ICS557-01
PCI-E
XPRESS
C
LOCK
S
OURCE
Applications Information
External Components
A minimum number of external components are
required for proper operation.
Output Structures
IREF
=2.3 mA
6*IREF
Decoupling Capacitors
Decoupling capacitors of 0.01
µF
should be connected
between VDD and the ground plane (pin 4) 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
A 25 MHz fundamental mode parallel resonant crystal
with C
L
= 16 pF should be used. This crystal must have
less than 300 ppm of error across temperature in order
for the ICS557-01 to meet PCI Express specifications.
R
R
475
Ω
See Output Termination
Sections - Pages 3 ~ 5
Crystal Capacitors
Crystal capacitors are connected from pins X1 to
ground and X2 to ground to optimize the accuracy of
the output frequency.
C
L
= Crystal’s load capacitance in pF
Crystal Capacitors (pF) = (C
L
- 8) * 2
For example, for a crystal with a 16 pF load cap, each
external crystal cap would be 16 pF. (16-8)*2=16.
General PCB Layout Recommendations
For optimum device performance and lowest output
phase noise, the following guidelines should be
observed.
1. Each 0.01µF decoupling capacitor should be
mounted on the component side of the board as close
to the VDD pin as possible.
2. No vias should be used between decoupling
capacitor and VDD pin.
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.
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 ICS557-01.This
includes signal traces just underneath the device, or on
layers adjacent to the ground plane layer used by the
device.
Current Source (Iref) Reference Resistor - R
R
If board target trace impedance (Z) is 50Ω, then R
R
=
475Ω (1%), providing IREF of 2.32 mA. The output
current (I
OH
) is equal to 6*IREF.
Output Termination
The PCI-Express differential clock outputs of the
ICS557-01 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 ICS557-01can also be configured for LVDS
compatible voltage levels. See the
LVDS Compatible
Layout Guidelines
section
MDS 557-01 F
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