ICS270
TRIPLE PLL FIELD PROGRAMMABLE VCXO CLOCK EPROM VCXO AND SYNTHESIZER
IDT™ / ICS™
TRIPLE PLL FIELD PROGRAMMABLE VCXO CLOCK 3
ICS270 REV F 051310
External Components
The ICS270 requires a minimum number of external
components for proper operation.
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.
Decoupling Capacitors
As with any high-performance mixed-signal IC, the ICS270
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. 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 ICS270 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
ICS270 incorporates on-chip variable load capacitors that
“pull” (change) the frequency of the crystal. The crystal
specified for use with the ICS270 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 ±20 ppm
Temperature Stability ±30 ppm
Aging ±20 ppm
Load Capacitance 14 pf
Shunt Capacitance, C0 7 pF Max
C0/C1 Ratio 250 Max
Equivalent Series Resistance 35Ω Max
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 ICS270. 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 ICS270 to 3.3 V. Connect pin 1 of the
ICS270 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:
Where:
f
target
= nominal crystal frequency
Error 10
6
x
f
3.0V
f
tetarg
–()f
0V
f
tetarg
–()+
f
tetarg
-----------------------------------------------------------------------
error
xtal
–=
