Choosing the Appropriate ESD Protection Device for Displayport

SI07-12
Surging Ideas
TVS Diode Application Note
PROTECTION PRODUCTS
DisplayPort Protection
VESA (Video Electronic Standard Association) has set
forth DisplayPort as the new license-free digital
interface standard used primarily between a computer
and its display(s) or a computer and a home theater
system. The DisplayPort consists of a main link,
auxiliary channel and a hot plug detect as shown in
Figure 1. The DisplayPort connector supports 1 to 4
data pairs as the main link. These data pairs are
referred to as lanes in the DisplayPort Specification.
Unlike HDMI, the clock signal is embedded in these
data pairs. Both audio and video can be transmitted
on these data pairs. DisplayPort supports video
speeds of 1.62Gbps and 2.7Gbps. The auxiliary
channel serves as the main link management and
device control. It runs at a constant 1Mbps rate.
Due to the high-speed nature of the DisplayPort main
link, choosing the appropriate protection device is
crucial to maintain signal integrity on both the
transmitter (source) and receiver (sink) side of the
system. The DisplayPort Compliance Test Specification
will test for signal integrity and good interoperability.
Designing with a protection device that offers superior
protection to the IEC61000-4-2 specification with
minimal effects on the high speed differential pairs is
not easily accomplished. The electrical and
mechanical properties of the protection device can
impact the high speed signal. Both effects will be seen
as a discontinuity on the balanced differential pairs
that will result in an impedance change or mismatch.
Changes or mismatches will result in signal
degradation, reflection and loss.
The DisplayPort connector is an external port that is
exposed to the user. All these exposed signal pairs are
vulnerable to electrostatic discharge (ESD) either
directly from a user or from plugging a charged cable
into the port. In order to ensure the functionality of
this port, the DisplayPort needs to be ESD hardened to
the IEC61000-4-2 ESD standard as indicated in the
DisplayPort Specification. A level 4 discharge would
require withstanding a ±15kV air and ±8kV contact
discharge. In order to meet this stringent standard,
external protection devices are required at the port
entrance.
Semtech RClamp0524P provides ESD protection in
excess of IEC61000-4-2 level 4 without adversely
affecting signal integrity of the DisplayPort high speed
differential signal. The RClamp0524P has a typical
capacitance of only 0.30pF between I/O pins. This
reduces its electrical affect on the high speed signal
and allows it to be used on circuits operating in excess
of 3GHz without signal attenuation. Its circuit diagram
is shown in Figure 2.
The RClamp0524P is housed in a SLP2510P8 package
designed specifically to reduce its affect on the high
Source Device
Sink Device
Main Link
DisplayPort
DisplayPort
TX
TX
(4 High Speed Lanes)
AUX CH
DisplayPort
DisplayPort
RX
RX
Hot Plug Detect
Figure 1 - Make-up of DisplayPort
Revision 10/24/2007
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SI07-12
Surging Ideas
TVS Diode Application Note
PROTECTION PRODUCTS
Pin 1
Pin 2
Pin 4
routed as a tightly coupled differential pair at
0.5mm pitch for good common mode rejection. It also
allows different lanes to be routed as a loosely
coupled differential pair at 1.0mm pitch between the
pairs for reduced crosstalk. Figure 4 shows an example of the flow through trace routing of the high
speed differential signal through the RClamp0524P
straight out from the connector pads.
Pin 5
3, 8
The combination of small size, low capacitance, and high
level of ESD protection makes the RClamp0524P a
superior solution to protect the sensitive DisplayPort
interface. Figure 5 shows the application schematic
recommendation for protecting the main link, auxiliary
channel, hot plug and power of the DisplayPort interface. To verify signal integrity of a DisplayPort source,
an eye mask specification is required by the DisplayPort
Specification. This eye mask definition is shown in
Figure 6. Table 1 shows the vertices for a reduced bit
rate of 1.62Gbps, and Table 2 shows the vertices for
the high bit rate of 2.7Gbps. Eye pattern testing of a
DisplayPort protected by the RClamp0524P at 1.62Gbps
and 2.7Gbps was conducted and the results are shown
in Figures 7 and 8. In both cases the eye mask was not
violated.
Figure 2 - RClamp0524P Circuit Diagram
10
9
8
7
6
1
2
3
4
5
Figure 3 - Pinout of RClamp0524P
speed differential signals. The SLP2510P8 measures
2.5 x 1.0 x 0.58mm as shown in Figure 3 and is
designed for easy PCB layout by allowing the traces to
run straight through the device. Trace layout is crucial
in high speed signal layout design because bends and
turns result in signal attenuation as a loss and reflection. The RClamp0524P is also designed so traces
within a lane of the DisplayPort connector can be
Conclusion
Based on data presented on the previous pages of
this application note, it can be concluded that the
RClamp0524P is a solution that offers superior
protection to the IEC61000-4-2 level 4 while
maintaining the signal integrity of DisplayPort.
RClamp0524P
Loosely Coupled (1.0 mm pitch)
for reduced Crosstalk
Tightly Coupled (0.5mm pitch) for
Good Common Mode Rejection
Figure 4 - Example of Straight flow-through layout of RClamp0524P
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SI07-12
Surging Ideas
TVS Diode Application Note
PROTECTION PRODUCTS
DISPLAY PORT
CONNECTOR
Ohm
µ
Figure 5 - DisplayPort Protection Application Schematic
 2007 Semtech Corp.
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Figure 6 - Eye Mask at the Source Connector Pins
Point
A0
Voltage(V)
1
0.210
0.000
2
0.355
0.140
3
0.500
0.175
4
0.645
0.175
5
0.790
0.000
6
0.645
-0.175
7
0.500
-0.175
8
0.210
-0.140
Table 1 - Mask Vertices for Reduced Bit Rate (1.62Gbps)
Point
A0
Voltage(V)
1
0.127
0.000
2
0.291
0.160
3
0.500
0.200
4
0.709
0.200
5
0.873
0.000
6
0.709
-0.200
7
0.500
-0.200
8
0.291
-0.160
Table 2 - Mask Vertices for High Bit Rate (2.7Gbps)
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SI07-12
Surging Ideas
TVS Diode Application Note
PROTECTION PRODUCTS
Figure 7 - DisplayPort at 1.62 Gbps with RClamp0524P Protection
Figure 8 - DisplayPort at 2.7Gbps with RClamp0524P Protection
 2007 Semtech Corp.
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