ICS680-01
NETWORKING CLOCK SYNTHESIZER AND ZERO DELAY BUFFER ZDB AND SYNTHESIZER
IDT™ / ICS™
NETWORKING CLOCK SYNTHESIZER AND ZERO DELAY BUFFER 3
ICS680-01 REV H 051310
External Components
The ICS680-01 requires a minimum number of external
components for proper operation.
Decoupling Capacitor
A decoupling capacitor of 0.01µF must be connected
between VDD (pins 5 and 16) and GND (pins 6 and 15), as
close to these pins as possible. For optimum device
performance, the decoupling capacitor should be mounted
on the component side of the PCB. Avoid the use of vias in
the decoupling circuit.
Series Termination Resistor
When the PCB trace between the clock outputs and the
loads are over 1 inch, series termination should be used. To
series terminate a 50Ω trace (a commonly used trace
impedance) place a 33Ω resistor in series with the clock line,
as close to the clock output pin as possible. The nominal
impedance of the clock output is 20Ω.
Crystal Information
The crystal used should be a fundamental mode (do not use
third overtone), parallel resonant. Crystal capacitors should
be connected from pins X1 to ground and X2 to ground to
optimize the initial accuracy. The value of these capacitors
is given by the following equation crystal caps (pF) =
(CL-6)x2
In the equation, CL is the crystal load capacitance. So for a
crystal with a 16 pF load capacitance, two 20 pF[(16-6)x2]
capacitors should be used
PCB Layout Recommendations
For optimum device performance and lowest output phase
noise, the following guidelines should be observed.
1) The 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.
2) The external crystal should be mounted just next to the
device with short traces. The X1 and X2 traces should not
be routed next to each other with minimum spaces, instead
they should be separated and away from other traces.
3) To minimize EMI, the 33Ω series termination resistor (if
needed) should be placed close to the clock output.
4) An optimum layout is one with all components on the
same side of the board, minimizing vias through other signal
layers. Other signal traces should be routed away from the
ICS680-01. This includes signal traces just underneath the
device, or on layers adjacent to the ground plane layer used
by the device.
18 VDD Power Connect to voltage supply.
19 ICLK Input Zero Delay Buffer Input. Weak Internal pull-up.
20 25M Output 25 MHz reference output clock. Weak internal pull-down when tri-state.
21 S1 Input Select pin 1. See table above.
22 PDTS Power Power-down tri-state. Powers down entire chip and tri-states outputs
when low. Internal pull-up resistor.
23 VDD Power Connect to voltage supply.
24 X2 XO Crystal output. Connect this pin to a crystal. Float for clock input.
Pin
Number
Pin
Name
Pin
Type
Pin Description