Spread Spectrum Clock Generator

MDS 7151 E

3

Revision 012306

Integrated Circuit Systems, Inc.

ICS7151

External Components

The ICS7151 requires a minimum number of external

components for proper operation.

Decoupling Capacitor

A decoupling capacitor of 0.01µF must be connected

between GND and VDD on pin 7, as close to this pin as

possible. For optimum device performance, the

decoupling capacitor should be mounted on the

component side of the PCB. Avoid the use of vias in the

decoupling circuit.

Series Termination Resistor

Series termination should be used on the clock output.

To series terminate a 50

Ω trace (a commonly used

trace impedance) place a 27

Ω resistor in series with

the clock line, as close to the clock output pin as

possible. The nominal impedance of the clock output is

25

Ω.

PCB Layout Recommendations

For optimum device performance and lowest output

phase noise, the following guidelines should be

observed.

1) The 0.01µF decoupling capacitor should be mounted

on the component side of the board as close to the

VDD pin as possible. No vias should be used between

the 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.

2) To minimize EMI, the 27

Ω series termination resistor

(if needed) should be placed close to the clock output.

3) An optimum layout is one with all components on the

same side of the board, minimizing vias through other

signal layers. Other signal traces should be routed

away from the ICS7151. This includes signal traces just

underneath the device, or on layers adjacent to the

ground plane layer used by the device.

Crystal Information

The crystal used should be a fundamental mode (do

not use third overtone), parallel resonant. Crystal

capacitors should be connected from pins X1 to ground

and X2 to ground to optimize the initial accuracy. The

value of these capacitors is given by the following

equation:

for a crystal with a 16 pF load capacitance, two 20 pF

[(16-6) x 2] capacitors should be used.

Spread Spectrum Profile

The ICS7151 low EMI clock generator uses a triangular

frequency modulation profile for optimal down stream

tracking of zero delay buffers and other PLL devices.

The frequency modulation amplitude is constant with

variations of the input frequency.

Time

Modulation Rate