ICS275
TRIPLE PLL FIELD PROGRAMMABLE VCXO CLOCK SYNTHESIZER
EPROM VCXO AND SYNTHESIZER
External Components
The ICS275 requires a minimum number of external
components for proper operation.
Series Termination Resistor
Ω
Ω
The external crystal must be connected as close to the chip
as possible and should be on the same side of the PCB as
the ICS275. There should be no via’s between the crystal
pins and the X1 and X2 device pins. There should be no
signal traces underneath or close to the crystal. See
application note MAN05.
Crystal Tuning Load Capacitors
The crystal traces should include pads for small fixed
capacitors, one between X1 and ground, and another
between X2 and ground. Stuffing of these capacitors on the
PCB is optional. The need for these capacitors is
determined at system prototype evaluation, and is
influenced by the particular crystal used (manufacture and
frequency) and by PCB layout. The typical required
capacitor value is 1 to 4 pF.
To determine the need for and value of the crystal
adjustment capacitors, you will need a PC board of your final
layout, a frequency counter capable of about 1 ppm
resolution and accuracy, two power supplies, and some
samples of the crystals which you plan to use in production,
along with measured initial accuracy for each crystal at the
specified crystal load capacitance, CL.
To determine the value of the crystal capacitors:
1. Connect VDD of the ICS275 to 3.3 V. Connect pin 1 of the
ICS275 to the second power supply. Adjust the voltage on
pin 1 to 0V. Measure and record the frequency of the CLK
output.
2. Adjust the voltage on pin 1 to 3.3 V. Measure and record
the frequency of the same output.
To calculate the centering error:
Ω
Decoupling Capacitors
Decoupling capacitors of 0.01µF must be connected
between each VDD and the PCB ground plane. For optimum
device performance, the decoupling capacitor should be
mounted on the component side of the PCB. Aboid the use
of vias on the decoupling circuit.
Quartz Crystal
The ICS275 VCXO function consists of the external crystal
and the integrated VCXO oscillator circuit. To assure the
best system performance (frequency pull range) and
reliability, a crystal device with the recommended
parameters (shown below) must be used, and the layout
guidelines discussed in the following section shown must be
followed.
The frequency of oscillation of a quartz crystal is determined
by its “cut” and by the load capacitors connected to it. The
ICS275 incorporates on-chip variable load capacitors that
“pull” (change) the frequency of the crystal. The crystal
specified for use with the ICS275 is designed to have zero
frequency error when the total of on-chip + stray
capacitance is 14 pF.
Recommended Crystal Parameters:
Initial Accuracy at 25
°
C
Temperature Stability
Aging
Load Capacitance
Shunt Capacitance, C0
C0/C1 Ratio
Equivalent Series Resistance
±20 ppm
±30 ppm
±20 ppm
14 pf
7 pF Max
250 Max
35Ω Max
6
(
f
3.0V
–
f
t arg et
)
+
(
f
0V
–
f
t arg et
)
-
Error = 10 x ----------------------------------------------------------------------
–
error
xtal
f
t arg et
Where:
f
target
= nominal crystal frequency
IDT™ / ICS™
TRIPLE PLL FIELD PROGRAMMABLE VCXO CLOCK SYNTHESIZER 3
ICS275
REV D 102808
IDT [ INTEGRATED DEVICE TECHNOLOGY ]
