RClamp0544M
PROTECTION PRODUCTS
Applications Information
(continued)
The HDMI Compliance Test Specification (CTS) requires
sink (receiver) ports maintain a differential impedance
of 100 Ohms +/- 15%. The measurement is taken
using a Time Domain Reflectometry (TDR) method that
utilizes a pulse with a risetime <= 200ps.
ESD protection devices have an inherent junction
capacitance. Even a small amount of added capaci-
tance on an HDMI port will cause the impedance of
the differential pair to drop. As such, some form of
compensation to the layout will be required to bring
the differential pairs back within the required 100
Ohm +/- 15% range. The higher the added capaci-
tance, the more extreme the modifications will need to
be. If the added capacitance is too high, compensa-
tion may not even be possible. The RClamp0544M
presents <1pF capacitance between the pairs while
being rated to handle >8kV ESD contact discharges
(>15kV air discharge) as outlined in IEC 61000-4-2.
As such, it is possible to make minor adjustments to
the board layout parameters to compensate for the
added capacitance of the RClamp0544M. Figure 7
shows how to implement the RClamp0544M in an
HDMI application (transmitter and receiver). Figure 8
shows impedance test results using a Semtech evalua-
tion board with layout compensation. As shown, the
device meets the HDMI CTS impedance requirements.
PRELIMINARY
B
A
C
X-axis
Y-axis
A
1.640
99.3
B
1.796
105.3
C
1.953
98.7
(nsec)
(Ohm)
Figure 8 - TDR Measurement using Semtech
Evaluation Board
1
1
1
1
Figure 7 - HDMI High Speed Signal Protection Application Diagram
2006 Semtech Corp.
7
www.semtech.com
SEMTECH [ SEMTECH CORPORATION ]
