PCS3P625Z05B, PCS3P625Z05C, PCS3P625Z09B, PCS3P625Z09C

http://onsemi.com

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Figure 1. General Block Diagrams

PLL

CLKIN

DLY_CTRL

CLKOUT1

CLKOUT2

CLKOUT3

CLKOUT4

PCS3P625Z05B/C

S2

S1

Select

Input

Decoding

PLL

CLKIN

MUX

DLY_CTRL

CLKOUTA1

CLKOUTA2

CLKOUTA3

CLKOUTA4

CLKOUTB1

CLKOUTB2

CLKOUTB3

CLKOUTB4

PCS3P625Z09B/C

Spread Spectrum Frequency Generation

The clocks in digital systems are typically square waves

with a 50% duty cycle and as frequencies increase the edge

rates also get faster. Analysis shows that a square wave is

composed of fundamental frequency and harmonics. The

fundamental frequency and harmonics generate the energy

peaks that become the source of EMI. Regulatory agencies

test electronic equipment by measuring the amount of peak

energy radiated from the equipment. In fact, the peak level

allowed decreases as the frequency increases. The standard

methods of reducing EMI are to use shielding, filtering,

multi−layer PCBs, etc. These methods are expensive.

Spread spectrum clocking reduces the peak energy by

reducing the Q factor of the clock. This is done by slowly

modulating the clock frequency. The PCS3P625Z05/09 uses

the center modulation spread spectrum technique in which

the modulated output frequency varies above and below the

reference frequency with a specified modulation rate. With

center modulation, the average frequency is the same as the

unmodulated frequency and there is no performance

degradation.

Zero Delay and Skew Control

All outputs should be uniformly loaded to achieve Zero

Delay between input and output. Since the DLY_CTRL pin

is the internal feedback to the PLL, its relative loading can

adjust the input−output delay.

For applications requiring zero input−output delay, all

outputs, including DLY_CTRL, must be equally loaded.

Even if DLY_CTRL is not used, it must have a capacitive

load equal to that on other outputs, for obtaining zero

input−output delay.

Timing−Safe Technology

Timing−Safe technology is the ability to modulate a clock

source with Spread Spectrum technology and maintain

synchronization with any associated data path.