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W181
Peak-Reducing EMI Solution
Features
•
Cypress PREMIS™ family offering
• Generates an EMI optimized clocking signal
at the output
• Selectable input to output frequency
• Single 1.25% or 3.75% down or center spread output
• Integrated loop filter components
• Operates with a 3.3V or 5V supply
• Low-power CMOS design
• Available in 8-pin small outline integrated circuit
(SOIC) or 14-pin thin shrink small outline package
(TSSOP select options only)
Simplified Block Diagram
3.3 or 5.0V
Pin Configurations
SOIC
W181-01/51
CLKIN or X1
NC or X2
GND
SS%
1
2
3
4
8
7
6
5
FS2
FS1
VDD
CLKOUT
X1
XTAL
Input
X2
40 MHz
Max.
W181
Spread Spectrum
Output
(EMI suppressed)
CLKIN or X1
NC or X2
GND
3.3 or 5.0V
SS%
1
2
3
4
8
7
6
5
SSON#
FS1
VDD
CLKOUT
TSSOP
FS2
CLKIN or X1
Oscillator or
Reference Input
1
2
3
4
5
6
7
14
13
12
11
10
9
8
NC
NC
FS1
NC
VDD
NC
CLKOUT
W181-02/03
W181-52/53
W181-01
W181
Spread Spectrum
Output
(EMI suppressed)
NC or X2
GND
NC
SS%
NC
Cypress Semiconductor Corporation
Document #: 38-07152 Rev. *D
•
3901 North First Street
•
San Jose
,
CA 95134
•
408-943-2600
Revised July 06, 2004
W181
Pin Definitions
Pin Name
CLKOUT
CLKIN or X1
Pin No.
(SOIC)
5
1
Pin No.
(TSSOP)(-01)
8
2
Pin
Type
O
I
Pin Description
Output Modulated Frequency:
Frequency modulated copy of the
unmodulated input clock (SSON# asserted).
Crystal Connection or External Reference Frequency Input:
This pin
has dual functions. It may either be connected to an external crystal, or
to an external reference clock.
Crystal Connection:
If using an external reference, this pin must be left
unconnected.
Spread Spectrum Control (Active LOW):
Asserting this signal (active
LOW) turns the internal modulation waveform on. This pin has an internal
pull-down resistor.
Frequency Selection Bit(s) 1 and 2:
These pins select the frequency
range of operation. Refer to
Table 2.
These pins have internal pull-up
resistors.
Modulation Width Selection:
When Spread Spectrum feature is turned
on, this pin is used to select the amount of variation and peak EMI
reduction that is desired on the output signal. This pin has an internal
pull-up resistor.
Power Connection:
Connected to 3.3V or 5V power supply.
Ground Connection:
Connect all ground pins to the common system
ground plane.
No Connection
NC or X2
SSON#
2
8(02/03/52/53)
3
--
I
I
FS1:2
7, 8 (01/51)
12, 1
I
SS%
4
6
I
VDD
GND
NC
6
3
10
4
5, 7, 9, 11, 13,
14
P
G
NC
Key Specifications
Supply Voltages: .........................................V
DD
= 3.3V ± 5%
.................................................................or V
DD
= 5V ± 10%
Frequency Range: ............................ 28 MHz
≤
F
in
≤
75 MHz
Crystal Reference Range.................. 28 MHz
≤
F
in
≤
40 MHz
Cycle to Cycle Jitter: ....................................... 300 ps (max.)
Selectable Spread Percentage: ................... 1.25% or 3.75%
Output Duty Cycle: ............................... 40/60% (worst case)
Output Rise and Fall Time: .................................. 5 ns (max.)
Table 1. Modulation Width Selection
SS%
0
1
W181-01, 02, 03 Output W181-51, 52, 53 Output
–1.25%
(Down Spread)
–3.75%
(Down Spread)
±0.625
(Center Spread)
±1.875%
(Center Spread)
Overview
The W181 products are one series of devices in the Cypress
PREMIS family. The PREMIS family incorporates the latest
advances in PLL spread spectrum frequency synthesizer
techniques. By frequency modulating the output with a
low-frequency carrier, peak EMI is greatly reduced. Use of this
technology allows systems to pass increasingly difficult EMI
testing without resorting to costly shielding or redesign.
In a system, not only is EMI reduced in the various clock lines,
but also in all signals which are synchronized to the clock.
Therefore, the benefits of using this technology increase with
the number of address and data lines in the system. The
Simplified Block Diagram on page 1 shows a simple imple-
mentation.
Functional Description
The W181 uses a phase-locked loop (PLL) to frequency
modulate an input clock. The result is an output clock whose
frequency is slowly swept over a narrow band near the input
signal. The basic circuit topology is shown in
Figure 1.
The
input reference signal is divided by Q and fed to the phase
detector. A signal from the VCO is divided by P and fed back
to the phase detector also. The PLL will force the frequency of
the VCO output signal to change until the divided output signal
and the divided reference signal match at the phase detector
input. The output frequency is then equal to the ratio of P/Q
times the reference frequency. (Note: For the W181 the output
frequency is equal to the input frequency.) The unique feature
of the Spread Spectrum Frequency Timing Generator is that a
modulating waveform is superimposed at the input to the VCO.
This causes the VCO output to be slowly swept across a
predetermined frequency band.
Page 2 of 9
Table 2. Frequency Range Selection
W181 Option#
FS2
0
0
1
1
FS1
0
1
0
1
-01, 51
(MHz)
28
≤
F
IN
≤
38
38
≤
F
IN
≤
48
46
≤
F
IN
≤
60
58
≤
F
IN
≤
75
-02, 52
(MHz)
28
≤
F
IN
≤
38
38
≤
F
IN
≤
48
N/A
N/A
-03, 53
(MHz)
N/A
N/A
46
≤
F
IN
≤
60
58
≤
F
IN
≤
75
Document #: 38-07152 Rev. *D
W181
Because the modulating frequency is typically 1000 times
slower than the fundamental clock, the spread spectrum
process has little impact on system performance.
Frequency Selection With SSFTG
In Spread Spectrum Frequency Timing Generation, EMI
reduction depends on the shape, modulation percentage, and
frequency of the modulating waveform. While the shape and
frequency of the modulating waveform are fixed for a given
frequency, the modulation percentage may be varied.
V
DD
Clock Input
Reference Input
Freq.
Divider
Q
Phase
Detector
Charge
Pump
Using frequency select bits (FS1:2 pins), the frequency range
can be set. Spreading percentage is set to be 1.25% or 3.75%
(see
Table 1).
A larger spreading percentage improves EMI reduction.
However, large spread percentages may either exceed
system maximum frequency ratings or lower the average
frequency to a point where performance is affected. For these
reasons, spreading percentages between 0.5% and 2.5% are
most common.
Σ
Modulating
Waveform
VCO
Post
Dividers
CLKOUT
(EMI suppressed)
Feedback
Divider
P
PLL
GND
Figure 1. Functional Block Diagram
Spread Spectrum Frequency Timing
Generation
The device generates a clock that is frequency modulated in
order to increase the bandwidth that it occupies. By increasing
the bandwidth of the fundamental and its harmonics, the ampli-
tudes of the radiated electromagnetic emissions are reduced.
This effect is depicted in
Figure 2.
As shown in
Figure 2,
a harmonic of a modulated clock has a
much lower amplitude than that of an unmodulated signal. The
reduction in amplitude is dependent on the harmonic number
and the frequency deviation or spread. The equation for the
reduction is:
dB = 6.5 + 9*log
10
(P) + 9*log
10
(F)
where
P
is the percentage of deviation and
F
is the frequency
in MHz where the reduction is measured.
The output clock is modulated with a waveform depicted in
Figure 3.
This waveform, as discussed in “Spread Spectrum
Clock Generation for the Reduction of Radiated Emissions” by
Bush, Fessler, and Hardin produces the maximum reduction
in the amplitude of radiated electromagnetic emissions.
Figure 3
details the Cypress spreading pattern. Cypress does
offer options with more spread and greater EMI reduction.
Contact your local Sales representative for details on these
devices.
Document #: 38-07152 Rev. *D
Page 3 of 9
W181
EMI Reduction
SSFTG
Typical Clock
Amplitude (dB)
Amplitude (dB)
Spread
Spectrum
Enabled
Non-
Spread
Spectrum
Frequency Span (MHz)
Center spread
Frequency Span (MHz)
Down Spread
Figure 2. Clock Harmonic with and without SSCG Modulation Frequency Domain Representation
MAX.
FREQUENCY
10%
20%
30%
40%
50%
60%
70%
80%
90%
10%
20%
30%
40%
50%
60%
70%
80%
100%
90%
MIN.
Figure 3. Typical Modulation Profile
Document #: 38-07152 Rev. *D
100%
Page 4 of 9
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