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参数资料
| 型号： | MPC9230FNR2 |
| 厂商： | MOTOROLA INC |
| 元件分类： | 时钟产生/分配 |
| 英文描述： | 800 MHz, OTHER CLOCK GENERATOR, PQCC28 |
| 封装： | PLASTIC, LCC-28 |
| 文件页数： | 8/9页 |
| 文件大小： | 133K |
| 代理商： | MPC9230FNR2 |
4
MPC9230
MOTOROLA ADVANCED CLOCK DRIVERS DEVICE DATA
415
Figure 4. Serial Interface Timing Diagram
S_CLOCK
S_DATA
S_LOAD
M[8:0]
N[1:0]
P_LOAD
T2
T1
T0
N1
N0
M8 M7 M6 M5 M4 M3 M2 M1
M0
M, N
First
Bit
Last
Bit
Power Supply Filtering
The MPC9230 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 pin impacts the device characteristics. The
MPC9230 provides separate power supplies for the digital
circuitry (VCC) and the internal PLL (VCC_PLL) 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 VCC_PLL pin for the MPC9230.
Figure 5 illustrates a typical power supply filter scheme. The
MPC9230 is most susceptible to noise with spectral content in
the 1 kHz to 1 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 that will
be seen between the VCC supply and the MPC9230 pin of the
MPC9230. From the data sheet, the VCC_PLL current (the
current sourced through the VCC_PLL pin) is maximum 20 mA,
assuming that a minimum of 2.835 V must be maintained on the
VCC_PLL pin. The resistor shown in Figure 5 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 VCC_PLL pin, a low DC
resistance inductor is required (less than 15
).
Figure 5. VCC_PLL Power Supply Filter
VCC_PLL
VCC
MPC9230
C1, C2 = 0.01...0.1 F
VCC
CF = 22 F
RF = 10-15
C2
C1
Layout Recommendations
The MPC9230 provides sub–nanosecond output edge rates
and thus a good power supply bypassing scheme is a must.
Figure 6 shows a representative board layout for the MPC9230.
There exists many different potential board layouts and the one
pictured is but one. The important aspect of the layout in Figure
6 is the low impedance connections between VCC and GND for
the bypass capacitors. Combining good quality general pur-
pose chip capacitors with good PCB layout techniques will
produce effective capacitor resonances at frequencies ade-
quate to supply the instantaneous switching current for the
MPC9230 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 MPC9230 has
several design features to minimize the susceptibility to power
supply noise (isolated power and grounds and fully differential
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