DS99R124Q Datasheet, PDF (22/24 Pages) National Semiconductor (TI) – 5 - 43 MHz 18-bit Color FPD-Link II to FPD-Link Converter

Power Up Requirements and PDB Pin
The VDD (V_DDn), V_DDTX and V_DDIO supply ramps should be
faster than 1.5 ms with a monotonic rise. Supplies may power
up in any order, however device operation should be initiated
only after all supplies are in their valid operating ranges. The
optional serial bus address selection is done upon power up
also. Thus, if using this optional feature, the PDB signal must
be delayed to allow time for the ID setting to occur. The delay
maybe done by simply holding the PDB pin at a Low, or with
an external RC delay based off the V_DDIO rail which would then
need to lag the others in time. If the PDB pin is pulled to
V_DDIO, it is recommended to use a 10 kΩ pull-up and a 22 uF
cap to GND to delay the PDB input signal.
TRANSMISSION MEDIA
The Ser/Des chipset is intended to be used in a point-to-point
configuration, through a PCB trace, or through twisted pair
cable. The Ser and Des provide internal terminations provid-
ing a clean signaling environment. The interconnect for LVDS
should present a differential impedance of 100 Ohms. Use
cables and connectors that have matched differential
impedance to minimize impedance discontinuities. Shielded
or un-shielded cables may be used depending upon the noise
environment and application requirements.
LIVE LINK INSERTION
The Ser and Des devices support live pluggable applications.
The automatic receiver lock to random data “plug & go” hot
insertion capability allows the DS99R124Q to attain lock to
the active data stream during a live insertion event.
PCB LAYOUT AND POWER SYSTEM CONSIDERATIONS
Circuit board layout and stack-up for the LVDS Ser/Des de-
vices should be designed to provide low-noise power feed to
the device. Good layout practice will also separate high fre-
quency or high-level inputs and outputs to minimize unwanted
stray noise pickup, feedback and interference. Power system
performance may be greatly improved by using thin di-
electrics (2 to 4 mils) for power / ground sandwiches. This
arrangement provides plane capacitance for the PCB power
system with low-inductance parasitics, which has proven es-
pecially effective at high frequencies, and makes the value
and placement of external bypass capacitors less critical. Ex-
ternal bypass capacitors should include both RF ceramic and
tantalum electrolytic types. RF capacitors may use values in
the range of 0.01 uF to 0.1 uF. Tantalum capacitors may be
in the 2.2 uF to 10 uF range. Voltage rating of the tantalum
capacitors should be at least 5X the power supply voltage
being used.
Surface mount capacitors are recommended due to their
smaller parasitics. When using multiple capacitors per supply
pin, locate the smaller value closer to the pin. A large bulk
capacitor is recommend at the point of power entry. This is
typically in the 50uF to 100uF range and will smooth low fre-
quency switching noise. It is recommended to connect power
and ground pins directly to the power and ground planes with
bypass capacitors connected to the plane with via on both
ends of the capacitor. Connecting power or ground pins to an
external bypass capacitor will increase the inductance of the
path.
A small body size X7R chip capacitor, such as 0603, is rec-
ommended for external bypass. Its small body size reduces
the parasitic inductance of the capacitor. The user must pay
attention to the resonance frequency of these external bypass
capacitors, usually in the range of 20-30 MHz. To provide ef-
fective bypassing, multiple capacitors are often used to
achieve low impedance between the supply rails over the fre-
quency of interest. At high frequency, it is also a common
practice to use two vias from power and ground pins to the
planes, reducing the impedance at high frequency.
Some devices provide separate power and ground pins for
different portions of the circuit. This is done to isolate switch-
ing noise effects between different sections of the circuit.
Separate planes on the PCB are typically not required. Pin
Description tables typically provide guidance on which circuit
blocks are connected to which power pin pairs. In some cas-
es, an external filter many be used to provide clean power to
sensitive circuits such as PLLs.
Use at least a four layer board with a power and ground plane.
Locate LVCMOS signals away from the LVDS lines to prevent
coupling from the LVCMOS lines to the LVDS lines. Closely-
coupled differential lines of 100 Ohms are typically recom-
mended for LVDS interconnect. The closely coupled lines
help to ensure that coupled noise will appear as common-
mode and thus is rejected by the receivers. The tightly cou-
pled lines will also radiate less.
Information on the LLP style package is provided in National
Application Note: AN-1187.
LVDS INTERCONNECT GUIDELINES
See AN-1108 and AN-905 for full details.
• Use 100Ω coupled differential pairs
• Use the S/2S/3S rule in spacings
– S = space between the pair
– 2S = space between pairs
– 3S = space to LVCMOS signal
• Minimize the number of Vias
• Use differential connectors when operating above
500Mbps line speed
• Maintain balance of the traces
• Minimize skew within the pair
• Terminate as close to the TX outputs and RX inputs as
possible
Additional general guidance can be found in the LVDS
Owner’s Manual - available in PDF format from the National
web site at: www.national.com/lvds
www.national.com
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