IT6605
backend scalers.
C. TTL output traces to the scaler should be kept as short as possible
D. 33Ω resistors could be placed in series to the output pins. This slow down the signal rising
edges, reduces current spikes and lower the reflections.
E. The PCLK signal should be kept away from other signal traces to avoid crosstalk interference.
A general guideline is 2X the dielectric thickness. For example, if the dielectric layer between
the signal layer and the immediate power/ground layer is 7 mil, then the PCLK trace should
be kept at least 14 mil away from all other signal traces.
4. The characteristic impedance of all differential PCB traces should be kept at 100Ω all the way from
the HDMI connector to the IT6605. This is crucial to the system performance at high speeds. When
layouting these differential transmission lines, the following guidelines should be followed:
A. The signals traces should be on the outside layers (TOP layer or BOTTOM layer) while
beneath it there should be a continuous ground plane in order to maintain the so-called
micro-strip transmission line structure, giving stable and well-defined characteristic
impedances.
B. Carefully choose the width and spacing of the differential transmission lines as their
characteristic impedance depends on various parameters of the PCB: trace width, trace
spacing, copper thickness, dielectric constant, dielectric thickness, etc. Careful 3D EM
simulation is the best way to derive a correct dimension that enables a nominal 100Ω
differential impedance.
C. Cornering, through holes, crossing and any irregular signal routing should be minimized so as
to prevent from disrupting the EM field and creating discontinuity in characteristic impedance.
D. The IT6605 should be placed as close to the HDMI connector as possible. If the distance
between the chip and the connector is under 2 cm, the reflections could be kept small even if
the PCB traces do not have an 100Ω characteristic impedance. The extra signal attenuation
contributed by the PCB traces could be minimized, too.
5. Special care should be taken when adding discrete ESD devices to all differential PCB traces
(RX2P/M, RX1P/M, RX0P/M, RXCP/M). The IT6605 is designed to provide ESD protection for up to
2kV at these pins, which is good enough to prevent damages during assembly. To meet the system
EMC specification, external discrete ESD diodes might be added. But note that adding discrete ESD
diodes inevitably add capacitive loads, therefore degrade the electrical performance at high speeds. If
not chosen carefully, these diodes coupled with less-than-optimal layout would prevent the system
from passing the SINK TMDS-Differential Impedance test in the HDMI Compliance Test (Test ID 8-8).
One should only use low-capacitance ESD diode to protect these high-speed pins. Commercially
available devices such as Semtech's RClamp0524p that take into consideration of all aspects of
designing and protecting high-speed transmission lines are recommended. (http://www.semtech.com/
Feb-2012 Rev:0.92 35/38
www.ite.com.tw
ITE [ ITE TECH.INC. ]
