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参数资料
| 型号： | MC12429FNR2 |
| 厂商： | INTEGRATED DEVICE TECHNOLOGY INC |
| 元件分类： | 时钟产生/分配 |
| 英文描述： | 400 MHz, OTHER CLOCK GENERATOR, PQCC28 |
| 封装： | PLASTIC, LCC-28 |
| 文件页数： | 8/9页 |
| 文件大小： | 131K |
| 代理商： | MC12429FNR2 |
4
MC12429
MOTOROLA ADVANCED CLOCK DRIVERS DEVICE DATA
377
APPLICATIONS INFORMATION
Using the On–Board Crystal Oscillator
The MC12429 features a fully integrated on–board crystal
oscillator to minimize system implementation costs. The oscil-
lator 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 MC12429 as possible to avoid any board level parasitics.
To facilitate co–location surface mount crystals are recom-
mended, but not required. Because the series resonant design
is affected by capacitive loading on the xtal terminals loading
variation introduced by crystals from different vendors could be
a potential issue. For crystals with a higher shunt capacitance
it may be required to place a resistance across the terminals to
suppress the third harmonic. Although typically not required it
is a good idea to layout the PCB with the provision of adding
this external resistor. The resistor value will typically be be-
tween 500 and 1K
.
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 paral-
lel 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
MC12429 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 MC12429.
Table 5. Recommended Crystal Specifications
Parameter
Value
Crystal Cut
Fundamental AT Cut
Resonance
Series Resonance*
Frequency Tolerance
±75ppm at 25°C
Frequency/Temperature Stability
±150pm 0 to 70°C
Operating Range
0 to 70
°C
Shunt Capacitance
5–7pF
Equivalent Series Resistance (ESR)
50 to 80
Correlation Drive Level
100
W
Aging
5ppm/Yr (First 3 Years)
* See accompanying text for series versus parallel resonant discus-
sion.
Power Supply Filtering
The MC12429 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 sus-
ceptible to random noise, especially if this noise is seen on the
power supply pins. The MC12429 provides separate power
supplies for the digital ciruitry (VCC) and the internal PLL
(PLL_VCC) of the device. The purpose of this design tech-
nique is to try and isolate the high switching noise digital out-
puts 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
PLL_VCC pin for the MC12429.
Figure 6 illustrates a typical power supply filter scheme. The
MC12429 is most susceptible to noise with spectral content in
the 1KHz to 1MHz range. Therefore the filter should be de-
signed 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
MC12429. From the data sheet the IPLL_VCC current (the cur-
rent sourced through the PLL_VCC pin) is typically 15mA
(20mA maximum), assuming that a minimum of 3.0V must be
maintained on the PLL_VCC pin very little DC voltage drop can
be tolerated when a 3.3V VCC supply is used. The resistor
shown in Figure 6 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 20KHz. As the noise frequen-
cy crosses the series resonant point of an individual capacitor
it’s overall impedance begins to look inductive and thus in-
creases with increasing frequency. The parallel capacitor com-
bination shown ensures that a low impedance path to ground
exists for frequencies well above the bandwidth of the PLL.
Figure 6. Power Supply Filter
PLL_VCC
VCC
MC12429
0.01F
22F
L=1000H
R=15
0.01F
3.3V or
5.0V
RS=10-15
3.3V or
5.0V
A higher level of attenuation can be acheived by replacing
the resistor with an appropriate valued inductor. Figure 6
shows a 1000
H choke, this value choke will show a signifi-
cant impedance at 10KHz 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/capacitor filter will be
cheaper, easier to implement and provide an adequate level of
supply filtering.
The MC12429 provides sub–nanosecond output edge rates
and thus a good power supply bypassing scheme is a must.
Figure 7 shows a representaive board layout for the MC12429.
There exists many different potential board layouts and the
one pictured is but one. The important aspect of the layout in
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