CDP6872H 데이터 시트보기 (PDF) - Intersil
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CDP6872H Datasheet PDF : 15 Pages
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CDP6872
Typical values of the capacitors in Figure 2 are shown below.
Some trial and error may be required before the best combi-
nation is determined. The values listed are total capacitance
including parasitic or other sources. Remember that in the
10kHz to 100kHz frequency range setting the CDP6872
switches in two internal 15pF capacitors.
CRYSTAL
FREQUENCY
32kHz
1MHz
2MHz
LOAD CAPS
C1, C2
33pF
33pF
25pF
TRIMMER CAP
C3
5-50pF
5-50pF
5-50pF
Layout Considerations
Due to the extremely low current (and therefore high imped-
ance) the circuit board layout of the CDP6872 must be given
special attention. Stray capacitance should be minimized.
Keep the oscillator traces on a single layer of the PCB. Avoid
putting a ground plane above or below this layer. The traces
between the crystal, the capacitors, and the OSC pins
should be as short as possible. Completely surround the
oscillator components with a thick trace of VDD to minimize
coupling with any digital signals. The final assembly must be
free from contaminants such as solder flux, moisture, or any
other potential source of leakage. A good solder mask will
help keep the traces free of moisture and contamination over
time.
4MHz
22pF
5-100pF
Further Reading
Crystal Pullability
Figure 4 shows the basic equivalent circuit for a crystal and
its loading circuit.
C1
CM
LM
RM
VDD
C2
C0
2
OSC IN
3
OSC OUT
FIGURE 4.
Where:
CM = Motional Capacitance
LM = Motional Inductance
RM = Motional Resistance
C0 = Shunt Capacitance
CCL = ----C-----1-----1-------1+-----C-----1----2------- = Equivalent Crystal Load
Al Little “HA7210 Low Power Oscillator: Micropower Clock
Oscillator and Op Amps Provide System Shutdown for
Battery Circuits”. Intersil Application Note AN9317.
Robert Rood “Improving Start-Up Time at 32KHz for the
HA7210 Low Power Crystal Oscillator”. Intersil Application
Note AN9334.
S. S. Eaton “Timekeeping Advances Through COS/MOS
Technology”. Intersil Application Note ICAN-6086.
E. A. Vittoz et. al. “High-Performance Crystal Oscillator cir-
cuits: Theory and Application”. IEEE Journal of Solid-State
Circuits, Vol. 23, No3, June 1988, pp774-783.
M. A. Unkrich et. al. “Conditions for Start-Up in Crystal Oscil-
lators”. IEEE Journal of Solid-State Circuits, Vol. 17, No1,
Feb. 1982, pp87-90.
Marvin E. Frerking “Crystal Oscillator Design and Tempera-
ture Compensation”. New York: Van Nostrand-Reinhold,
1978. Pierce Oscillators Discussed pp56-75.
If loading capacitance is connected to a Series Mode Crys-
tal, the new Parallel Mode frequency of resonance may be
calculated with the following equation:
FP = FS 1 + 2--------C----0--C---+--M---C-----C----L-----
Where: FP = Parallel Mode Resonant Frequency
FS = Series Mode Resonant Frequency
In a similar way, the Series Mode resonant frequency may
be calculated from a Parallel Mode crystal and then you may
calculate how much the frequency will “pull” with a new load.
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