), very low impedance (100 Ω) and industry-leading ESD protection (IEC61000-4-2 Level 4). It supports bandwidths up to 120 Mbps and is suitable for applications such as high-resolution camera modules and displays in portable products such as mobile phones, mobile interconnect devices (MIDs), portable media players (PMPs), digital cameras, laptops or netbooks.

In data applications such as the high-resolution display screen and camera interface of the mobile phone mentioned above, high-speed LC filters, such as ON Semiconductor's NUF2900MN, can also be used. Compared with traditional RC filters, high-speed LC filters have higher cutoff frequencies, greater broadband attenuation, and lower insertion loss. For example, the typical cutoff frequency of NUF2900MN is 350 MHz, and it provides greater than -30 dB attenuation at frequencies from 800 MHz to 6.0 GHz, and its insertion loss performance is also more advantageous, see Figure 4.

Figure 4: Comparison of the response performance of a typical RC EMI filter and a high-speed LC EMI filter (NUF2900).

It is worth mentioning that there are more and more high-bandwidth applications in mobile phones, larger display screens with higher resolutions, and limited space, which drives parallel data to serial data. The solution is to use low-voltage differential signaling (LVDS) to serialize MDDI, MPPI, USB 2.0, and uSerDes data, and accordingly use common-mode choke (CMC) EMI/RFI filters to pass high-bandwidth differential signals while filtering out unwanted common-mode EMI/RFI signals. In this regard, ON Semiconductor's NUC2401MN high-speed serial data filter can be used. This is the industry's first CMC filter that integrates ultra-low capacitance (0.8 pF) ESD protection and common-mode filtering technology. It uses a small 2.0×2.2 mm DFN package. The single component is suitable for a variety of high-speed designs such as USB2.0 (480 Mbps), IEEE1394 (400 Mbps), MDDI (up to 550 Mbps), MIPI (up to 1.0 Gbps) and HDMI 1.2 (up to 1.32 Gbps per channel). It eliminates common-mode noise for these applications and provides pure data streams.

Figure 5: The integrated CMC filter NUF2401MN with ultra-low capacitance ESD protection is used in high-speed applications such as USB2.0.

It is worth mentioning that compared with silicon competing devices with the same capacitance below 1 pF, NUC2401MN provides better ESD clamping performance, which is very suitable for EMI filtering and ESD protection applications of high-speed data lines.

Summarize

As the data rate and clock frequency of portable devices such as mobile phones increase, they increasingly need high-performance EMI filtering and ESD protection solutions. As a global leading supplier of high-performance, energy-efficient silicon solutions, ON Semiconductor provides new integrated EMI filters that provide excellent EMI/RFI filtering performance while also providing ESD protection that complies with the IEC61000-4-2 Level 4 standard. A single IC replaces 10 or even dozens of discrete components, making it very suitable for audio and data line applications in portable applications such as mobile phones.