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MPC952 Datasheet(PDF) 5 Page - Motorola, Inc

Part No. MPC952
Description  LOW VOLTAGE PLL CLOCK DRIVER
Download  7 Pages
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Maker  MOTOROLA [Motorola, Inc]
Homepage  http://www.freescale.com
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MPC952 Datasheet(HTML) 5 Page - Motorola, Inc

   
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MPC952
ECLinPS and ECLinPS Lite
DL140 — Rev 3
5
MOTOROLA
VL = VS (Zo / Rs + Ro +Zo) = 3.0 (25/53.5) = 1.40V
At the load end the voltage will double, due to the near
unity reflection coefficient, to 2.8V. It will then increment
towards the quiescent 3.0V in steps separated by one round
trip delay (in this case 4.0ns).
Figure 4. Single versus Dual Waveforms
TIME (nS)
3.0
2.5
2.0
1.5
1.0
0.5
0
2
4
6
8
10
12
14
OutB
tD = 3.9386
OutA
tD = 3.8956
In
Since this step is well above the threshold region it will not
cause any false clock triggering, however designers may be
uncomfortable with unwanted reflections on the line. To
better match the impedances when driving multiple lines the
situation in Figure 5 should be used. In this case the series
terminating resistors are reduced such that when the parallel
combination is added to the output buffer impedance the line
impedance is perfectly matched.
Figure 5. Optimized Dual Line Termination
7
MPC952
OUTPUT
BUFFER
RS = 36Ω
ZO = 50Ω
RS = 36Ω
ZO = 50Ω
7
Ω + 36Ω k 36Ω = 50Ω k 50Ω
25
Ω = 25Ω
SPICE level output buffer models are available for
engineers who want to simulate their specific interconnect
schemes. In addition IV characteristics are in the process of
being generated to support the other board level simulators in
general use.
Power Supply Filtering
The MPC952 is a mixed analog/digital product and as
such it exhibits some sensitivities that would not necessarily
be seen on a fully digital product. Analog circuitry is naturally
susceptible to random noise, especially if this noise is seen
on the power supply pins. The MPC952 provides separate
power supplies for the output buffers (VCCO) and the internal
PLL (VCCA) of the device. The purpose of this design
technique is to try and isolate the high switching noise digital
outputs from the relatively sensitive internal analog
phase–locked loop. In a controlled environment such as an
evaluation board this level of isolation is sufficient. However,
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 simplest form of isolation is a
power supply filter on the VCCA pin for the MPC952.
Figure 6. Power Supply Filter
VCCA
VCC
MPC952
0.01
µF
22
µF
0.01
µF
3.3V
RS=5–15Ω
Figure 6 illustrates a typical power supply filter scheme.
The MPC952 is most susceptible to noise with spectral
content in the 1KHz to 1MHz 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 that will be seen between the VCC supply and the VCCA
pin of the MPC952. From the data sheet the IVCCA current
(the current sourced through the VCCA pin) is typically 15mA
(20mA maximum), assuming that a minimum of 3.0V must be
maintained on the VCCA pin very little DC voltage drop can
be tolerated when a 3.3V VCC supply is used. The resistor
shown in Figure 6 must have a resistance of 10–15
Ω 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 20KHz. As the noise
frequency crosses the series resonant point of an individual
capacitor it’s 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 MPC952 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.


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