Datasheet

MAX31180
Spread-Spectrum Crystal Multiplier
7Maxim Integrated
Crystal Capacitor Selection
The load capacitors C
L1
and C
L2
are selected based
on the crystal specifications (from the data sheet of the
crystal used). The crystal parallel load capacitance is
calculated as follows:
L1 L2
L
IN
L1 L2
C C
C C
C C
×
= +
+
Equation 1
For the MAX31180 use C
L1
= C
L2
= C
LX
.
In this case, the equation then reduces to:
LX
L
IN
C
C C
2
= +
Equation 2
where C
L1
= C
L2
= C
LX
.
Equation 2 is used to calculate the values of C
L1
and
C
L2
based on values on C
L
and C
IN
noted in the
Recommended Operating Conditions and DC Electrical
Characteristics.
Power-Supply Decoupling
To achieve best results, it is highly recommended that
a decoupling capacitor is used on the IC power-sup-
ply pins. Typical values of decoupling capacitors are
0.001μF and 0.1μF. Use a high-quality, ceramic, surface-
mount capacitor, and mount it as close as possible to the
V
CC
and GND pins of the IC to minimize lead inductance.
Layout Considerations
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 open as a
programming option (SMSEL and CMSEL). Coupling on
inputs due to clocks should be minimized.
Figure 1. Spread-Spectrum Frequency Modulation
+1.5%
+1.0%
+0.5%
-0.5%
f
0
-1.0%
-1.5%
DITHER CYCLE RATE = f
DITHER
= f
IN
/992
f
SSO
t