MPC9992 データシートの表示（PDF） - Motorola => Freescale
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MPC9992 Datasheet PDF : 12 Pages
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MPC9992
Power Supply Filtering
The MPC9992 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 VCC_PLL power supply impacts the device
characteristics, for instance I/O jitter. The MPC9992 provides
separate power supplies for the output buffers (VCC) and the
phase-locked loop (VCC_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
VCC_PLL pin for the MPC9992. Figure 4. illustrates a typical
power supply filter scheme. The MPC9992 frequency and
phase stability is most susceptible to noise with spectral
content in the 100kHz to 20MHz 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 RF. From the data sheet
the ICC_PLL current (the current sourced through the VCC_PLL
pin) is typically 3 mA (5 mA maximum), assuming that a
minimum of 2.325V (VCC=3.3V or VCC=2.5V) must be
maintained on the VCC_PLL pin. The resistor RF shown in
Figure 4. “VCC_PLL Power Supply Filter” must have a
resistance of 9-10W (VCC=2.5V) to meet the voltage drop
criteria.
RF = 9-10Ω
CF = 22 µF
RF
VCC
VCC_PLL
CF
10 nF
MPC9992
VCC
33...100 nF
Figure 4. VCC_PLL Power Supply Filter
The minimum values for RF and the filter capacitor CF 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 4. “VCC_PLL Power Supply Filter”, 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 MPC9992 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.
Differential
Pulse Generator
Z = 50 W
Z = 50 Ω
Z = 50 Ω
RT = 50 Ω
MPC9992 DUT
RT = 50 Ω
VTT
VTT
Figure 5. MPC9992 AC test reference
MOTOROLA
8
TIMING SOLUTIONS
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