
3.3V Communications Clock PLL
MDS 2049-45 G
5
Revision 101904
Integrated Circuit Systems, Inc.
● 525 Race Street, San Jose, CA 95126 ● tel (408) 297-1201 ● www.icst.com
MK2049-45
Charge Pump Current Table
Special considerations must be made in choosing loop
http://www.icst.com/products/telecom/loopfiltercap.htm
Series Termination Resistor
Clock output traces over one inch should use series
termination. To series terminate a 50
trace (a
commonly used trace impedance), place a 33
resistor
in series with the clock line, as close to the clock output
pin as possible. The nominal impedance of the clock
output is 20
. (The optional series termination resistor
is not shown in the External Component Schematic.)
Decoupling Capacitors
As with any high performance mixed-signal IC, the
MK2049-45 must be isolated from system power
supply noise to perform optimally.
Decoupling capacitors of 0.01F must be connected
between each VDD and the PCB ground plane. To
further guard against interfering system supply noise,
the MK2049-45 should use one common connection to
the PCB power plane as shown in the diagram on the
next page. The ferrite bead and bulk capacitor help
reduce lower frequency noise in the supply that can
lead to output clock phase modulation.
Recommended Power Supply Connection
for Optimal Device Performance
Crystal Load Capacitors
The device crystal connections should include pads for
small capacitors from X1 to ground and from X2 to
ground, shown as CL in the External Component
Schematic. These capacitors are used to adjust the
stray capacitance of the board to match the nominally
required crystal load capacitance. Because load
capacitance can only be increased in this trimming
process, it is important to keep stray capacitance to a
minimum by using very short PCB traces (and no vias)
been the crystal and device.
Please refer to
MAN05 for the procedure to determine
capacitor values.
PCB Layout Recommendations
For optimum device performance and lowest output
phase noise, the following guidelines should be
observed. Please also refer to the Recommended PCB
Layout drawing on Page 7.
1) Each 0.01 F decoupling capacitor should be
mounted on the component side of the board as close
to the VDD pin as possible. No via’s should be used
between decoupling capacitor and VDD pin. The PCB
trace to VDD pin should be kept as short as possible,
as should the PCB trace to the ground via. Distance of
the ferrite bead and bulk decoupling from the device is
less critical.
2) The loop filter components must also be placed
close to the CHGP and VIN pins. CP should be closest
to the device. Coupling of noise from other system
RSET
(k
)
Charge Pump Current
(ICP) (A)
13.02
139
15
125
16
119
18
109
20
100
22
93
24
86
27
68
36
56
47
43
56
35
75
28
100
22
150
15
200
12
Connection to 3.3V
Power Plane
Ferrite
Bead
Bulk Decoupling Capacitor
(such as 1 F Tantalum)
VDD Pin
0.01 F Decoupling Capacitors