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參數(shù)資料
| 型號： | SK12439PJT |
| 元件分類： | 時鐘產(chǎn)生/分配 |
| 英文描述： | 800 MHz, OTHER CLOCK GENERATOR, PQCC28 |
| 封裝： | PLASTIC, LCC-28 |
| 文件頁數(shù)： | 17/18頁 |
| 文件大?。?/td> | 233K |
| 代理商： | SK12439PJT |
8
Revision 1/February 9, 2001
www.semtech.com
HIGH-PER.ORMANCE PRODUCTS
SK12439
ADVANCED
Application Information (continued)
Using the On-Board Crystal Oscillator
The SK12439 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
SK12439 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, 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 SK12439 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
inaccuracies. In a general computer application, this level
of inaccuracy is immaterial. Table 1 below specifies the
performance requirements of the crystals to be used with
the SK12439.
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Table 1.
Crystal Specifications
Power Supply Filtering
The SK12439 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
SK12439 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
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 SK12439.
Figure 5 illustrates a typical power supply filter scheme.
The SK12439 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 SK12439. From the datasheet,
the IPLL_VCC current (the current sourced through the
PLL_VCC pin) is typically 15 mA (20 mA 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 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.
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