ICS650-47
SPREAD SPECTRUM CLOCK SYNTHESIZER CLOCK SYNTHESIZER
IDT™ / ICS™
SPREAD SPECTRUM CLOCK SYNTHESIZER 3
ICS650-47 REV B 082305
Pin Descriptions
External Components
Decoupling Capacitor
As with any high-performance mixed-signal IC, the
ICS650-47 must be isolated from system power supply
noise to perform optimally.
A decoupling capacitor of 0.01µF must be connected
between each VDD and the PCB ground plane.
Series Termination Resistor
Clock output traces over one inch should use series
termination. To series terminate a 50Ω trace (a commonly
used trace impedance), place a 33Ω resistor in series with
the clock line, as close to the clock output pin as possible.
The nominal impedance of the clock output is 20Ω.
Crystal Load Capacitors
The device crystal connections should include pads for
small capacitors from X1 to ground and from X2 to ground.
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) between the crystal and device. Crystal
capacitors must be connected from each of the pins X1 and
X2 to ground.
The value (in pF) of these crystal caps should equal (C
L
-6
pF)*2. In this equation, C
L
= crystal load capacitance in pF.
Example: For a crystal with a 16 pF load capacitance, each
crystal capacitor would be 20 pF [(16-6) x 2] = 20.
PCB Layout Recommendations
For optimum device performance and lowest output phase
noise, the following guidelines should be observed.
Pin
Number
Pin Name
Pin
Type
Pin Description
1 X1/CLKIN Input Crystal input. Connect this pin to a 25 MHz crystal or external input
clock.
2 FS0 Input Select pin 0. Internal pull-up resistor. See table on page 2.
3 FS1 Input Select pin 1. Internal pull-up resistor. See table on page 2.
4 SS_EN Input Spread spectrum enable pin. Internal pull-up resistor. Enabled = high.
5 VDD Power Connect to +3.3 V.
6 GND Power Connect to ground.
7 FS3 Input Select pin 3. Internal pull-up resistor. See table on page 2.
8 NC — No connect. No internal connection.
9 50M Output Spread Spectrum output. Weak internal pull-down when tri-stated.
10 VDDO Power Connect to +2.5 V.
11 GND Power Connect to ground.
12 VDD Power Connect to +3.3 V.
13 FS2 Input Select pin 2. Internal pull-up resistor. See table on page 2.
14 PDTS
Input Powers down entire chip. Tri-states CLK outputs when low. Internal
pull-up.
15 VDD Power Connect to +3.3 V.
16 X2 Output Crystal Output. Connect this pin to a 25 MHz crystal. Do not connect if
clock input is used.
