CDP6872E INTERSIL [Intersil Corporation], CDP6872E Datasheet - Page 6

• Manufacturer Part Number: CDP6872E
• Description: Low Power Crystal Oscillator
• Manufacturer: INTERSIL [Intersil Corporation]

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 Pullability
Figure 4 shows the basic equivalent circuit for a crystal and
its loading circuit.
Where: C
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:
Where: F
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.
FREQUENCY
CRYSTAL
32kHz
1MHz
2MHz
4MHz
F
C
L
R
C
F
C
P
M
P
S
M
CL
M
0
1
= Parallel Mode Resonant Frequency
= Shunt Capacitance
= Series Mode Resonant Frequency
= Motional Inductance
= Motional Capacitance
= Motional Resistance
=
=
2
OSC IN
F
S
---------------------------- -
C
-------
C
M
1
1
1
+
1
+
LOAD CAPS
------------------------------------ -
2 C
-------
C
L
FIGURE 4.
1
C
M
C1, C2
2
0
33pF
33pF
25pF
22pF
0
C
=
+
M
R
C
Equivalent Crystal Load
M
CL
C
3
OSC OUT
2
TRIMMER CAP
V
5-100pF
DD
5-50pF
5-50pF
5-50pF
C3
CDP6872
6
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 V
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.
Further Reading
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.
DD
to minimize