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MPC9239 Datasheet(HTML) 9 Page - Freescale Semiconductor, Inc

Part No. MPC9239
Description  900 MHz Low Voltage LVPECL Clock Synthesizer
Download  14 Pages
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Maker  FREESCALE [Freescale Semiconductor, Inc]
Homepage  http://www.freescale.com
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MPC9239 Datasheet(HTML) 9 Page - Freescale Semiconductor, Inc

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Advanced Clock Drivers Devices
Freescale Semiconductor
9
MPC9239
Layout Recommendations
The MPC9239 provides sub-nanosecond output edge
rates and thus a good power supply bypassing scheme is a
must. Figure 6 shows a representative board layout for the
MPC9239. There exists many different potential board
layouts and the one pictured is but one. The important aspect
of the layout in Figure 6 is the low impedance connections
between VCC and GND for the bypass capacitors. Combining
good quality general purpose chip capacitors with good PCB
layout techniques will produce effective capacitor resonances
at frequencies adequate to supply the instantaneous
switching current for the MPC9239 outputs. It is imperative
that low inductance chip capacitors are used; it is equally
important that the board layout does not introduce back all of
the inductance saved by using the leadless capacitors. Thin
interconnect traces between the capacitor and the power
plane should be avoided and multiple large vias should be
used to tie the capacitors to the buried power planes. Fat
interconnect and large vias will help to minimize layout
induced inductance and thus maximize the series resonant
point of the bypass capacitors. Note the dotted lines circling
the crystal oscillator connection to the device. The oscillator
is a series resonant circuit and the voltage amplitude across
the crystal is relatively small. It is imperative that no actively
switching signals cross under the crystal as crosstalk energy
coupled to these lines could significantly impact the jitter of
the device. Special attention should be paid to the layout of
the crystal to ensure a stable, jitter free interface between the
crystal and the on—board oscillator. Although the MPC9239
has several design features to minimize the susceptibility to
power supply noise (isolated power and grounds and fully
differential PLL), there still may be applications in which
overall performance is being degraded due to system power
supply noise. The power supply filter and bypass schemes
discussed in this section should be adequate to eliminate
power supply noise related problems in most designs.
Figure 6. PCB Board Layout Recommendation
for the PLCC28 Package
Using the On-Board Crystal Oscillator
The MPC9239 features a fully integrated on-board crystal
oscillator to minimize system implementation costs. The
oscillator is a series resonant, multivibrator type design as
opposed to the more common parallel resonant oscillator
design. The series resonant design provides better stability
and eliminates the need for large on chip capacitors. The
oscillator is totally self contained so that the only external
component required is the crystal. As the oscillator is
somewhat sensitive to loading on its inputs the user is
advised to mount the crystal as close to the MPC9239 as
possible to avoid any board level parasitics. To facilitate co-
location surface mount crystals are recommended, but not
required. Because the series resonant design is affected by
capacitive loading on the XTAL terminals loading variation
introduced by crystals from different vendors could be a
potential issue. For crystals with a higher shunt capacitance
it may be required to place a resistance across the terminals
to suppress the third harmonic. Although typically not
required it is a good idea to layout the PCB with the provision
of adding this external resistor. The resistor value will typically
be between 500 and 1K
Ω.
The oscillator circuit is a series resonant circuit and thus
for optimum performance a series resonant crystal should be
used. Unfortunately most crystals are characterized in a
parallel resonant mode. Fortunately there is no physical
difference between a series resonant and a parallel resonant
crystal. The difference is purely in the way the devices are
characterized. As a result a parallel resonant crystal can be
used with the MPC9239 with only a minor error in the desired
frequency. A parallel resonant mode crystal used in a series
resonant circuit will exhibit a frequency of oscillation a few
hundred ppm lower than specified, a few hundred ppm
translates to kHz inaccuracies. In a general computer
application this level of inaccuracy is immaterial. Table 12
below specifies the performance requirements of the crystals
to be used with the MPC9239.
* See accompanying text for series versus parallel resonant
discussion.
1
C2
CF
XTAL
C1
C1
= VCC
= GND
= Via
Table 12. Recommended Crystal Specifications
Parameter
Value
Crystal Cut
Fundamental AT Cut
Resonance
Series Resonance*
Frequency Tolerance
±75 ppm at 25°C
Frequency/Temperature Stability
±150 pm 0 to 70°C
Operating Range
0 to 70
°C
Shunt Capacitance
5-7 pF
Equivalent Series Resistance (ESR)
50 to 80
Correlation Drive Level
100
µW
Aging
5ppm/Yr (First 3 Years)


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