cd00119493

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
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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
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