MPC9350

TIMING SOLUTIONS

7

MOTOROLA

Power Supply Filtering

The MPC9350 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 MPC9350 provides separate

power supplies for the output buffers (VCCO) and the

phase-locked loop (VCCA) 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 ofisolation may be required. The simplest form of

isolation is a power supply filter on the VCCA pin for the

MPC9350. Figure 3. illustrates a typical power supply filter

scheme. The MPC9350 is most susceptible to noise with

spectral content in the 10kHz to 5MHz 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 VCCA pin of the MPC9350. From the data sheet the IVCCA

current (the current sourced through the VCCA pin) is typically

10 mA (15 mA maximum), assuming that a minimum of 3.0V

must be maintained on the VCCA pin. Very little DC voltage

drop can be tolerated when a 3.3V VCC supply is used. The

resistor shown in Figure 3. “Power Supply Filter” must have

a resistance of 10-15

W to meet the voltage drop criteria for

VCC=3.3V. For VCC=2.5V operation, RS must be selected to

maintain the minimum VCC specification of 2.375V for the PLL

supply pin for proper operation. 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 its 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. It is recommended that the user start

with an 8-10

Ω resistor to avoid potential VCC drop problems

and only move to the higher value resistors when a higher level

of attenuation is shown to be needed.

Figure 3. Power Supply Filter

VCCA

VCC

MPC9350

0.01

µF

22

µF

0.01

µF

2.5V or 3.3V

RS=5–15Ω

Although the MPC9350 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

schemes discussed in this section should be adequate to

eliminate power supply noise related problems in most

designs.

Driving Transmission Lines

The MPC9350 clock driver was designed to drive high

speed signals in a terminated transmission line environment.

To provide the optimum flexibility to the user the output drivers

were designed to exhibit the lowest impedance possible. With

an output impedance of less than 15

Ω the drivers can drive

either parallel or series terminated transmission lines. For

more information on transmission lines the reader is referred

to Motorola application note AN1091. In most high

performance clock networks point-to-point distribution of

signals is the method of choice. In a point-to-point scheme

either series terminated or parallel terminated transmission

lines can be used. The parallel technique terminates the signal

at the end of the line with a 50

Ω resistance to VCC÷2.

This technique draws a fairly high level of DC current and

thus only a single terminated line can be driven by each output

of the MPC9350 clock driver. For the series terminated case

however there is no DC current draw, thus the outputs can

drive multiple series terminated lines. Figure 4. “Single versus

Dual Transmission Lines” illustrates an output driving a single

series terminated line versus two series terminated lines in

parallel. When taken to its extreme the fanout of the MPC9350

clock driver is effectively doubled due to its capability to drive

multiple lines.

Figure 4. Single versus Dual Transmission Lines

14

Ω

IN

MPC9350

OUTPUT

BUFFER

RS = 36Ω

ZO = 50Ω

OutA

14

Ω

IN

MPC9350

OUTPUT

BUFFER

RS = 36Ω

ZO = 50Ω

OutB0

RS = 36Ω

ZO = 50Ω

OutB1

Freescale Semiconductor, Inc.

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