nbc12439a ON Semiconductor, nbc12439a Datasheet - Page 12

• Manufacturer Part Number: nbc12439a
• Description: 3.3 V/5 V Programmable Pll Synthesized Clock Generator
• Manufacturer: ON Semiconductor

Using the On-Board Crystal Oscillator
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 device as possible to avoid any board level parasitics. To
facilitate
recommended, but not required. Because the series resonant
design is affected by capacitive loading on the crystal
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 between 500 W
and 1 KW.
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 device 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 inaccuracy). Table 12 below specifies the
performance requirements of the crystals to be used with the
device.
* See accompanying text for series versus parallel resonant
Table 12. Crystal Specifications
Crystal Cut
Resonance
Frequency Tolerance
Frequency/Temperature Stability
Operating Range
Shunt Capacitance
Equivalent Series Resistance (ESR)
Correlation Drive Level
Aging
The NBC12439 and NBC12439A feature a fully
The oscillator circuit is a series resonant circuit and thus,
discussion.
co-location,
Parameter
surface
mount
Fundamental AT Cut
Series Resonance*
±75 ppm at 25°C
±150 ppm 0 to 70°C
0 to 70°C
5-7 pF
50 to 80 W
100 mW
5 ppm/Yr
(First 3 Years)
APPLICATIONS INFORMATION
Value
crystals are
NBC12439, NBC12439A
http://onsemi.com
12
Power Supply Filtering
analog/digital products and as such, exhibit 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 NBC12439 and NBC1239A provide
separate power supplies for the digital circuitry (V
the internal PLL (PLL_V
this design technique is to try and isolate the high switching
noise of the 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_V
NBC12349A.
The NBC12439 and NBC12439A are 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
V
NBC12439A. From the data sheet, the PLL_V
(the current sourced through the PLL_V
23 mA (28 mA maximum). Assuming that a minimum of
2.8 V must be maintained on the PLL_V
DC voltage drop can be tolerated when a 3.3 V V
is used. The resistor shown in Figure 8 must have a
resistance of 10-15 W 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.
NBC12439A
CC
The
Figure 8 illustrates a typical power supply filter scheme.
NBC12439
PLL_V
supply and the PLL_V
NBC12439
V
CC
CC
Figure 8. Power Supply Filter
0.01 mF
R
S
= 10-15 W
CC
and
CC
22 mF
pin for the NBC12439 and
) of the device. The purpose of
CC
3.3 V or
0.01 mF
NBC12439A
5.0 V
pin of the NBC12439 and
CC
CC
3.3 V or
5.0 V
pin) is typically
pin, very little
are
L=1000 mH
R=15 W
CC
CC
CC
current
supply
mixed
) and