mk3771-17 Integrated Device Technology, mk3771-17 Datasheet - Page 4

• Manufacturer Part Number: mk3771-17
• Description: Vcxo And Hdtv Set-top Clock Source
• Manufacturer: Integrated Device Technology

External Component Selection
The MK3771-17 requires a minimum number of external
components for proper operation.
Decoupling Capacitors
Decoupling capacitors of 0.01µF should be connected
between VDD and GND on pins 3 and 6, and on pins 13 and
14, as close to the MK3771-17 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 traces 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 MK3771-17 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 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
MK3771-17 incorporates on-chip variable load capacitors
that “pull” (change) the frequency of the crystal. The crystal
specified for use with the MK3771-17 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 MK3771-17. There should be no vias between the
crystal pins and the X1 and X2 device pins. There should be
no signal traces underneath or close to the crystal.
Please see application note MAN05 for recommended
crystal parameters and suppliers.
Crystal Tuning Load Capacitors
The crystal traces should include pads for small fixed
IDT™ / ICS™ VCXO AND HDTV SET-TOP CLOCK SOURCE
MK3771-17
VCXO AND HDTV SET-TOP CLOCK SOURCE
4
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 MK3771-17 to 3.3 V. Connect pin 4
of the MK3771-17 to the second power supply. Adjust the
voltage on pin 4 to 0V. Measure and record the frequency of
the CLK 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:
Where:
f
error
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 for details.) If the
centering error is more than 25ppm positive, add identical
fixed centering capacitors from each crystal pin to ground.
The value for each of these caps (in pF) is given by:
target
Error
xtal
= nominal crystal frequency
=actual initial accuracy (in ppm) of the crystal being
=
10
6
x
(
----------------------------------------------------------------------------- -
f
3.3V
–
f
t
arg
et
f
t
)
arg
+
VCXO AND SYNTHESIZER
et
(
f
0V
MK3771-17
–
f
t
arg
et
)
–
error
REV C 083104
xtal