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MC12439FN

High Frequency Clock Generator

Motorola => Freescale 

APPLICATIONS INFORMATION

MC12439

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 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 MC12439.

Table 1. 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 80Ω

Correlation Drive Level

100µW

Aging

5ppm/Yr (First 3 Years)

* See accompanying text for series versus parallel resonant

discussion.

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 ciruitry (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 MC12439.

Figure 5 illustrates a typical power supply filter scheme.

The MC12439 is most susceptible to noise with spectral

content in the 1KHz to 1MHz 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 (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 5 must have a

resistance of 10–15Ω 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 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.

3.3V or

5.0V

RS=10–15Ω

PLL_VCC

MC12439

22µF

0.01µF

VCC

0.01µF

Figure 5. Power Supply Filter

TIMING SOLUTIONS

7

BR1333 — Rev 6

MOTOROLA

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