MPC9331 REVISION 7 March 11, 2016 9 ©2016 Integrated Device Technology, Inc.
MPC9331 Data Sheet 3.3 V 1:6 LVCMOS PLL CLOCK GENERATOR
APPLICATIONS INFORMATION
Power Supply Filtering
The MPC9331 is a mixed analog/digital product. Its analog
circuitry is naturally susceptible to random noise, especially if
this noise is seen on the power supply pins. Random noise
on the V
CC_PLL
power supply impacts the device
characteristics, for instance, I/O jitter. The MPC9331 provides
separate power supplies for the output buffers (V
CC
) and the
phase-locked loop (V
CC_PLL
) of the device.The purpose of
this design technique is to isolate the high switching noise
digital outputs from the relatively sensitive internal analog
phase-locked loop. In a digital system environment where it
is more difficult to minimize noise on the power supplies, a
second level of isolation may be required. The simple but
effective form of isolation is a power supply filter on the
V
CC_PLL
pin for the MPC9331. Figure 3 illustrates a typical
power supply filter scheme. The MPC9331 frequency and
phase stability is most susceptible to noise with spectral
content in the 100 kHz to 20 MHz range. Therefore the filter
should be designed to target this range. The key parameter
that needs to be met in the final filter design is the DC voltage
drop across
the series filter resistor R
F
. From the data sheet,
the I
CC_PLL
current (the current sourced through the V
CC_PLL
pin) is typically 8 mA (12 mA maximum), assuming that a
minimum of 3.0 V must be maintained on the V
CC_PLL
pin.
Figure 3. V
CC_PLL
Power Supply Filter
The minimum values for RF and the filter capacitor C
F
are
defined by the required filter characteristics: the RC filter
should provide an attenuation greater than 40 dB for noise
whose spectral content is above 100 kHz. In the example RC
filter shown in Figure 3, the filter cut-off frequency is around
3-5 kHz and the noise attenuation at 100 kHz is better than
42 dB.
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. Although the MPC9331 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 schemes discussed in
this section should be adequate to eliminate power supply
noise related problems in most designs.
Driving Transmission Lines
The MPC9331 clock driver was designed to drive high-
speed signals in a terminated transmission line environment.
To provide the optimum flexibility to the user, the output
drivers were designed to exhibit the lowest impedance
possible. With an output impedance of less than 20 the
drivers can drive either parallel or series terminated
transmission lines. For more information on transmission
lines, the reader is referred to Freescale application note
AN1091. In most high performance clock networks,
point-to-point distribution of signals is the method of choice.
In a point-to-point scheme, either series terminated or parallel
terminated transmission lines can be used. The parallel
technique terminates the signal at the end of the line with a
50 resistance to V
CC
2.
This technique draws a fairly high level of DC current and
thus only a single terminated line can be driven by each
output of the MPC9331 clock driver. For the series terminated
case, however, there is no DC current draw, thus the outputs
can drive multiple series terminated lines. Figure 4 illustrates
an output driving a single series terminated line versus two
series terminated lines in parallel. When taken to its extreme
the fanout of the MPC9331 clock driver is effectively doubled
due to its capability to drive multiple lines.
Figure 4. Single versus Dual Transmission Lines
The waveform plots in Figure 5 show the simulation results
of an output driving a single line versus two lines. In both
cases, the drive capability of the MPC9331 output buffer is
more than sufficient to drive 50 transmission lines on the
incident edge. Note from the delay measurements in the
simulations, a delta of only 43 ps exists between the two
differently loaded outputs. This suggests that the dual line
driving need not be used exclusively to maintain the tight
output-to-output skew of the MPC9331. The output waveform
in Figure 5 shows a step in the waveform; this step is caused
by the impedance mismatch seen looking into the driver. The
parallel combination of the 36 series resistor plus the
output impedance does not match the parallel combination of
V
CC_PLL
V
CC
MPC9331
10 nF
R
F
= 10 – 15
C
F
33...100 nF
R
F
V
CC
C
F
= 22 F
14
IN
MPC9331
Output
Buffer
R
S
= 36
Z
O
= 50
OutA
14
IN
MPC9331
Output
Buffer
R
S
= 36
Z
O
= 50
OutB0
R
S
= 36
Z
O
= 50
OutB1
