ICS7152A
SPREAD SPECTRUM CLOCK GENERATOR SSCG
IDT™ / ICS™
SPREAD SPECTRUM CLOCK GENERATOR 3
ICS7152A REV E 051310
External Components
The ICS7152A 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 pins 3 and 6, as close to these
pins 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 50Ω trace (a commonly used trace
impedance) place a 27Ω resistor in series with the clock line,
as close to the clock output pin as possible. The nominal
impedance of the clock output is 25Ω.
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 27Ω 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
ICS7152A. 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, 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 caps (pF) = (C
L
- 6) x 2
In the equation, C
L
is the crystal load capacitance. So, for a
crystal with a 16 pF load capacitance, two 20 pF [(16-6) x 2]
capacitors should be used.
Spread Spectrum Profile
The ICS7152A 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
Frequency
Modulation Rate
