TA0328 Technical article Display and camera ESD protection in 3G handsets Introduction The introduction of multimedia services in 3G phones is driving the increase in resolution of graphic displays and cameras. The interface between displays and camera modules to the baseband circuit or multimedia processor through the flex cable involves tough cabling constraints and EMI becomes a key concern. To address these issues, several display manufacturers, semiconductor and camera module maker companies have developed high speed serial bus technology to liberate phone designers from crowded and noisy parallel interface designs. Contents 1 The EMI constraints in clamshell phones . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1 Pixel format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2 Display and camera resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3 Required bandwidth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.4 Solutions to EMI constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.5 1.4.1 Larger bus width with higher number of data lines, and increased bus frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.4.2 Serial bus implementation with high data clock frequency. . . . . . . . . . . . 6 The benefits of the serial bus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2 The need for protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3 STMicroelectronics solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.2 Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.3 Order codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 December 2006 Rev 1 1/13 www.st.com The EMI constraints in clamshell phones 1 TA0328 The EMI constraints in clamshell phones The number of wires and the bus clock frequency are linked together and depend on the display and camera resolution. The bus clock frequency and the required bandwidth when using a parallel interface is defined by several key parameters: 1.1 ■ pixel format ■ display and camera resolution ■ number of wires (bus width) Pixel format The number of distinct colors that can be represented by a pixel depends on the number of bits per pixel (bpp). The maximum number of colors a pixel can have can be found by the following calculation: Number of colors = 2[color depth] The color depth being the way the red, green and blue colors are coded. As an example: ● 16 bpp (5,6,5 RGB), each pixel using two bytes ● 18 bpp (6,6,6 RGB), packed ● 18 bpp (6,6,6 RGB), loosely packed into three bytes ● 24 bpp (8,8,8 RGB), each pixel using three bytes Table 1. Commonly available color definition schemes Bits per pixel 1 2 (monochrome) 2 4 (CGA) 4 16 (EGA) 8 28 = 256 (VGA) 16 216 = 63536 (High Color, XGA) 24 224 = 16777216 (True Color, SVGA) 32 16,777,216 (True Color + Alpha Channel) Figure 1. 2/13 Number of colors Parallel interface TA0328 1.2 The EMI constraints in clamshell phones Display and camera resolution The display and camera resolutions are increasing drastically in order to cover the new scope of 3G phones capability such as TV on mobile, high resolution camera, video phones… Table 2. 1.3 Pixel geometry for common resolutions Resolution name Horizontal resolution Vertical resolution QQVGA 160 120 QCIF 176 144 QCIF+ 176 208 QCIF+ 176 220 QVGA 320 240 1/2 VGA 320 480 CIF 352 288 CIF+ 352 416 2/3 VGA 640 320 VGA 640 480 WVGA 800 480 SVGA 800 600 XVGA 1024 768 Required bandwidth Knowing the resolution, the color depth and the refresh rate, required bandwidth to transfer the data between the display (or camera) and the multimedia processor can be calculated: Bandwidth = Line x Column x fps x bits per pixel x overhead As an example, for a VGA resolution (640 x 480) with 18 bpp (262144 colors) and 60 fps, the required bandwidth is 640 x 480 x 18 x 60 x 1.5 = 381.15 Mb/s. Using an 8-bit parallel bus, the required bus clock is 41.5 Mb/s = 21 MHz. Figure 2. Display color bandwidth versus display resolution showing increasing interface throughput Bandwidth (Mb/s) 1200 1000 800 18 bpp color depth, 60 fps 600 400 200 0 QCIF+ QVGA 1/2 VGA 3/4 VGA 1/2 VGA SVGA 3/4 SVGA 1/2 XGA SVGA XGA WXGA 3/13 The EMI constraints in clamshell phones 1.4 TA0328 Solutions to EMI constraints To support the increased resolution of display and camera in mobile handset, two solutions can be implemented: 1.4.1 1. Larger bus width with higher number of data lines, and increased bus frequency 2. Serial bus implementation with high data clock frequency Larger bus width with higher number of data lines, and increased bus frequency In this case the EMI constraint becomes critical, especially for clamshell and flip phone. The high resolution color LCD and camera module are connected to the base board via a flexible cable or a long track PCB. The length of the cable is often fixed by the mechanical constraints (phone design, shape…), but is not impacted by the style of the I/O used. The cable is subjected to parasitic GSM bursts from the antenna. The fast edge rates and wide swings propagating over cables contribute to radiation emission. We have seen that the number of signals used in the cable varies with the performances targeted for the display and the camera. In some case the camera is also radiating and EMI is coupled into the cable and disturbs the ICs. Once more, in clamshell or slide phone, ESD events can damage the ICs via the hinge of the phone. To reduce EMI and ESD susceptibility as well as EMI emission, filters are implemented at the output of the ICs. Figure 3. 4/13 Clamshell Phone EMI constraints TA0328 The EMI constraints in clamshell phones Figure 4. Frequency response of EMIF06-VIDO1xx compared to discrete C- R- C filter dB 0 -10 -20 -30 -40 Discrete (C-R-C) Discrete T FilterFilter EMIF device EMIF06-VID01xx -50 F (Hz) -60 100k Figure 5. 1M 10M 100M 1G EMIF0x-1502Mx: EMI Filters and ESD protection implementation for camera parallel interface VDD EMI Filter & ESD Protection SDA SCL RST Sensor Controller Viewfinder FrontEnd JPEG Encoder DCLCK BB interface Image Sensor IMAGE PROCESSING EMIF04-1502M8 EMIF06-1502M12 EMIF08-1502M16 Horizontal synchro Vertical synchro I2C I2CBUS BUS(115kbs (115 khz) BB BBICIC VIDEO INPUTS FLASH MEMORY Hinge Customer expectations in term of performances, variety of available functions and compactness of the phone, mean that the wide spread discrete EMI filters solutions can no longer be implemented. The discrete solutions board space consumption as well as the poor filtering performances are no longer compatible with new generation phones requirements. For example, a discrete filter implementation for a VGA resolution display with 18 bpp and 60 fps using 8-bit parallel bus will require 8 resistors, 16 capacitors and 8 protection diodes consuming around 26 mm². In comparison, the same EMI filter using integrated IPAD technology from STMicroelectronics will consume less than 5.5 mm². The bus frequency should be in the range of 40Mbps (20MHz) and the cut-off frequency of the filter has to be at least 100MHz. This requires high EMI and ESD performance filters having low line capacitance. Table 3. below summarizes the the cut-off frequency and bandwidth compatibility of different filter solutions. 5/13 The EMI constraints in clamshell phones Table 3. TA0328 Cut-off frequency and bandwidth comparisions R Serial (Ω) Capacitance (pF) Cut-off frequency (MHz) Discrete filter 100 27 59 EMIF0x-1502Mx 150 7 150 EMIF0x-VID01F2 100 8.5 190 20 MHz clock compatibility Due to the parasitic inductances of the PCB tracks, the filtering performance using discrete filters will be degraded in high frequencies, making this solution difficult to implement in 3G phones using higher RF frequencies (see Figure 4.). 1.4.2 Serial bus implementation with high data clock frequency. When high resolution display and camera with high refresh rate is implemented, a serial interface between display and base unit IC is usually chosen. This solution offers several valuable benefits. 1.5 The benefits of the serial bus As the resolution of display and camera increase, the required data bandwidth in the handsets increases, so does the power required. In a clamshell phone, the major user of power in the flip is the display and camera module. Serial interfaces using Low Voltage Differential Signaling types are lowering the power needed to transfer high bandwidth data. Compared to parallel interfaces, serial interfaces offer the following benefits. Low pin count The serial bus can translate a wide, low-speed parallel data stream into a narrow high speed serial data stream. Reduced EMI and power consumption: I/Os are moving from a low-speed single ended technology to a high-speed differential technology, reducing EMI and power consumption. Standardized interfaces: Several IC makers are promoting their own serial interface protocol. Most are based on LVDS technology. 6/13 TA0328 Table 4. The EMI constraints in clamshell phones Comparisons of Camera Serial Interfaces for 3G mobile handsets Consideration CSI-1 CCP2 MDDI CSI-2 internal only internal only internal and external internal only Unidirectional Unidirectional Half-duplex Bidirectional Unidirectional Embedded codes I2C control I/F Embedded codes I2C control I/F Packet based Packet based I2C control I/F Arbitrary data No No Yes Yes Scalable No No 1,2, 4, or 8 lanes 1, 2, 3, or 4 lanes Speed 1 - 208 Mb/s 1 - 650 Mb/s ~400 Mb/s per lane 80 - 1000 Mb/s per lane Vswing 200 mV 150 mV 400 mV 200 mV 1.8 V 1.8 V 1.8 V 1.2 V Wires 6 6 4 (Min. configuration) 6 (Min. configuration) EMS Partial source termination Partial source termination No source termination Fully terminated Scope Type Protocol Min Vdd Figure 6. MDDI Topology Lower Clamshell Hinge Upper Clamshell Analog Earpiece Audio Power Data+ DataStrobe+ Strobe- Power DSILC6-4P6 MDDI Data (Host) MDDI Strobe (Host) Base band IC GND GND MDDI Client & LCD Controller Chip (With Frame Buffer) PRIMARY LCD SECONDARY LCD 7/13 The need for protection 2 TA0328 The need for protection With clamshell or slide phones, ESD events can damage the ICs via the hinge of the phone. To reduce ESD susceptibility, low capacitance ESD protection must be implemented at the output of the ICs, as close as possible to the display and camera connector. The data frequency on the bus needs low capacitance ESD protection. Indeed, for frequencies above 400 Mbps, and taking into consideration the parasitic capacitance of the cable routing, it is considered that the capacitance of the ESD protection should not exceed 3 pF. It has been estimated that customers may find that 90% of ESD failures result in junction damage or burn-out or conductor / resistor fusing. Figure 7. ESD hazards in clamshell handsets Figure 8. Oxide dielectric breakdown Hinge ESD HAZARD Photo of a junction short Photo of a fused metal line The cost of such damage is high: 8/13 ● Engineer time, including travel time and expenses ● Rework to assemblies ● Cost of replacement parts ● Customer service staff costs ● Additional facility costs ● Customer dissatisfaction, loss of reputation and possible lost future sales TA0328 3 STMicroelectronics solution STMicroelectronics solution STMicroelectronics has introduced a new family of low capacitance ESD protection devices providing up to 20 kV ESD protection (IEC61000-4-2) and capacitances as low as 1.8 pF from I/O to Ground with less than 2% tolerance. The DSILC6-4P6/F2 belongs to this product family and is specifically intended to address Display Serial Interface ESD protection. 3.1 Features ● 1.8 pF max line capacitance for high speed serial interface compliance ● 0.9 pF max line to line ● 4-line protection in one package for high integration ● VBR = 6.1 V ● No insertion loss to 2.0 GHz ● Low leakage current ● Flip-Chip and Micro Package for ultra low space consumption Figure 9. Example of ESD protection implementation for MDDI bus Lower Clamshell Hinge Upper Clamshell Analog Earpiece Audio Power Power DSILC6-4P6 Data+ DataStrobe+ Strobe- MDDI Data (Host) MDDI Strobe (Host) Base band IC GND 3.2 GND MDDI Client & LCD Controller Chip (With Frame Buffer) PRIMARY LCD SECONDARY LCD Benefits ● MIPI, MDDI, MPL… compliance ● PCB Area < 1.45 mm2 to 2.56 mm2 ● < 0.6 mm height 9/13 STMicroelectronics solution TA0328 Figure 10. Remaining voltage after ESD positive surge 5 V/div Vin Vout 200 ns/div Figure 11. Remaining voltage after ESD negative surge 5 V/div Vin Vout 200 ns/div 10/13 TA0328 STMicroelectronics solution Figure 12. Frequency response 0.00 S21(dB) -5.00 -10.00 -15.00 F(Hz) -20.00 100.0k 3.3 1.0M 10.0M 100.0M 1.0G Order codes CI/O-GND MAX VBR CI/O-I/O MAX Part number VRM DSILC6-4P6 5 V 6.1 V 1.8 pF 0.9 pF SOT666 DSILC6-4F2 5 V 6.1 V 2 pF 1.2 pF Flip-Chip VR = 1.65 V, VCC = 4.3 V, VR = 1.65 V, VCC = 4.3 V, VOSC = 400 mV, F = 1 MHz VOSC = 400 mV, F = 1 MHz Package 11/13 Conclusion 4 TA0328 Conclusion The trend to higher resolution displays and camera phones imposes either wider parallel interfaces or high speed serial interfaces. Whatever the solution implemented by mobile phone makers, STMicroelectronics provides highly integrated ESD and EMI solutions. For high resolution displays and camera clamshell phones, the ESD susceptible hinge drives the need for low capacitance ESD protection. The DSILC6-4xx fully answers these needs. It is tailored for high speed display and camera serial interfaces where very low capacitance ESD protection is needed to support high bandwidth. 5 12/13 Revision history Date Revision 08-Dec-2006 1 Changes Initial release TA0328 Please Read Carefully: Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any time, without notice. All ST products are sold pursuant to ST’s terms and conditions of sale. Purchasers are solely responsible for the choice, selection and use of the ST products and services described herein, and ST assumes no liability whatsoever relating to the choice, selection or use of the ST products and services described herein. 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