MPC9774 View Datasheet(PDF) - Motorola => Freescale
Part Name
Description
Manufacturer
MPC9774 Datasheet PDF : 16 Pages
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MPC9774
Power Supply Filtering
The MPC9774 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 MPC9774 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 MPC9774. Figure 10 illustrates a typical
power supply filter scheme. The MPC9774 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 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
MPC9774 must have a resistance of 9-10Ω (VCC = 2.5V) to
meet the voltage drop criteria.
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 MPC9774, the filter cut-off frequency is around
3-5 kHz and the noise attenuation at 100 kHz is better than
42 dB.
RF = 9–10Ω
CF = 22 µF
RF
VCC
VCC_PLL
CF
10 nF
MPC9774
VCC
33...100 nF
Figure 10. VCC Power Supply Filter
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 MPC9774 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.
Pulse
Generator
Z = 50W
Z = 50Ω
MPC9774 DUT
RT = 50Ω
VTT
Z = 50Ω
RT = 50Ω
VTT
Figure 11. CCLK MPC9774 AC test reference for Vcc = 3.3V and Vcc = 2.5V
TIMING SOLUTIONS
11
MOTOROLA
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