ICS308
SERIAL PROGRAMMABLE QUAD PLL VERSACLOCK SYNTHESIZER SER PROG CLOCK SYNTHESIZER
IDT™ / ICS™
SERIAL PROGRAMMABLE QUAD PLL VERSACLOCK SYNTHESIZER 4
ICS308 REV L 051310
External Components
Series Termination Resistor
Clock output traces over one inch should use series
termination. 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.
STROBE Pull-up Resistor
In order for the device to start up in the default state, a
250 kOhm pull-up resistor is required.
Decoupling Capacitors
As with any high-performance mixed-signal IC, the
ICS308 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.
Crystal Load Capacitors
The device crystal connections should include pads for
small capacitors from X1 to ground and from X2 to
ground. These capacitors are used to adjust the stray
capacitance of the board to match the nominally
required crystal load capacitance. Because load
capacitance can only be increased in this trimming
process, it is important to keep stray capacitance to a
minimum by using very short PCB traces (and no vias)
between the crystal and device. Crystal capacitors must
be connected from each of the pins X1 and X2 to
ground.
The value (in pF) of these crystal caps should equal (C
L
-6 pF)*2. In this equation, C
L
= crystal load capacitance
in pF. Example: For a crystal with a 16 pF load
capacitance, each crystal capacitor would be 20 pF
[(16-6) x 2 = 20].
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. 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 each clock output.
4) An optimum layout is one with all components on the
same side of the board, minimizing vias through other
signal layers.
ICS308 Configuration Capabilities
The architecture of the ICS308 allows the user to easily
configure the device to a wide range of output
frequencies, for a given input reference frequency.
The frequency multiplier PLL provides a high degree of
precision. The M/N values (the multiplier/divide values
available to generate the target VCO frequency) can be
set within the range of M = 1 to 2048 and N = 1 to 1024.
The ICS308 also provides separate output divide
values, from 2 through 20, to allow the two output clock
banks to support widely differing frequency values from
the same PLL.
Each output frequency can be represented as:
Output Freq. = (Ref. Freq)*(M/N)/Output Divide
