SI05-03 - Semtech

SI05-03
Surging Ideas
TVS Diode Application Note
PROTECTION PRODUCTS
Layout Guidelines for adding ESD Protection in HDMI
Receiver Applications
capacitance) on the lines will have a larger affect on the
impedance of the line with a faster TDR measurement
pulse risetime. If a protection device with a typical
differential capacitance value of 1.5pF was added to
the 100Ω differential transmission lines without any
compensation, the differential impedance of the lines
may decrease by 55Ω or more at the position of the
protection device depending on the risetime of the test
pulse. The measurements in this application note are
made at a 200ps risetime.
The High Definition Multimedia Interface (HDMI) video
signals are transmitted on very high speed differential
pairs. These lines are susceptible 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, consumer
electronics manufacturers require that HDMI ports be
ESD hardened, often to the IEC61000-4-2 ESD standard.
A level 4 discharge would require withstanding a ±15kV
air and ±8kV contact discharge. In order to meet this
stringent standard, external protection devices at the
port entrance are required.
The idea behind compensating for the added CJ is best
described by Dr. Howard Johnson’s “Potholes” analogy.
The idea is to reduce the effect of the pothole by filling
it with a rock that is approximately the size of a pothole.
The results may not completely negate the effect but it
can be reduced to a tolerable amount. Figure 1 shows
a transmission line with the added capacitance of the
protection device labeled as C(TVS). Equations 1 and 2
can be used as a good means of determining if it is even
possible to compensate for the additional capacitance
presented by the added protection component. The
equations are for common mode impedance while the
HDMI application specifies the differential impedance
of a transmission line pair. It is common, for example,
to design Z0 to be 50Ω in order to achieve the 100Ω
differential impedance HDMI requirement. However,
this is not necessarily true especially as high speed
differential traces are tightly coupled for good common
mode reject. To determine the actual dimensions of
traces, dielectrics thickness, trace spacing, etc., the
PCB layout software will need to include a controlled
impedance calculator add-on option. Layout software
like Mentor Graphic’s “Expedition” or Cadence’s “Allegro”
come equipped with an impedance calculator. However,
a designer should still defer to the PCB manufacturer’s
software and calculation, because they have their own
design rules, tolerances and constraints. Many PCB
manufacturers use “Polar” as their controlled impedance
calculator because of its accuracy.
Semtech offers very low capacitance protection devices
for high speed applications. However, adding even a
small amount of capacitance across the differential
pair can cause the impedance of the differential
pair to drop as much as 40Ω depending on the
measurement method. This is not desirable because
HDMI sink (receiver) ports are required to maintain a
differential impedance of 100Ω ±15% on each of the
four differential pairs, per the HDMI Compliance Test
Specification (CTS). Some compensation becomes
necessary to remain within the HDMI CTS impedance
requirement for a sink device. Sink ports include
applications such as LCD-TV, PTV and HDTV. Source
(transmit) ports include applications like Set-top-box
and DVD players and are not subject to the HDMI CTS
impedance requirement.
In addition to protection, filtering for EMI suppression
is often needed. Taking advantage of the cancellation
effect of the EMI filter inductance and/or by using some
board layout techniques to increase the impedance
of the differential pairs, compensation of the TVS
capacitance can be done. This application note will
outline these methods that will essentially cancel the
effects of the added capacitance and inductance of
the ESD/EMI protection components. Examples will be
provided based on the uses of Semtech RClamp0502B
and RClamp0544M ESD protection devices.
To use the following method, a designer may start
by making a board with 50Ω common mode traces
that should result in a differential impedance of
approximately 100Ω. The designer can then measure
the amount of drop caused by the addition of the
imperfection (added protection device and filtering
on the lines). Z1 will then need to be this measured
impedance added to Z0. Since the HDMI impedance
The HDMI CTS specifies that the impedance of the
receiver HDMI port be measured using a Time Domain
Reflectometry (TDR) method that utilizes a pulse
with a risetime of <=200ps. Imperfections (added
Revision 08/22/2006
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SI05-03
Surging Ideas
TVS Diode Application Note
PROTECTION PRODUCTS
PROTECTION PRODUCTS
typical differential capacitance of the RClamp0502B
is 0.6pF. Due to this low differential capacitance, it
was determined that the RClamp0502B on the HDMI
signals can still remain within the HDMI CTS impedance
requirement when minor layout compensation are
made. If a common mode choke is normally used in the
HDMI design, some considerations need to be made to
ensure that the impedance on the high side does not
exceed the HDMI CTS impedance requirement or a more
capacitive device should be used (RClamp0504M). The
capacitance of the RClamp0502B is so low that it may
not necessarily cancel the added inductance of the
common mode choke.
Figure 1 - Compensation of C(TVS) with trace length
Equation 1:
 Z0CTVS  k 
x=
 2

 τ  k + 1 
k=
Four Layer Evaluation Board Results of the
RClamp0502B
Z1
Z0
The “SC-75 HDMI EVAL Rev D” evaluation board
was designed with minor impedance adjustments
to compensate for the RClamp0502B low capacitive
loading to keep the differential impedance of the
HDMI signal traces within the 100Ω ± 15% HDMI CTS
requirement. Table 1 shows the summary of the board
and trace parameters. The TDR test results in Figure
2 shows were well within the HDMI CTS impedance
Equation 2:
Z0 is the surrounding transmission line.
k defines the unloaded impedance of the the
adjusted segment
Z1 is the impedance needed to compensate
for the add C(TVS)
τ is the effective delay of the adjusted
segment which is 180ps FR4.
x is the length of the adjusted segment and
will be given in inches if is given in ps and
C(TVS) in pF.
HDMI SC-75 Rev D Evaluation Board
RClamp0502BM with layout and board compensation
measurement is made differentially, Z1 must be the
amount of measured impedance drop divided by
two plus the Z0. Knowing how much Z1 is needed to
compensate for the added capacitance of the line,
k and x can be found. A designer should give this Z1
impedance requirement to the PCB manufacturer to
determine if the layout parameters needed to achieve
this impedance can be reliably produced.
Number layers
4
Copper thickness
1.5 oz finished thickness
Board thickness
0.062”
Dielectric thickness
between layer 1 and Gnd
0.010”
In order to perform an empirical evaluation of these
methods, Semtech has designed several boards of
varying layouts and measured the results. The boards
were designed specifically for the RClamp0504M, the
RClamp0502B, and the RClamp0514M protection
devices. The results of these evaluations are given in
the following section.
Trace pitch
(1.0mm) or 0.0393”
Trace thickness
0.004” for 0.080” before
and after the RClamp0402B
Trace thickness to achieve 0.013”
50 Ω nominally
Evaluation Board Results of the RClamp0502B
The RClamp0502B is a low capacitance protection
device that protects one high speed differential pair. The
 2006 Semtech Corp.
Table 1 - RClamp0502B four layer evaluation board
layout parameters
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SI05-03
Surging Ideas
TVS Diode Application Note
PROTECTION PRODUCTS
Evaluation board results of the RClamp0544M
requirements. On the high side, the impedance
measured at 106.1Ω and on the low side, the impedance
measured at 101.6Ω. Figure 3 shows the impedance
design guidelines a PCB board designer may use
in accordance with their board layout and design
parameters.
A
The RClamp0544M has a flow through pacakge design to help reduce discontinuities on high speed signal lines. The typical differential capacitance of the
RClamp0544M is 0.7pF. Like the RClamp0502B, the
RClamp0544M also has a very low differential capacitance. This allows it to be designed with minor layout
compensation onto HDMI signals to remain within the
HDMI CTS impedance requirement. Results for a two,
four and six layer board is presented in the following.
B
RClamp0544M Four Layer Board Evaluation
The “HDMI MSOP-10L Rev E” board is a four layer board
designed with minor trace compensation to keep the
impedance of the differential pairs within the HDMI CTS
impedance specification. A higher trace impedance of
140 Ω is placed 0.1575 inches before and after the
RClamp0544M to compensate for added capacitance.
This design guideline is shown in Figure 4. The board
and trace parameters of the evaluation made to these
design guidelines are summarized in Table 2.
C
X-axis
Y-axis
A
1.816
108
B
1.95
102
C
2.15
108
nsec
Ω
HDMI MSOP-10L Rev E Evaluation Board
RClamp0544M with layout and board compensation
Figure 2 - RClamp0502B TDR test results on four layer
evaluation board
Copper thickness
1.5 oz finished thickness
Board thickness
0.062”
RClamp0502B
0.08 inches
100Ω differentially
to the LSI
Figure 3 - HDMI SC-75 Rev D Evaluation Board Design
Guideline for RClamp0502B
 2006 Semtech Corp.
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Dielectric thickness
0.010”
between layer 1 and Gnd
0.08 inches
140Ω differentially
Number layers
Trace pitch
(0.500 mm) or 0.0197”
Trace thickness
0.004” for 0.1575” before
and after the RClamp0544M
Trace thickness to
achieve 50 Ω nominally
0.010”
Table 2 - RClamp0544M four layer Evaluation board
layout parameters
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SI05-03
Surging Ideas
TVS Diode Application Note
PROTECTION PRODUCTS
PROTECTION PRODUCTS
Two Layer Evaluation Board Results of the
RClamp0544M
In the pursuit of saving manufacturing cost, many designers are try to convert their design to two layers. This
may not alway be possible as a design number of trace
connections increase. As more functions are provided
in each design, it may become very difficult if not impossible to route all the required traces using only two layers. The “HDMI MSOP-10L Rev G” board is a two layer
board designed with minor trace compensation to keep
the impedance of the differential pairs within the HDMI
CTS impednace requirement. The two layer board design
guideline shown in Figure 6 provides the means to use
the RClamp0544M on the HDMI high speed signal while
keeping the differnential impedance of the signal traces
within the HDMI CTS impedance requriement. Table
3 summarizes the trace parameters for the two layer
evaluation board that was made (HDMI MSOP-10L Rev
G). The TDR result is shown in Figure 7. As shown, the
impedance of the differential pair is well within the HDMI
CTS spec of 100 Ω ± 15%.
0.1575 inches
140Ω differentially
RClamp0544M
0.1575 inches
100Ω differentially
to LSI
Figure 4 - RClamp0544M Four Layer Design Guideline
(HDMI MSOP-10L Rev E)
A
Two Layer Evaluation Board (HDMI MSOP-10L Rev G)
RClamp0544M with layout and board compensation
C
B
Number layers
2
Copper thickness
1.0 oz finished thickness
Board thickness
0.062”
Dielectric thickness
0.010”
between layer 1 and Gnd
X-axis
Y-axis
A
1.816
108
B
1.95
102
C
2.15
108
nsec
Ω
Trace pitch
(0.500 mm) or 0.0197”
Trace thickness
0.006” for 0.1575” before and
after the RClamp0544M
Trace thickness to
0.010”
achieve 50 Ω nominally
Figure 5 - RClamp0544M TDR test result
on Four Layer Board
Table 3 - RClamp0544M two layer Evaluation board
parameters
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SI05-03
Surging Ideas
TVS Diode Application Note
PROTECTION PRODUCTS
Six Layer Evaluation Board Results of the RClamp0544M
As HDMI design gets more complicated, a four layer
board may not provide enough room to make the necessary connection. When this happens, designers will
use a six layer board. The six layer board design guideline shown in Figure 8 provides the means to use the
RClamp0544M on the HDMI high speed signal while
keeping the differnential impedance of the signal traces
within the HDMI CTS impedance requirement. Table 4
summarizes the trace parameters for the six layer evaluation board. The TDR result is shown in Figure 9. As
shown, the impedance of the differential is well within
the HDMI CTS spec of 100 Ω ± 15%.
0.1575 inches
125Ω
differentially
RClamp0544M
0.1575
inches
100Ω
differentially
to LSI
Figure 6 - RClamp0544M Two Layer Design Guideline
(HDMI MSOP-10L Rev G Evaluation Board)
A
Six Layer HDMI Evaluation Board
RClamp0544M with layout and board compensation
C
Number layers
6
Copper thickness
1.5 oz finished thickness
Board thickness
0.062”
Dielectric thickness
0.0047”
between layer 1 and Gnd
Trace pitch
(0.500 mm) or 0.0197”
Trace thickness
0.005” for 0.220” before and
after the RClamp0544M
B
Trace thickness to
0.010”
achieve 50 Ω nominally
X-axis
Y-axis
A
1.794
107
B
1.98
95
C
2.356
108
Table 4 - RClamp0544M Six layer Evaluation board
parameters
nsec
Ω
Figure 7- RClamp0544M TDR test results
on two Layer Board
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SI05-03
Surging Ideas
TVS Diode Application Note
PROTECTION PRODUCTS
PROTECTION PRODUCTS
Conclusion
A
This application note discussed two protection solutions to meet the IEC61000-4-2 ESD standard; the
RClamp0502B and the RClamp0544M. Unfortunately,
the protection solution is inherently capacitive. Both
the RClamp0502B and RClamp0544M were designed
to be very low in capacitance for high speed signals.
Although the capacitive loading of these devices are
very low, they can still cause the differential impedance
of the signal trace go outside of the HDMI CTS impedance requirement. This application note provided design
guidelines to provide trace compensation around the
protection device in order to keep the impedance within
the HDMI CTS. Design guidelines were provided using the RClamp0502B on a two layer board and the
RClamp0544M in a two, four and six layer board. Table
5 summarizes the TDR test results of all four test configurations. In all cases, the TDR of the trace stayed well
within the HDMI CTS.
C
B
X-axis
Y-axis
A
1.68
104
B
1.993
90
C
2.134
100
nsec
Ω
Figure 8- RClamp0544M TDR test results
on six Layer Board
0.220 inches
120Ω
differentially
100Ω
differentially
to LSI
RClamp0544M
0.220 inches
Figure 9 - RClamp0544M Six Layer Design Guideline
(HDMI MSOP-10L Rev G)
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SI05-03
Surging Ideas
TVS Diode Application Note
PROTECTION PRODUCTS
Four Layer Evaluation Board (RClamp0502B)
Two Layer Evaluation Board (RClamp0544M)
Four Layer Evaluation Board (RClamp0544M)
Six Layer Evaluation Board (RClamp0544M)
Spec Max
(W)
115
115
115
115
Spec Min
(W)
85
85
85
85
Actual Max
(W)
108
108
106.1
104
Actual Min
(W)
102
95
95
90
Table 5 - Summary of TDR Test Results
References:
Johnson, Dr. Howard “Potholes”, EDN Magazine Nov
1999 and www.sigcon.com
Johnson, Dr. Howard, “ High Speed Signal Propagation- Advance Black Magic”
Geske, Hani and Yoshima, Satoko, “HDMI Compliant
ESD/CDE Protection for Real World Video Circuit.”
June 9, 2006.
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