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參數(shù)資料
| 型號： | MPC961CFA |
| 廠商： | FREESCALE SEMICONDUCTOR INC |
| 元件分類： | 時鐘及定時 |
| 英文描述： | 961 SERIES, PLL BASED CLOCK DRIVER, 17 TRUE OUTPUT(S), 0 INVERTED OUTPUT(S), PQFP32 |
| 封裝： | PLASTIC, LQFP-32 |
| 文件頁數(shù)： | 5/9頁 |
| 文件大?。?/td> | 159K |
| 代理商： | MPC961CFA |
MPC961C
FREESCALE SEMICONDUCTOR ADVANCED CLOCK DRIVERS DEVICE DATA
487
APPLICATIONS INFORMATION
Power Supply Filtering
The MPC961C 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 MPC961C provides separate power supplies
for the output buffers (VCC) and the phase-locked loop (VCCA)
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 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
MPC961C.
Figure 3 illustrates a typical power supply filter scheme. The
MPC961C is most susceptible to noise with spectral content in
the 10 kHz to 10 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 VCCA pin of the
MPC961C. From the data sheet the ICCA current (the current
sourced through the VCCA pin) is typically 2 mA
(5 mA maximum), assuming that a minimum of 2.375 V
(VCC =3.3 V or VCC = 2.5 V) must be maintained on the VCCA
pin. The resistor RF shown in Figure 3 must have a resistance
of 270
(V
CC = 3.3 V) or 5 to 15 (VCC = 2.5 V) 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 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.
Figure 3. Power Supply Filter
Although the MPC961C 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 MPC961C 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 15
the drivers can drive
either parallel or series terminated transmission lines. For more
information on transmission lines the reader is referred to the
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 VCC/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 MPC961C 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 vs
two series terminated lines in parallel. When taken to its
extreme the fanout of the MPC961C clock driver is effectively
doubled due to its capability to drive multiple lines.
Figure 4. Single versus Dual Transmission Lines
The waveform plots of Figure 5 show the simulation results
of an output driving a single line verses two lines. In both cases
the drive capability of the MPC961C 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
MPC961C. The output waveform in Figure 5 shows a step in
the waveform, this step is caused by the impedance mismatch
VCCA
VCC
MPC961C
10 nF
RF = 270 for VCC = 3.3 V
RF = 5–15 for VCC = 2.5 V
CF
33...100 nF
RF
VCC
14
IN
MPC961
OUTPUT
BUFFER
RS = 36
ZO = 50
OutA
14
IN
MPC961
OUTPUT
BUFFER
RS = 36
ZO = 50
OutB0
RS = 36
ZO = 50
OutB1
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