ICS252
FIELD PROGRAMMABLE DUAL OUTPUT SS VERSACLOCK SYNTHESIZER
EPROM CLOCK SYNTHESIZER
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 the 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) 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
ICS252. This includes signal traces just underneath the
device, or on layers adjacent to the ground plane layer used
by the device.
Output Drive Control
The ICS252 has two output drive settings. Low drive should
be selected when outputs are less than 100 MHz. High drive
should be selected when outputs are greater than 100 MHz.
(Consult the AC Electrical Characteristics for output rise and
fall times for each drive option.)
IDT VersaClock Software
IDT applies years of PLL optimization experience into a user
friendly software that accepts the user’s target reference
clock and output frequencies and generates the lowest jitter,
lowest power configuration, with only a press of a button.
The user does not need to have prior PLL experience or
determine the optimal VCO frequency to support multiple
output frequencies.
VersaClock software quickly evaluates accessible VCO
frequencies with available output divide values and provides
an easy to understand, bar code rating for the target output
frequencies. The user may evaluate output accuracy,
performance trade-off scenarios in seconds.
Spread Spectrum Modulation
The ICS252 utilizes frequency modulation (FM) to distribute
energy over a range of frequencies. By modulating the
output clock frequencies, the device effectively lowers
energy across a broader range of frequencies; thus,
lowering a system’s electro-magnetic interference (EMI).
The modulation rate is the time from transitioning from a
minimum frequency to a maximum frequency and then back
to the minimum.
Spread Spectrum Modulation can be applied as either
“center spread” or “down spread”. During center spread
modulation, the deviation from the target frequency is equal
in the positive and negative directions. The effective
average frequency is equal to the target frequency. In
applications where the clock is driving a component with a
maximum frequency rating, down spread should be applied.
In this case, the maximum frequency, including modulation,
is the target frequency. The effective average frequency is
less than the target frequency.
The ICS252 operates in both center spread and down
spread modes. For center spread, the frequency can be
modulated between +/- 0.125% to +/-2.0%. For down
spread, the frequency can be modulated between -0.25% to
-4.0%.
ICS252 Configuration Capabilities
The architecture of the ICS252 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 ICS252 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:
OutputFreq
=
REFFreq
-------------------------------------
-
OutputDivide
⋅
M
----
-
N
IDT®
FIELD PROGRAMMABLE DUAL OUTPUT SS VERSACLOCK SYNTHESIZER 3
ICS252
REV J 052010
IDT [ INTEGRATED DEVICE TECHNOLOGY ]
