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ICS557G-05ALFT Datasheet(PDF) 4 Page - Integrated Device Technology |
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ICS557G-05ALFT Datasheet(HTML) 4 Page - Integrated Device Technology |
4 / 12 page ICS557-05A QUAD DIFFERENTIAL PCI-EXPRESS CLOCK SOURCE PCIE SSCG IDT™ / ICS™ QUAD DIFFERENTIAL PCI-EXPRESS CLOCK SOURCE 4 ICS557-05A REV H 092407 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 = 475Ω (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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