What is the EMI and ESD noise suppression design solution based on LCD or camera?
As the video resolution of LCD and camera in mobile phones is higher, the frequency of data operation will exceed 40MHz. For the suppression of wireless EMI and ESD, traditional filter solutions have reached their technical limits. To accommodate the increased data rate without interrupting the video signal, designers can choose the new low-capacitance, high-filtering EMI filters discussed in this article.
As the wireless market continues to develop, the next generation of mobile phones will have more features, such as with multiple color screens (each phone has at least two color screens) and high-resolution cameras with megapixels and more.
Still driven by the trend towards compact designs, implementing high-resolution LCDs and cameras will present designers with multiple challenges. One of the main design considerations is the sensitivity of these new modules to electromagnetic interference (EMI).
For many popular mobile phones (especially clamshell phones), color LCD or camera CMOS sensors are connected to the baseband controller through a flexible or long wiring PCB connected between the two main parts of the mobile phone (upper and lower).
On the one hand, the connection line is subject to interference from parasitic GSM / CDMA frequencies radiated by the antenna. On the other hand, due to the introduction of high-resolution CMOS sensors and TFT modules, digital signals operate at higher frequencies, so that the connection line will generate EMI / RFI like an antenna or may cause dangerous events like ESD.
In short, in the above two cases, all these EMI and ESD interferences will destroy the integrity of the video signal and even damage the baseband controller circuit.
To suppress these EMI emissions and ensure normal data transmission, consider implementing several filter solutions, which can be achieved by using discrete RC filters or integrated EMI filters.
EMI and ESD noise suppression methods
If you take into account design constraints such as board space, high filtering performance on the phone's operating frequency, and preservation of signal integrity, currently known solutions are reaching their technical limits.
Discrete filters cannot provide any space savings for the solution, and they can only provide limited filtering performance for narrow-band attenuation, so most designers are currently considering integrated EMI filters.
In phones equipped with high-resolution LCDs and embedded cameras, signals are 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 more frequently the data works. So far, general data has been working at frequencies of approximately 6 to 20 MHz, and the race for resolution will also encourage camera module manufacturers to continue to increase this frequency to 40-60 MHz.
In order to adapt to the increase of the data rate without interrupting the video signal, the designer must choose a low-capacitance filter that takes into account the theoretical recommendations, that is, the filter cutoff frequency (1 / 2πRC) must be about 5 times the clock frequency.
In current wireless terminals, for camera modules with 300,000 to 600,000 pixels, the clock frequency is between approximately 6 to 12 MHz. Therefore, it is recommended to select the filter (upper and lower) cutoff frequency in the range of 30 to 50MHz. Many filter solutions follow this theoretical recommendation, but as resolution increases and clock frequencies exceed 40 MHz, the filter cutoff frequency must be in the 200 MHz range. Therefore, it is foreseeable that some filter solutions are reaching their limits.
The test gives the comparison of several filter capacitor values with the cutoff frequency, and the clock compatibility. This shows that the low-capacitance filter is the most suitable solution for high-frequency, high-speed data signal transmission.
However, the designer knows that there is an unsolvable trade-off between the value of the filter capacitor and the attenuation characteristics at the GSM / CDMA frequency. The low-capacitance structure will affect the high-frequency performance of the filter, and currently most low-capacitance filters cannot provide attenuation performance better than -25dB at a frequency of 900MHz. The effect of EMI filter capacitors on GSM frequency attenuation is shown.
In addition to affecting filtering performance, low-capacitance filters also affect ESD performance. Considering that lower diode capacitance can significantly reduce ESD surge capability, it is extremely challenging to find the best compromise between good attenuation, ESD performance, and low capacitance filter structure.
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