NBC12439FN datasheet(13/20 Pages) ONSEMI

13 / 20 page

NBC12439

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Figure 7. Power Supply Filter

PLL_VCC

VCC

NBC12439

0.01

L=1000

R=15

0.01

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 7

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 provides sub-nanosecond output edge

rates and therefore a good power supply bypassing scheme

is a must. Figure 8 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 8 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 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 8. PCB Board Layout for NBC12439 (28 PLCC)

Xtal

R1 = 10-15

C1 = 0.01

C2 = 22

C3 = 0.1

= VCC

= GND

= Via

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 has 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.

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