NBC12439, NBC12439A
http://onsemi.com
12
APPLICATIONS INFORMATION
Using the On−Board Crystal Oscillator
The NBC12439 and NBC12439A feature 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 load capacitors. The oscillator is totally self contained
so that the only external component required is the crystal
per Figure 8 (do not use cyrstal load caps). 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 co−location,
surface mount crystals are 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, optional R
shunt
,
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.
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 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.
Figure 8. Crystal Application
Table 12. Crystal Specifications
Parameter Value
Crystal Cut Fundamental AT Cut
Resonance Series Resonance*
Frequency Tolerance ±75 ppm at 25°C
Frequency/Temperature Stability ±150 ppm 0 to 70°C
Operating Range 0 to 70°C
Shunt Capacitance 5−7 pF
Equivalent Series Resistance (ESR)
50 to 80 W
Correlation Drive Level
100 mW
Aging 5 ppm/Yr
(First 3 Years)
* See accompanying text for series versus parallel resonant
discussion.
Power Supply Filtering
The NBC12439 and NBC12439A are mixed
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
CC
) and
the internal PLL (PLL_V
CC
) of the device. The purpose of
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
CC
pin for the NBC12439 and
NBC12349A.
Figure 9 illustrates a typical power supply filter scheme.
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
CC
supply and the PLL_V
CC
pin of the NBC12439 and
NBC12439A. From the data sheet, the PLL_V
CC
current
(the current sourced through the PLL_V
CC
pin) is typically
23 mA (28 mA maximum). Assuming that a minimum of
2.8 V must be maintained on the PLL_V
CC
pin, very little
DC voltage drop can be tolerated when a 3.3 V V
CC
supply
is used. The resistor shown in Figure 9 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
