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參數(shù)資料
| 型號(hào)： | MC12439FNR2 |
| 廠商： | MOTOROLA INC |
| 元件分類： | 時(shí)鐘產(chǎn)生/分配 |
| 英文描述： | 800 MHz, OTHER CLOCK GENERATOR, PQCC28 |
| 封裝： | PLASTIC, LCC-28 |
| 文件頁(yè)數(shù)： | 11/11頁(yè) |
| 文件大小： | 128K |
| 代理商： | MC12439FNR2 |
4
MC12439
MOTOROLA ADVANCED CLOCK DRIVERS DEVICE DATA
396
APPLICATIONS INFORMATION
Using the On–Board Crystal Oscillator
The MC12439 features a fully integrated on–board crystal
oscillator to minimize system implementation costs. The
oscillator is a series resonant, multivibrator type design as
opposed to the more common parallel resonant oscillator
design. The series resonant design provides better stability and
eliminates the need for large on chip capacitors. The oscillator
is totally self contained so that the only external component
required is the crystal. As the oscillator is somewhat sensitive
to loading on its inputs, the user is advised to mount the crystal
as close to the MC12439 as possible to avoid any board level
parasitics. To facilitate co–location, surface mount crystals are
recommended but not required.
The oscillator circuit is a series resonant circuit and thus for
optimum performance a series resonant crystal should be used.
Unfortunately, most crystals are characterized in a parallel
resonant mode. Fortunately, there is no physical difference
between a series resonant and a parallel resonant crystal. The
difference is purely in the way the devices are characterized. As
a result, a parallel resonant crystal can be used with the
MC12439 with only a minor error in the desired frequency. A
parallel resonant mode crystal used in a series resonant circuit
will exhibit a frequency of oscillation a few hundred ppm lower
than specified, a few hundred ppm translates to kHz inaccura-
cies. In a general computer application, this level of inaccuracy
is immaterial. Table 1 below specifies the performance require-
ments of the crystals to be used with the MC12439.
Table 1. Crystal Specifications
Parameter
Value
Crystal Cut
Fundamental AT Cut
Resonance
Series Resonance*
Frequency Tolerance
±75 ppm at 25°C
Frequency/Temperature Stability
±150 pm 0 to 70°C
Operating Range
0 to 70
°C
Shunt Capacitance
5–7 pF
Equivalent Series Resistance (ESR)
50 to 80
Correlation Drive Level
100
W
Aging
5 ppm/Yr (First 3 Years)
* See accompanying text for series versus parallel resonant discus-
sion.
Power Supply Filtering
The MC12439 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 MC12439 provides separate power supplies
for the digital circuitry (VCC) and the internal PLL (PLL_VCC) 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 environ-
ment 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
PLL_VCC pin for the MC12439.
Figure 4 illustrates a typical power supply filter scheme. The
MC12439 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 PLL_VCC pin of the
MC12439. From the data sheet, the IPLL_VCC current (the
current sourced through the PLL_VCC pin) is typically 15mA
(20 mA maximum), assuming that a minimum of 3.0 V must be
maintained on the PLL_VCC pin very little DC voltage drop can
be tolerated when a 3.3 V VCC supply is used. The resistor
shown in Figure 4 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.
Figure 4. Power Supply Filter
PLL_VCC
VCC
MC12439
0.01F
22F
0.01F
3.3V or
5.0V
RS=10-15
A higher level of attenuation can be acheived 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 PLL_VCC pin, a low DC resistance
inductor is required (less than 15
). Generally, the resistor/ca-
pacitor filter will be cheaper, easier to implement and provide an
adequate level of supply filtering.
The MC12439 provides sub–nanosecond output edge rates
and thus a good power supply bypassing scheme is a must.
Figure 5 shows a representative board layout for the MC12439.
There exist many different potential board layouts and the one
pictured is but one. The important aspect of the layout in
Figure 5 is the low impedance connections between VCC and
GND for the bypass capacitors. Combining good quality
general purpose chip capacitors with good PCB layout tech-
niques will produce effective capacitor resonances at frequen-
cies adequate to supply the instantaneous switching current for
the 12439 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
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