September 2003

ASM3P2185B

rev 1.0

Alliance Semiconductor

2575, Augustine Drive

• Santa Clara, CA • Tel: 408.855.4900 • Fax: 408.855.4999 • www.alsc.com

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

EMI Reduction IC

Features

FCC approved method of EMI attenuation.

Provides up to 15dB EMI reduction.

Generates a 1X low EMI spread spectrum clock of

the input frequency.

Input frequency range: 25MHz to 50 MHz.

Internal loop filter minimizes external components

and board space.

Down Spread and Center Spread.

2 spread frequency deviation selections: -1.25%

and ± 0.8%.

Low inherent cycle-to-cycle jitter.

3.3V operating voltage range.

TTL or CMOS compatible inputs and outputs.

Low power CMOS design.

Supports notebook VGA and other LCD timing

controller applications.

Products are available for industrial temperature

range.

Available in 8-pin SOIC and TSSOP.

Product Description

The ASM3P2185B is a versatile spread spectrum

frequency modulator designed specifically for a wide

range of input clock frequencies from 25MHz to 50MHz.

The ASM3P2185B can generate an EMI reduced clock

from an OSC or a system generated clock.

The ASM3P2185B reduces electromagnetic interference

(EMI) at the clock source, allowing system wide EMI

reduction for all the down stream clocks and data

dependent signals. The ASM3P2185B allows significant

system cost savings by reducing the number of circuit

board layers, ferrite beads, shielding, and other passive

components that are traditionally required to pass EMI

regulations.

The ASM3P2185B uses the most efficient and optimized

modulation profile approved by the FCC and is

implemented in a proprietary all digital method.

The ASM3P2185B modulates the output of a single PLL

in order to “spread” the bandwidth of a synthesized clock,

and more importantly, decreases 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 frequency generators. Lowering

EMI by increasing a signal’s bandwidth is called ‘spread

spectrum clock generation’.

Applications

The ASM3P2185B is targeted towards EMI management

for memory and LVDS interfaces in mobile graphic

chipsets and high-speed digital applications such as PC

peripheral devices, consumer electronics, and embedded

controller systems.

Block Diagram

VSS

XIN

Crystal

Oscillator

Frequenc

y

Feedback

Divider

Modulation

Phase

Detector

Loop

Filter

VCO

Output

Divider

ModOUT

PLL

VDD

SS%

XOUT