November 2003

ASM3P2531A

rev 1.0

Low Frequency EMI Reduction

1 of 8

Notice: The information in this document is subject to change without notice.

Features

FCC approved method of EMI attenuation.

Generates a low EMI spread spectrum of the

input clock frequency.

Optimized for input frequency range between

35MHz – 55MHz.

Internal loop filter minimizes external

components and board space.

Frequency Deviation: ±1.65%.

Low inherent cycle-to-cycle jitter.

3.3 V or 5 V operating voltage.

CMOS/TTL compatible inputs and outputs.

Ultra low power CMOS design: 5.50 mA @3.3 V.

Supports notebook VGA and other LCD timing

controller applications.

Available in 8-pin SOIC and TSSOP.

Product Description

The ASM3P2531A is a versatile spread spectrum

frequency modulator designed specifically for a wide

range of clock frequencies. It reduces electromagnetic

interference (EMI) at the clock source allowing system-

wide reduction of EMI of downstream clock and data

dependent signals. It allows significant system cost

savings by reducing the number of circuit board layers

and shielding traditionally required to pass EMI

regulations.

The ASM3P2531A modulates the output of a single PLL

in order to spread the bandwidth of a synthesized clock,

thereby decreasing the peak amplitudes of its harmonics.

This results in significantly lower system EMI compared

to the typical narrow band signal produced by oscillators

and most clock generators. Lowering EMI by increasing

a signal’s bandwidth is called spread spectrum clock

generation.

The ASM3P2531A uses the most efficient and optimized

modulation profile approved by the FCC and is

implemented by using a proprietary all-digital method.

Applications

The ASM3P2531A is targeted toward the notebook VGA

chip and other displays using an LVDS interface, PC

peripheral devices, and embedded systems

Block Diagram

CLKIN

Crystal

Oscillator

Frequency

Divider

Feedback

Divider

Modulation

Phase

Detector

Loop

Filter

VCO

Output

Divider

MODOUT

PLL

VDD

VSS

STOP