ICS557GI-05ALF datasheet(4/12 Pages) ICST

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Quad Differential PCI-Express Clock Source

MDS 557-05A E

Revision 011606

Integrated Circuit Systems, Inc.

● 525 Race Street, San Jose, CA 95126 ● tel (408) 297-1201 ● www.icst.com

ICS557-05A

Application Information

Decoupling Capacitors

As with any high-performance mixed-signal IC, the

ICS557-05A must be isolated from system power

supply noise to perform optimally.

Decoupling capacitors of 0.01µF must be connected

between each VDD and the PCB ground plane.

PCB Layout Recommendations

For optimum device performance and lowest output

phase noise, the following guidelines should be

observed.

Each 0.01µF decoupling capacitor should be mounted

on the component side of the board as close to the

VDD pin as possible. No vias should be used between

decoupling capacitor and VDD pin. 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.

2) An optimum layout is one with all components on the

same side of the board, minimizing vias through other

signal layers (the ferrite bead and bulk decoupling

capacitor can be mounted on the back). Other signal

traces should be routed away from the ICS557-05A.

This includes signal traces just underneath the device,

or on layers adjacent to the ground plane layer used by

the device.

External Components

A minimum number of external components are

required for proper operation. Decoupling capacitors of

0.01

µF should be connected between VDD and GND

pairs (1,9 and 15,16) as close to the device as possible.

On chip capacitors- Crystal capacitors should be

connected from pins X1 to ground and X2 to ground to

optimize the initial accuracy. The value (in pf) of these

crystal caps equal (CL-12)*2 in this equation,

CL=crystal load capacitance in pf. For example, for a

crystal with a 16 pF load cap, each external crystal cap

would be 8 pF. [(16-12)x2]=8.

Current Reference Source Rr (Iref)

If board target trace impedance (Z) is 50

Ω, then Rr =

475

Ω (1%), providing IREF of 2.32 mA, output current

(IOH) is equal to 6*IREF.

Load Resistors RL

Since the clock outputs are open source outputs, 50

ohm external resistors to ground are to be connected at

each clock output.

Output Termination

The PCI-Express differential clock outputs of the

ICS557-05A 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 ICS557-05A can also be configured for LVDS

compatible voltage levels. See the LVDS Compatible

Layout Guidelines section.

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