ICS7151

SPREAD SPECTRUM CLOCK GENERATOR

SSCG

IDT™ / ICS™ SPREAD SPECTRUM CLOCK GENERATOR

3

ICS7151

REV J 051310

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

Ωresistor 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:

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