Spread-Spectrum Crystal Multiplier
Typical Operating Circuit
CRYSTAL
C
L1
C
L2
X1
GND
X2
V
1
2
3
4
8
7
6
5
CC
V
CC
DECOUPLING
CAPACITOR
CMSEL
SMSEL
SSO
PDN
DS1080L
f
SSO
V
CC
NOTE: IN THE ABOVE CONFIGURATION WITH PDN CONNECTED TO V , SMSEL CONNECTED TO GND
CC
AND CMSEL FLOATING, THE DEVICE IS IN NORMAL OPERATION WITH 2x CLOCK MULTIPLICATION, AND
SPREAD-SPECTRUM MAGNITUDE OF ±0.5%.
where C = C = C
LX.
L1
L2
Applications Information
Equation 2 is used to calculate the values of C and
L1
Crystal Selection
The DS1080L requires a parallel resonating crystal
operating in the fundamental mode, with an ESR of less
than 90Ω. The crystal should be placed very close to
the device to minimize excessive loading due to para-
sitic capacitances.
C
based on values on C and C noted in the data
L2
L IN
sheet electrical specifications.
Power-Supply Decoupling
To achieve best results, it is highly recommended that
a decoupling capacitor is used on the IC power-supply
pins. Typical values of decoupling capacitors are
0.001µF and 0.1µF. Use a high-quality, ceramic, sur-
face-mount capacitor, and mount it as close as possi-
Oscillator Input
When driving the DS1080L using an external oscillator
clock, consider the input (X1) to be high impedance.
ble to the V
and GND pins of the IC to minimize lead
CC
inductance.
Crystal Capacitor Selection
The load capacitors C and C are selected based
L1
L2
Layout Considerations
on the crystal specifications (from the data sheet of the
crystal used). The crystal parallel load capacitance is
calculated as follows:
As noted earlier, the crystal should be placed very
close to the device to minimize excessive loading due
to parasitic capacitances. Care should also be taken to
minimize loading on pins that could be floated as a pro-
gramming option (SMSEL and CMSEL). Coupling on
inputs due to clocks should be minimized.
C
x C
L2
+ C
L2
L1
C
=
C
Equation 1
Equation 2
L
IN
C
L1
For the DS1080L use C = C = C .
LX
L1
L2
In this case, the equation then reduces to:
C
LX
2
C
=
+ C
IN
L
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7
DALLAS [ DALLAS SEMICONDUCTOR ]
