MPC92439 Data Sheet 900MHZ, LOW VOLTAGE, LVPECL CLOCK SYNTHESIZER
MPC92439 REVISION 5 FEBRUARY 6, 2013 10 ©2013 Integrated Device Technology, Inc.
the V
CC
supply and the MPC92439 pin of the MPC92439. From the
data sheet, the V
CC_PLL
current (the current sourced through the
V
CC_PLL
pin) is maximum 20 mA, assuming that a minimum of
2.835 V must be maintained on the V
CC_PLL
pin. The resistor shown
in Figure 8 must have a resistance of 10–15 to meet the voltage
drop criteria. The RC filter pictured will provide a broadband filter with
approximately 100:1 attenuation for noise whose spectral content is
above 20 kHz. As the noise frequency crosses the series resonant
point of an individual capacitor its overall impedance begins to look
inductive and thus increases with increasing frequency. The parallel
capacitor combination shown ensures that a low impedance path to
ground exists for frequencies well above the bandwidth of the PLL.
Generally, the resistor/capacitor filter will be cheaper, easier to
implement and provide an adequate level of supply filtering. A higher
level of attenuation can be achieved by replacing the resistor with an
appropriate valued inductor. A 1000 H choke will show a significant
impedance at 10 kHz frequencies and above. Because of the current
draw and the voltage that must be maintained on the V
CC_PLL
pin, a
low DC resistance inductor is required (less than 15 ).
Figure 8. V
CC_PLL
Power Supply Filter
Layout Recommendations
The MPC92439 provides sub-nanosecond output edge rates and
thus a good power supply bypassing scheme is a must. Figure 9
shows a representative board layout for the MPC92439. There exists
many different potential board layouts and the one pictured is but
one. The important aspect of the layout in Figure 9 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 MPC92439 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 MPC92439 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 9. PCB Board Layout Recommendation
for the PLCC28 Package
The On-Chip Crystal Oscillator
The MPC92439 features an integrated on-chip crystal oscillator to
minimize system implementation cost. The integrated oscillator is a
Pierce-type that uses the crystal in its parallel resonance mode. It is
recommended to use a 10 to 20 MHz crystal with a load specification
of C
L
= 10 pF. Crystals with a load specification of C
L
= 20 pF may be
used at the expense of an slightly higher frequency than specified for
the crystal. Externally connected capacitors on both the XTAL_IN
and XTAL_OUT pins are not required but can be used to fine-tune the
crystal frequency as desired.
The crystal, the trace and optional capacitors should be placed on
the board as close as possible to the MPC92439 XTAL_IN and
XTAL_OUT pins to reduce crosstalk of active signals into the
oscillator. Short and wide traces further reduce parasitic inductance
and resistance. It is further recommended to guard the crystal circuit
by placing a ground ring around the traces and oscillator
components. See Table 12 for recommended crystal specifications.
V
CC_PLL
V
CC
MPC92439
C
1
, C
2
= 0.01...0.1 F
V
CC
C
F
= 22 F
R
F
= 10-15
C
2
C
1
Table 12. Recommended Crystal Specifications
Parameter Value
Crystal Cut Fundamental AT Cut
Resonance Mode Parallel
Crystal Frequency 10 - 20 MHz
Shunt Capacitance C
0
5 - 7 pF
Load Capacitance C
L
10 pF
Equivalent Series Resistance ESR 20–60
1
C2CF
XTAL
C1 C1
= V
CC
= GND
= Via
