MPC990 MPC991

TIMING SOLUTIONS

BR1333 — Rev 6

7

MOTOROLA

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 MPC990/991. From the data sheet the IVCCA

current (the current sourced through the VCCA pin) is

typically 15mA (20mA 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 4 must have a resistance

of 5–15

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.

Although the MPC990/991 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.

Figure 4. Power Supply Filter

VCCA

VCC

MPC990/991

0.01

µF

22

µF

0.01

µF

3.3V

RS=5–15Ω