Anti-electromagnetic interference and ESD protection technology in mobile phone design

Currently, for many popular mobile phones (especially clamshell phones), the color LCD of the mobile phone, the OLED display, or the camera module CMOS sensor are connected to the baseband controller through a flexible circuit or a long trace PCB. The cable will be disturbed by the parasitic GSM/CDMA frequencies radiated by the antenna. At the same time, due to the introduction of high-resolution CMOS sensors and TFT modules, digital signals are required to operate at higher frequencies, and these connection lines can cause EMI interference like the antenna or may cause ESD hazardous events.

Such EMI and ESD disturbances as described above will destroy the integrity of the video signal and even damage the baseband controller circuit. Driven by compact design trends, discrete filters cannot provide any space savings for the solution due to design constraints such as circuit board space, high filtering performance on mobile phone operating frequencies, and preservation of signal integrity, and can only provide for narrowband attenuation. With limited filtering capabilities, most designers today use integrated EMI filters.

With the increase in the resolution of LCD displays in portable devices such as mobile phones and cameras, the transmission rate of video signals is also getting higher and higher, and traditional filter solutions have slowly reached their technical limits. In mobile phones equipped with a high-resolution display and an embedded camera, the signal is transmitted from the baseband ASIC to the LCD and the embedded camera at a specific frequency (depending on the resolution). The higher the video resolution, the higher the frequency of data work. For example, for a camera module of 30 to 600,000 pixels, the clock frequency is approximately between 6 and 12 MHz. It is therefore recommended that the filter (upper and lower) cutoff frequency be selected within the range of 30 to 50 MHz. With the increase of resolution to several megapixels, the clock frequency has exceeded 60MHz, which requires the filter cutoff frequency up to 300MHz.

Faced with these trends in the mobile phone industry, traditional RC filter solutions are reaching their limits. In order to meet the increasing demand for mobile video signals and stronger anti-ESD surge capability, Infineon has developed a new generation of EMI filters based on LC structures. This integrated LC filter structure can be used to provide cut-off frequencies up to 350 MHz and can support data rates with clock frequencies in excess of 60 MHz. At the same time it can provide excellent filtering performance, better than -25dB attenuation in the 800MHz to 2.5GHz frequency range. Figure 4 shows the S21 parameter specification using this filter's basic unit architecture. In addition to the filtering function, the integrated input TVS tube also suppresses air discharge ESD surges up to 15kV, achieving the performance level required by the IEC61000-4-2 Level 4 industry standard.

Infineon's low-capacitance EMI filters, UM4411, UM6411, and UM8411, support 4, 6, and 8-wire configurations. Each configuration includes a PI-type RC filter network with a TVS tube. The device uses a 0.4mm pitch QFN package to provide designers with ultra-thin handsets with ample design space. Especially on the PCB layout board, some current display I/O connection bases have a 0.4mm pitch. EMI filters with a 0.4mm pitch DFN package will help system engineers to layout. The selection of the device should be based on the number of data channels to be reasonably selected. Figure 5 shows some of the advantages of using an 8-channel filter and two 4-channel filters on the board; an 8-channel filter is also used. It is also less expensive than two 4-channel filters.