November 2003
rev 1.0
ASM3P2531A
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.
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.
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
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
STOP
VDD
Modulation
CLKIN
Crystal
Oscillator
PLL
Frequency
Divider
Feedback
Divider
Phase
Detector
Loop
Filter
VCO
Output
Divider
MODOUT
VSS
Low Frequency EMI Reduction
Notice: The information in this document is subject to change without notice.
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November 2003
rev 1.0
Pin Configuration
ASM3P2531A
CLKIN
VDD
VSS
ModOUT
1
2
3
4
ASM3P2531A
8
7
6
5
NC
NC
NC
STOP
Spread Range Selection, VDD = 3.3 V
CLKIN frequency
35 MHz – 55 MHz
Pin Description
Pin#
1
2
3
4
5
6
7
8
Pin Name
CLKIN
VDD
VSS
ModOUT
STOP
NC
NC
NC
Type
I
P
P
O
I
-
-
-
External reference frequency input.
Power supply for the entire chip.
Ground to entire chip.
Spread spectrum clock output or reference output.
Refer Standby Mode Selection.
Active LOW signal. When HIGH, enables ModOUT and when LOW, ModOUT
would be LOW.
No connect.
No connect.
No connect.
Description
Spreading range
±1.65%
Modulation rate
(CLKIN/1280) KHz
Low Frequency EMI Reduction
Notice: The information in this document is subject to change without notice.
2 of 8
November 2003
rev 1.0
Absolute Maximum Ratings
Symbol
V
DD
, V
IN
T
STG
T
A
Parameter
Voltage on any pin with respect to GND
Storage temperature
Operating temperature
Rating
-0.5 to +7.0
-65 to +125
0 to +70
ASM3P2531A
Unit
V
°C
°C
DC Electrical Characteristics
Symbol
V
IL
V
IH
I
IL
I
IH
V
OL
V
OH
I
DD
I
CC
V
DD
t
ON
Z
OUT
Input low voltage
Input high voltage
Input low current
Input high current
Output low voltage
Output high voltage
V
DD
= 3.3 V, I
OL
= 20 mA
V
DD
= 5.0 V, I
OL
= 20 mA
V
DD
= 3.3 V, I
OL
= 20 mA
V
DD
= 5.0 V, I
OL
= 20 mA
Normal mode
Dynamic supply current
Operating voltage
Power-up time (first locked cycle after power up)
Clock output impedance
3.3 V and 10 pF loading
5.0 V and 10 pF loading
2.5
4.5
0.6
f
IN
-min
3.2
6.2
2.7
3.3
0.18
50
f
IN
-typ
f
IN
-max
7.0
13.6
5.5
mA
mA
V
mS
Ω
Parameter
Min
GND - 0.3
2.0
Typ
Max
0.8
VDD + 0.3
-35
35
0.4
Unit
V
V
µA
µA
V
V
V
V
mA
Static supply current standby mode
AC Electrical Characteristics
Symbol
f
IN
f
OUT
t
LH 1
t
HL 1
t
JC
T
D
Input frequency
Output frequency
Output rise time
Output fall time
Jitter (cycle to cycle)
Output duty cycle
45
50
Measured at 0.8 V to 2.0 V
Measured at 1.2 V to 3.75 V
Measured at 2.0 V to 0.8 V
Measured at 1.2 V to 3.75 V
0.6
Parameter
Min
35
35
0.7
0.9
0.75
0.8
0.75
360
55
1.0
Typ
Max
55
55
1.1
Unit
MHz
MHz
ns
ns
ns
ns
ps
%
Low Frequency EMI Reduction
Notice: The information in this document is subject to change without notice.
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