NBC12439FN datasheet(13/21 Pages) ONSEMI

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NBC12439, NBC12439A

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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.

Figure 9. Power Supply Filter

PLL_VCC

VCC

NBC12439

NBC12439A

0.01 mF

22 mF

L=1000 mH

R=15 W

0.01 mF

3.3 V or

5.0 V

RS = 10−15 W

3.3 V or

5.0 V

A higher level of attenuation can be achieved by replacing

the resistor with an appropriate valued inductor. Figure 9

shows a 1000

mH choke. This value choke will show a

significant impedance at 10 KHz 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

W). Generally, the

resistor/capacitor filter will be cheaper, easier to implement,

and provide an adequate level of supply filtering.

The

NBC12439

and

NBC12439A

provide

sub−nanosecond output edge rates and therefore a good

power supply bypassing scheme is a must. Figure 10 shows

a representative board layout for the NBC12439. There

exists many different potential board layouts and the one

pictured is but one. The important aspect of the layout in

Figure 10 is the low impedance connections between VCC

and GND for the bypass capacitors. Combining good quality

general purpose chip capacitors with good PCB layout

techniques will produce effective capacitor resonances at

frequencies adequate to supply the instantaneous switching

current for the NBC12439 and NBC12439A outputs. It is

imperative that low inductance chip capacitors are used. It

is equally important that the board layout not introduce any

of the inductance saved by using the leadless capacitors.

Thin interconnect traces between the capacitor and the

power plane should be avoided and multiple large vias

should be used to tie the capacitors to the buried power

planes. Fat interconnect and large vias will help to minimize

layout induced inductance and thus maximize the series

resonant point of the bypass capacitors.

Figure 10. PCB Board Layout for (PLCC−28)

Xtal

R1 = 10−15 W

C1 = 0.01 mF

C2 = 22 mF

C3 = 0.1 mF

= VCC

= GND

= Via

RSHUNT

Note the dotted lines circling the crystal oscillator

connection to the device. The oscillator is a series resonant

circuit and the voltage amplitude across the crystal is

relatively small. It is imperative that no actively switching

signals cross under the crystal as crosstalk energy coupled

to these lines could significantly impact the jitter of the

device. Special attention should be paid to the layout of the

crystal to ensure a stable, jitter free interface between the

crystal and the on−board oscillator. Note the provisions for

placing a resistor across the crystal oscillator terminals as

discussed in the crystal oscillator section of this data sheet.

Although the NBC12439 and NBC12439A have several

design features to minimize the susceptibility to power

supply noise (isolated power and grounds and fully

differential PLL), there still may be applications in which

overall performance is being degraded due to system power

supply noise. The power supply filter and bypass schemes

discussed in this section should be adequate to eliminate

power supply noise−related problems in most designs.

Jitter Performance

Jitter is a common parameter associated with clock

generation and distribution. Clock jitter can be defined as the

deviation in a clock’s output transition from its ideal

position.

Cycle−to−Cycle Jitter (short−term) is the period

variation between adjacent periods over a defined number of

observed cycles. The number of cycles observed is

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