Chrontel CH7034B Notas de aplicación - Página 5

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CHRONTEL
U3
XI/FIN
CH7034
QFN
Reference Crystal Oscillator
CH7034B includes an oscillator circuit that allows a predefined-frequency crystal to be connected directly.
Alternatively, an externally generated clock source may be supplied to CH7034B. If an external clock source is used,
it should have CMOS level specifications. The clock should be connected to the XI pin, and the XO pin should be
left open. The external source must exhibit ± 20ppm or better frequency accuracy, and have low jitter characteristics.
If a crystal is used, the designer should ensure that the following conditions are met:
The crystal is specified to be predefined-frequency, ± 20 ppm fundamental type and in parallel resonance (NOT
series resonance). The crystal should also have a load capacitance equal to its specified value (C
External load capacitors have their ground connection very close to CH7034B (C
To be able to tune, a variable capacitor may be connected from XI to ground.
Note that the XI and XO pins each has approximately 10 PF (C
calculate the proper external load capacitance to be added to the XI and XO pins, the following calculation should
be used:
C
= (2 x C
) - C
- 2C
ext
L
int
S
Where
C
= external load capacitance required on XI and XO pins.
ext
C
= crystal load capacitance specified by crystal manufacturer.
L
C
= capacitance internal to CH7034B (approximately 10-15 pF on each of XI and XO pins).
int
C
= stray capacitance of the circuit (i.e. routing capacitance on the PCB, associated capacitance of crystal holder
S
from pin to pin etc.).
In general,
C
XI = C
XO = C
int
int
int
C
XI = C
XO = C
ext
ext
ext
Such that
C
= (C
+ C
) / 2 + C
and C
L
int
ext
S
Therefore C
must be specified greater than C
L
206-1000-013
Rev1.4,
68
XI/FIN
67
XO
XO
Figure 6: Crystal Pins
= 2 (C
- C
) - C
=2C
ext
L
S
int
/2 + C
int
06/30/2020
X1
4
GND
P2
1
P1
GND
535-9118-1-ND ( 27 MHz)
C1
18pF
) of shunt capacitance internal to the device. To
int
- (2C
+ C
)
L
S
int
in order to select C
properly.
S
ext
AN-B013
3
2
C2
18pF
).
L
).
ext
5