MK3720 Integrated Circuit System, MK3720 Datasheet - Page 3

• Manufacturer Part Number: MK3720
• Description: 27 MHZ And 54 MHZ 3.3v Vcxo
• Manufacturer: Integrated Circuit System

MDS 3720 F
Int egrat ed C ircuit Syste ms
External Component Selection
The MK3720 requires a minimum number of external
components for proper operation.
Decoupling Capacitor
A decoupling capacitor of 0.01µF must be connected
between VDD (pin 2) and GND (pin 4), as close to
these pins as possible. For optimum device
performance, the decoupling capacitor 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 output (CLK,
pin 5) and the load is over 1 inch, series termination
should be used. To series terminate a 50 trace (a
commonly used trace impedance) place a 33 resistor
in series with the clock line, as close to the clock output
pin as possible. The nominal impedance of the clock
output is 20 .
Quartz Crystal
The MK3720 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 MK3720 incorporates on-chip
variable load capacitors that “pull” (change) the
frequency of the crystal. The crystal specified for use
with the MK3720 is designed to have zero frequency
error when the total of on-chip + stray capacitance is
14pF.
Recommended Crystal Parameters:
See application note MAN05 for crystal information.
MAN05 is available on the internet at
www.icst.com/pdf/man05.pdf.
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 MK3720. There should be no via’s between
the crystal pins and the X1 and X2 device pins. There
525 R ace S tr eet, San Jose, CA 95126
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should be no signal traces underneath or close to the
crystal.
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 MK3720 to 3.3V. Connect pin 3
of the MK3720 to the second power supply. Adjust the
voltage on pin 3 to 0V. Measure and record the
frequency of the CLK output.
2. Adjust the voltage on pin 3 to 3.3V. Measure and
record the frequency of the same output.
To calculate the centering error:
Where:
f
error
being measured
If the centering error is less than ±25 ppm, no
adjustment is needed. If the centering error is more
than 25ppm 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 MicroClock for details.) If the
centering error is more than 25ppm positive, add
target
Error
xtal
= nominal crystal frequency
=actual initial accuracy (in ppm) of the crystal
=
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