MC12430

TIMING SOLUTIONS

BR1333 — Rev 6

7

MOTOROLA

Power Supply Filtering

The MC12430 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 MC12430 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 MC12430.

NO TAG illustrates a typical power supply filter scheme.

The MC12430 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 MC12430. 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 NO TAG must have a

resistance of 10–15

Ω 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.

Figure 3. Power Supply Filter

PLL_VCC

VCC

MC12430

0.01

µF

22

µF

0.01

µF

3.3V or

5.0V

RS=10–15Ω

A higher level of attenuation can be acheived by replacing

the resistor with an appropriate valued inductor. A 1000

µH

choke will show a significant impedance at 10KHz

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

Ω). Generally

the resistor/capacitor filter will be cheaper, easier to

implement and provide an adequate level of supply filtering.

The MC12430 provides sub–nanosecond output edge

rates and thus a good power supply bypassing scheme is a

must. NO TAG shows a representaive board layout for the

MC12430. There exists many different potential board

layouts and the one pictured is but one. The important aspect

of the layout in NO TAG 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 12430 outputs. It is

imperative that low inductance chip capacitors are used; it is

equally important that the board layout does not introduce

back all 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 4. PCB Board Layout for MC12430

C2

1

C3

R1

Xtal

C1

C1

R1 = 10–15

Ω

C1 = 0.01

µF

C2 = 22

µF

C3 = 0.1

µF

= 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.

Although the MC12430 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