ICS557-03
PCI-EXPRESS CLOCK SOURCE
Applications Information
External Components
A minimum number of external components are
required for proper operation.
Output Structures
6*IREF
IREF
=2.3 mA
Decoupling Capacitors
Decoupling capacitors of 0.01 µF should be connected
between each VDD pin and the ground plane, as close
to the VDD pin as possible. Do not share ground vias
between components. Route power from power source
through the capacitor pad and then into ICS pin.
Crystal
See Output Termination
Sections - Pages 3 ~ 5
A 25 MHz fundamental mode parallel resonant crystal
should be used. This crystal must have less than 300
ppm of error across temperature in order for the
ICS557-03 to meet PCI Express specifications.
Ω
RR 475
General PCB Layout Recommendations
Crystal Capacitors
For optimum device performance and lowest output
phase noise, the following guidelines should be
observed.
Crystal capacitors are connected from pins X1 to
ground and X2 to ground to optimize the accuracy of
the output frequency.
1. Each 0.01µF decoupling capacitor should be
mounted on the component side of the board as close
to the VDD pin as possible.
C = Crystal’s load capacitance in pF
L
Crystal Capacitors (pF) = (C - 8) * 2
L
For example, for a crystal with a 16 pF load cap, each
external crystal cap would be 16 pF. (16-8)*2=16.
2. No vias should be used between decoupling
capacitor and VDD pin.
Current Source (Iref) Reference Resistor - RR
3. The PCB trace to VDD pin should be kept as short
as possible, as should the PCB trace to the ground via.
Distance of the ferrite bead and bulk decoupling from
the device is less critical.
If board target trace impedance (Z) is 50Ω, then R =
R
475Ω (1%), providing IREF of 2.32 mA. The output
current (I ) is equal to 6*IREF.
OH
4. An optimum layout is one with all components on the
same side of the board, minimizing vias through other
signal layers (any ferrite beads and bulk decoupling
capacitors can be mounted on the back). Other signal
traces should be routed away from the ICS557-03.This
includes signal traces just underneath the device, or on
layers adjacent to the ground plane layer used by the
device.
Output Termination
The PCI-Express differential clock outputs of the
ICS557-03 are open source drivers and require an
external series resistor and a resistor to ground. These
resistor values and their allowable locations are shown
in detail in the PCI-Express Layout Guidelines
section.
The ICS557-03 can also be configured for LVDS
compatible voltage levels. See the LVDS Compatible
Layout Guidelines section.
MDS 557-03 E
3
Revision 061005
Integrated Circuit Systems ● 525 Race Street, San Jose, CA 95126 ● tel (408) 297-1201 ● www.icst.com
ICSI [ INTEGRATED CIRCUIT SOLUTION INC ]
