ICS84427 Datasheet, PDF (10/13 Pages) Integrated Circuit Systems – CRYSTAL-TO-LVDS INTEGRATED FREQUENCY SYNTHESIZER/FANOUT BUFFER

Integrated
Circuit
Systems, Inc.
PRELIMINARY
ICS84427
CRYSTAL-TO-LVDS
INTEGRATED FREQUENCY SYNTHESIZER/FANOUT BUFFER
The following component footprints are used in this layout
example:
All the resistors and capacitors are size 0603.
POWER AND GROUNDING
Place the decoupling capacitors C3, C5 and C6, as close as
possible to the power pins. If space allows, placement of the
decoupling capacitor on the component side is preferred. This
can reduce unwanted inductance between the decoupling ca-
pacitor and the power pin caused by the via.
Maximize the power and ground pad sizes and number of vias
capacitors. This can reduce the inductance between the power
and ground planes and the component power and ground pins.
The RC filter consisting of R7, C11, and C16 should be placed
as close to the V_DDA pin as possible.
CLOCK TRACES AND TERMINATION
Poor signal integrity can degrade the system performance or
cause system failure. In synchronous high-speed digital systems,
the clock signal is less tolerant to poor signal integrity than other
signals. Any ringing on the rising or falling edge or excessive ring
back can cause system failure. The shape of the trace and the
trace delay might be restricted by the available space on the board
and the component location. While routing the traces, the clock
signal traces should be routed first and should be locked prior to
routing other signal traces.
• The differential 100Ω output traces should have the
same length.
• Avoid sharp angles on the clock trace. Sharp angle
turns cause the characteristic impedance to change
on the transmission lines.
• Keep the clock traces on the same layer. Whenever
possible, avoid placing vias on the clock traces. Place-
ment of vias on the traces can affect the trace charac-
teristic impedance and hence degrade signal integ-
rity.
• To prevent cross talk, avoid routing other signal traces
in parallel with the clock traces. If running parallel traces
is unavoidable, allow a separation of at least three
trace widths between the differential clock trace and
the other signal trace.
• Make sure no other signal traces are routed between
the clock trace pair.
• The matching termination resistors should be located as
close to the receiver input pins as possible.
CRYSTAL
The crystal X1 should be located as close as possible to the pins
20 (XTAL_IN) and 19 (XTAL_OUT). The trace length between
the X1 and U1 should be kept to a minimum to avoid unwanted
parasitic inductance and capacitance. Other signal traces
should not be routed near the crystal traces.
C6
C1
C5
R7
VDDA
C16
C11
X1
GND
VDD
Signals
VIA
C2
84427CM
C3
U1 ICS84427
Pin1
50 Ohm Traces
FIGURE 4B. PCB BOARD LAYOUT FOR ICS84427
www.icst.com/products/hiperclocks.html
10
REV. D NOVEMBER 30, 2005