ICS477-05 Datasheet, PDF (3/7 Pages) Integrated Circuit Systems – Quad PLL with VCXO for HDTV

ICS477-05
Quad PLL with VCXO for HDTV
External Components
The ICS477-05 requires a minimum number of external
components for proper operation.
Decoupling Capacitors
Decoupling capacitors of 0.01µF must be connected
between VDD and GND, as close to these pins as
possible. For optimum device performance, the
decoupling capacitors should be mounted on the
component side of the PCB. Avoid the use of vias in the
decoupling circuit.
Series Termination Resistor
When the PCB trace between the clock outputs and the
loads are over 1 inch, series termination should be
used. 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.
Quartz Crystal
The ICS477-05 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 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 ICS477-05 incorporates on-chip
variable load capacitors that “pull” (change) the
frequency of the crystal. The crystal specified for use
with the ICS477-05 is designed to have zero frequency
error when the total of on-chip + stray capacitance is 14
pF.
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 ICS477-05. 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 for complete crystal
specifications.
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 ICS477-05 to 3.3 V. Connect pin
4 of the ICS477-05 to the second power supply. Adjust
the voltage on pin 3 to 0V. Measure and record the
frequency of the 27 MHz output.
2. Adjust the voltage on pin 4 to 3.3 V. Measure and
record the frequency of the same output.
To calculate the centering error:
Error = 10⁶x
(_{---f--3--.--0---V----}–_{-----f--t--a--r--g---e---t--})_---+-----(_{---f-0---V-----}–_{----f--t--a---r--g---e---t-})_-
f_{t arget}
– error_xtal
Where:
f_target = nominal crystal frequency
error_xtal =actual initial accuracy (in ppm) of the crystal
being measured
If the centering error is less than ±25 ppm, no
adjustment is needed. If the centering error is more
than 25 ppm negative, the PC board has excessive
stray capacitance and a new PCB layout should be
considered to reduce stray capacitance. (Alternately,
the crystal may be re-specified to a higher load
capacitance. Contact ICS for details.) If the centering
error is more than 25 ppm positive, add identical fixed
centering capacitors from each crystal pin to ground.
The value for each of these caps (in pF) is given by:
MDS 477-05 H
3
Revision 062404
Integrated Circuit Systems, Inc. ● 525 Race Street, San Jose, CA 95126 ● tel [phone] ● www.icst.com