With the continued development of the wireless market, the next generation of mobile phones will have more functional characteristics, such as with multiple color screens (there are at least two color screens per cell phone) and high-resolution cameras above millions of pixels.
Still being driven by the recruitment design, achieving high-resolution LCDs and cameras will make designers face a variety of challenges, one of which is the sensitivity of these new modules for electromagnetic interference (EMI).
For many popular mobile phones (especially flip-type mobile phones), color LCDs or camera CMOS sensors are connected to the baseband controller by connecting the flexible or long trace PCB between the two main parts of the mobile phone (up and down).
On the one hand, the cable is subject to the interference of the parasitic GSM / CDMA frequency radiated from the antenna. On the other hand, due to the introduction of the high-resolution CMOS sensor and TFT module, digital signals operate on higher frequencies, so that the cable will generate EMI / RFI like antennas or may result in ESD hazardous events.
In summary, in both cases, all of these EMI and ESD interference will damage the integrity of the video signal, and even damage the baseband controller circuit.
To suppress these EMI radiation and ensure normal data transmission, you can consider implementing several filter solutions, which can be implemented by using a discrete blocking filter or integrated EMI filter.
EMI and ESD noise suppression method
If you take into account high-filter performance on board space, mobile phone operating frequency, and save signal integrity, etc., the currently known solution is reaching its technology limit.
Discrete filters cannot provide any space savings to the solution, but only limited filter performance for narrowband attenuation, so most designers are currently considering integrated EMI filters.
In a mobile phone with high resolution LCD and an embedded camera, the signal is transmitted from the baseband ASIC from the baseband ASIC from the baseband ASIC from the baseband ASIC at a camera through a specific frequency.
The higher the video resolution, the higher the frequency of data work. To date, general data works in frequencies of approximately 6 to 20 MHz, and the resolution competition will cause camera module manufacturers to increase this frequency to 40-60 MHz.
In order to adapt to the increase in data rates and do not interrupt the video signal, the designer must choose a filter that considers the theoretical proposal low capacitance, ie the filter cutoff frequency (1 / 2πrc) must be approximately 5 times the clock frequency.
In the current wireless terminal, the clock frequency is approximately between 6 and 12 MHz for a camera module of 30 to 600,000 pixels. Therefore, it is recommended to select the filter (up and down) cutoff frequency within 30 to 50 MHz. Many filter solutions follow this theoretical recommendations, but with the increase in resolution and the clock frequency exceeds 40 MHz, the filter cutoff must be in the 200MHz range. Therefore, it is foreseen that some filter solutions are reaching their limits.
The test gives a comparison of several filter capacitance values and cutoff frequencies, and clock compatibility. This indicates that the low capacitive filter is the most suitable solution for high frequency, high speed data signal transmission.
However, the designer knows that there is an unsolicable compromise between the decay characteristics of the filter capacitance value and the frequency of GSM / CDMA. The low capacitance structure affects the high frequency performance of the filter, and most of the low capacitance filters cannot provide superior to -25dB attenuation performance at 900 MHz. The effect of EMI filter capacitor on GSM frequency attenuation is shown.
In addition to the influence of filtering performance, low capacitive filters also affect ESD performance. Considering that lower diode capacitors can significantly reduce ESD surge capabilities, it is very challenging between good attenuation, ESD performance and low capacitance filter structure.
Low capacitance EMI filter after performance improvement
In order to meet the contradiction of the low capacitive filter but at the same time, it maintains high filter performance, a semiconductor company has developed a new generation of EMI filters with high frequency attenuation characteristics and ultra-low capacitance structures in 900 MHz frequently.
These new EMI filters based on iPad technology (integrated active, passive devices) use standard PI filter structures with integrated ESD protection. A base filter unit configuration with a series resistance and a capacitor is shown.
This new low capacitance structure is used to provide a cutoff frequency within a 200 MHz range that supports a data rate of more than 40MHz at clock frequencies.
Although the diode capacitance has been greatly reduced to 8.5pf, it provides excellent filtering properties, namely the attenuation characteristics in the frequency range of approximately 900 MHz are better than -35dB.
The figure shows the S21 parameter indicator using this filter basic unit architecture. The figure shows the attenuation characteristics of 35 dB in the 900 MHz frequency, which is an unprecedented performance of the EMI filter through the 17PF wire capacitance.
In addition to filtering, the integrated input Zener diode can also suppress an air discharge ESD impact up to 15kV, reaching the performance level required by IEC61000-4-2 Level 4 Industrial Standards.
High speed data compatibility
In order not to disturb the video signal, the new low capacitor filter uses an optimized linear capacitance value to support the clock frequency above 40MHz.
This structure has risen on the data signal, and the falling edge has only a small impact, and the device is input, and there is almost no delay between output.
Simulation of the input RT (10-90% rising) and FT (10-90% falling edge) with the maximum 2.8V, 1 ns, and the results show that the delay caused by the filter (the difference between the output and the input signal) No more than 1 ns. It can be affirmed, even for high-resolution LCD or camera applications, the integrity of the data can be fully maintained.
The figure shows that the 3V video signals working on 40 MHz frequencies are compared to the transmission of high and low capacitive filters, respectively. It can be found that the delay caused by the high capacitance structure is 5 to 6 times the low capacitance structure. In this case, the signal output voltage cannot be properly received.
High integrated solution
Compared to discrete design, a flip chip encapsulation type integrated with a laminated point can simplify the PCB layout and saves up to 80% of the board area.
The results show that the line integration rate (PCB area / line number) is approximately 0.6. This means that these new filters can provide EMI functions and ESD protection per line to provide EMI functionality and ESD protection per-line PCB area.
It is recommended that the new filter series uses 4, 6 and 8 "PI" configuration to provide design flexibility and meet most high-speed data line design requirements. Its PCB area is 2.4mm2, 3.7mm2 and 5.0mm2, so it is almost completely used in traditional SOT323 plastic packaging.
Semiconductor's new low capacitance EMI filters support 4, 6, and 8-wire configuration, each configuration contains a RC filtering network with Zener diodes. 100 ohm series resistance and 17PF linear capacitance values are used to reach the minimum 30 dB at 0.8 MHz to 2 GHz. The low capacitance of the device means they can be used for LCD displays and camera sensors for the next generation clock frequency over 40MHz.
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