Anti electromagnetic interference and ESD protecti

  • Detail

Anti electromagnetic interference and ESD protection technology in the design

at present, for many popular but high profit (especially flip type), color LCD, OLED display screen or camera module CMOS sensor and other components are connected to the baseband controller through flexible circuit or long-distance PCB, and these connecting lines will be interfered by the parasitic gsm/cdma frequency radiated by the antenna. At the same time, due to the introduction of high-resolution CMOS sensors and TFT modules, digital signals need to work at higher frequencies. These connecting lines will produce EMI interference like antennas or may cause ESD dangerous events

the above EMI and ESD interference will damage the integrity of the video signal, and even damage the baseband controller circuit. Driven by the trend of compact design, considering the design constraints such as circuit board space, high filtering performance on working frequency and preservation of signal integrity, discrete filters cannot provide any space savings for the solution to avoid further combustion of internal materials, and can only provide limited filtering performance for narrow-band attenuation. Therefore, most designers use integrated EMI filters at present

with the improvement of LCD display resolution in portable devices such as cameras, the transmission rate of video signals is also getting higher and higher, and the traditional filter schemes have slowly reached their technical limits. In those equipped with high-resolution display screen and embedded camera, the signal is transmitted from baseband ASIC to LCD and embedded camera through specific frequency (depending on the resolution). The higher the video resolution, the higher the frequency of data operation. For example, for a camera module with 300000 to 600000 pixels, the clock frequency is about 6 to 12Mhz. Therefore, it is recommended to select the filter (upper and lower) cut-off frequency within the range of 30 to 50MHz. With the improvement of the resolution to millions of pixels, the clock frequency has exceeded 60MHz, which requires the cut-off frequency of the filter to be as high as 300MHz

Figure 1: new filter unit structure (series resistance is 100 ohms, line capacitance is 20pF)

Figure 2: new RC filter S21 parameter curve

Figure 3: Yinglian electronic LC filter unit structure

facing these development trends of the industry, the traditional RC filter solution is reaching its limit. In order to meet the increasing demand of video signal and stronger anti ESD surge ability, Yinglian electronics has developed a new generation of EMI filter based on LC structure. This integrated LC filter structure can be used to provide a cut-off frequency of up to 350MHz, and can support data rates with clock frequencies exceeding 60MHz. At the same time, it can provide excellent filtering performance, and the attenuation characteristic is better than -25db in the frequency range of 800MHz to 2.5GHz. Figure 4 shows the S21 parameter index using the basic unit architecture of this filter. In addition to the filtering function, the integrated input TVs tube can also suppress air discharge up to 15kV. ESD impact length measurement method determines the wear amount according to the change of the normal size of the friction surface before and after the experiment, reaching the performance level required by IEC level 4 industrial standard

Fig. 4: S21 parameter curve of British electronic LC filter

low capacitance EMI filters um4411, um6411 and um8411 of British electronic support 4, 6 and 8-wire configurations, and each configuration includes PI type RC filter network with TVs tube on the side. The device adopts QFN package with 0.4mm pin spacing, which can provide more abundant design space for ultra-thin designers. Especially on the PCB layout, at present, the pin spacing of some display i/o connection bases is 0.4mm, so the EMI filter packaged with 0.4mm pin spacing DFN will help the system engineer layout. In terms of device selection, it should be reasonably selected according to the number of data channels. Figure 5 shows some advantages of using an 8-channel filter and two 4-channel filters on the distribution board; At the same time, the cost of selecting an 8-channel filter is also lower than that of two 4-channel filters

Figure 5 Effect of EMI filter pin spacing on the layout


Copyright © 2011 JIN SHI