MPC92433 datasheet(16/20 Pages) FREESCALE

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Advanced Clock Drivers Devices

Freescale Semiconductor

MPC92433

VCC_PLL Filter

The MPC92433 is a mixed analog/digital product. Its

analog circuitry is naturally susceptible to random noise,

especially if this noise is seen on the power supply pins.

Random noise on the VCC_PLL pin impacts the device AC

characteristics. The MPC92433 provides separate power

supplies for the digital circuitry (VCC) and the internal PLL

(VCC_PLL) of the device. The purpose of this design

technique is to isolate the high switching noise digital outputs

from the relatively sensitive internal analog phase-locked

loop. In digital system environments where it is more difficult

to minimize noise on the power supplies a second level of

isolation is recommended: a power supply filter on the

VCC_PLL pin for the MPC92433.

Figure 7. VCC_PLL Power Supply Filter

Figure 7 illustrates a recommended power supply filter

scheme.

The MPC92433 is most susceptible to noise with spectral

content in the 100 kHz to 1 MHz 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

VCC_PLL pin of the MPC92433. From the data sheet, the

VCC_PLL current (the current sourced through the VCC_PLL

pin) is maximum 10 mA, assuming that a minimum of 2.985 V

must be maintained on the VCC_PLL pin. The resistor shown

in Figure 7 must have a resistance of 10–15

Ω to meet the

voltage drop criteria. The minimum values for RF and the filter

capacitor CF are defined by the filter characteristics: the RC

filter should provide an attenuation greater than 40 dB for

noise whose spectral content is above 100 kHz. In the

recommended filter shown in Figure 7 the filter cut-off

frequency is around 3.0–4.5 kHz and the noise attenuation at

100 kHz is better than 42 dB.

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.

The On-Chip Crystal Oscillator

The MPC92433 features an integrated on-chip crystal

oscillator to minimize system implementation cost. The

integrated oscillator is a Pierce-type that uses the crystal in

its parallel resonance mode. It is recommended to use a 15

to 20 MHz crystal with a load specification of CL =10 pF.

Crystals with a load specification of CL = 20 pF may be used

at the expense of an resulting slightly higher frequency than

specified for the crystal. Externally connected capacitors on

both the XTAL_IN and XTAL_OUT pins are not required but

can be used to fine-tune the crystal frequency as desired.

The crystal, the trace and optional capacitors should be

placed on the board as close as possible to the MPC92433

XTAL_IN and XTAL_OUT pins to reduce crosstalk of active

signals into the oscillator. Short and wide traces further

reduce parasitic inductance and resistance. It is further

recommended to guard the crystal circuit by placing a ground

ring around the traces and oscillator components.

RF = 10–15 Ω

VCC

CF = 22 µF

10 nF

33...100 nF

VCC_PLL

VCC

MPC92433

Table 22. Recommended Crystal Specifications

Parameter

Value

Crystal Cut

Fundamental AT Cut

Resonance Mode

Parallel

Crystal Frequency

16–20 MHz

Shunt Capacitance C0

5–7 pF

Load Capacitance CL

10 pF

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