ICS487-25
QUAD PLL FOR DTV CLOCK SYNTHESIZER
IDT™ / ICS™
QUAD PLL FOR DTV 3
ICS487-25 REV B 092109
External Components
Decoupling Capacitor
As with any high-performance mixed-signal IC, the
ICS487-25 must be isolated from system power supply
noise to perform optimally.
A decoupling capacitor of 0.01µF must be connected
between each VDD and the PCB ground plane.
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.
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) The 0.01µF decoupling capacitors should be
mounted on the component side of the board as close
to the VDD pin as possible. No vias should be used
between the decoupling capacitors and VDD pins. The
PCB trace to VDD pins 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 ICS487-25. This includes signal traces just
underneath the device, or on layers adjacent to the
ground plane layer used by the device.
Absolute Maximum Ratings
Stresses above the ratings listed below can cause permanent damage to the ICS487-25. These ratings,
which are standard values for IDT commercially rated parts, are stress ratings only. Functional operation of
the device at these or any other conditions above those indicated in the operational sections of the
specifications is not implied. Exposure to absolute maximum rating conditions for extended periods can
affect product reliability. Electrical parameters are guaranteed only over the recommended operating
temperature range.
Item Rating
Supply Voltage, VDD 7 V
All Inputs and Outputs -0.5 V to VDD+0.5 V
Ambient Operating Temperature 0 to +70° C
