EMI and ESD noise suppression design solutions 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 valueswith 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.
Improved low-capacitance EMI filter
In order to meet the contradictory requirements of implementing a low-capacitance filter while maintaining high filtering performance, a semiconductor company has developed a new generation of EMI filters with high-frequency attenuation characteristics at 900 MHz and an ultra-low capacitance structure.
These new EMI filters based on IPAD technology (integrated active and passive components) use a standard PI filter structure with integrated ESD protection. The figure shows a basic filter unit configuration with a series resistor and capacitor.
This new low-capacitance structure is used to provide a cut-off frequency in the 200MHz range and can support data rates with clock frequencies in excess of 40MHz.
Although the diode capacitance has been greatly reduced to 8.5pF, it can provide excellent filtering performance, that is, attenuation characteristics better than -35dB in the frequency range of about 900MHz.
The figure shows the S21 parameter index using the basic unit architecture of this filter. The figure shows 35dB attenuation at 900MHz. This is an unprecedented performance achieved by integrating an EMI filter with a 17pF line capacitor.
In addition to the filtering function, the integrated input Zener diode can also suppress the ESD impact of air discharge up to 15kV, reaching the performance level required by the IEC61000-4-2 level 4 industry standard.
High-speed data compatibility
In order not to disturb the video signal, the new low-capacitance filter is designed with optimized line capacitance values to support chipsets with clock frequencies higher than 40MHz.
This structure has only a small effect on the rising and falling edges of the data signal, and there is almost no delay between the input and output of the device.
Using a maximum 2.8V, 1ns signal to simulate the input Rt (10-90% rising edge) and Ft (10-90% falling edge), the results show that the delay caused by the filter (the difference between the output and input signals) No more than 1ns. It is certain that the data integrity will be completely maintained even for high-resolution LCD or camera applications.
The figure shows a comparison of the transmission of a 3V video signal operating at a frequency of 40MHz through high and low capacitance filters. It can be found that the delay caused by the high-capacitance structure is 5 to 6 times that of the low-capacitance structure. In this case, the signal output voltage cannot be received correctly.
Highly integrated solutions
Compared with discrete designs, the use of integrated EMI filters designed as flip-chip packages with stacked bumps can simplify PCB layout and save up to 80% of board area.
The results show that the line integration rate (PCB area / number of lines) is approximately 0.6. This means that these new filters can take up 0.6mm2 of PCB area per line to provide EMI functionality and ESD protection.
The new filter series is recommended to use 4, 6, and 8 "PI" line configurations to provide design flexibility and meet most high-speed data line design requirements. Its PCB area occupies 2.4mm2, 3.7mm2 and 5.0mm2, respectively, so it can almost completely use the traditional SOT323 plastic package.
Semiconductor's new low-capacitance EMI filters support 4, 6, and 8-wire configurations, and each configuration includes an RC filter network with Zener diodes on the side. A series resistance of 100 ohms and a line capacitance of 17 pF are used to achieve a minimum 30dB attenuation in the 0.8MHz to 2GHz range. The low capacitance of the devices means that they can be used in next-generation LCD displays and camera sensors with clock frequencies exceeding 40MHz.
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