MC145170P2 데이터 시트보기 (PDF) - Freescale Semiconductor

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MC145170P2

PLL Frequency Synthesizer with Serial Interface

Freescale Semiconductor 

Design Considerations

Power is dissipated in the effective series resistance of the crystal, Re, in Figure 22. The maximum drive

level specified by the crystal manufacturer represents the maximum stress that the crystal can withstand

without damage or excessive shift in operating frequency. R1 in Figure 20 limits the drive level. The use

of R1 is not necessary in most cases.

To verify that the maximum dc supply voltage does not cause the crystal to be overdriven, monitor the

output frequency at the REFout pin (OSCout is not used because loading impacts the oscillator). The

frequency should increase very slightly as the dc supply voltage is increased. An overdriven crystal

decreases in frequency or becomes unstable with an increase in supply voltage. The operating supply

voltage must be reduced or R1 must be increased in value if the overdriven condition exists. The user

should note that the oscillator start-up time is proportional to the value of R1.

Through the process of supplying crystals for use with CMOS inverters, many crystal manufacturers have

developed expertise in CMOS oscillator design with crystals. Discussions with such manufacturers can

prove very helpful.

Frequency

Synthesizer

OSCin Rf

R1*

OSCout

C1

C2

5.0 to 10 pF

* May be needed in certain cases. See text.

Figure 20. Pierce Crystal Oscillator Circuit

OSCin

Cin

Ca

Cout

OSCout

Cstray

Figure 21. Parasitic Capacitances of the Amplifier and Cstray

RS

LS CS

1

2

1

2

CO

1

Re

Xe

2

NOTE: Values are supplied by crystal manufacturer (parallel resonant crystal).

Figure 22. Equivalent Crystal Networks

MC145170-2 Technical Data, Rev. 5

Freescale Semiconductor

19

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