The Cypress FS781/82/84 are Spread Spectrum clock
generator ICs (SSCG) designed for the purpose of reducing
electromagnetic interference (EMI) found in today’s
high-speed digital systems.
The FS781/82/84 SSCG clocks use a Cypress-proprietary
technology to modulate the input clock frequency, XIN, by
modulating the frequency of the digital clock. By modulating
the reference clock the measured EMI at the fundamental and
harmonic frequencies of FSOUT is greatly reduced. This
reduction in radiated energy can significantly reduce the cost
of complying with regulatory requirements without degrading
digital waveforms.
The Cypress FS781/82/84 clocks are very simple and
versatile devices to use. By programming the two range select
lines, S0 and S1, any frequency from 6- to 82-MHz operating
range can be selected. The FS781/2/4 are designed to
operate over a very wide range of input frequencies and
provides 1×, 2×, and 4× modulated clock outputs.
The FS78x devices have a simple frequency selection table
that allows operation from 6 MHz to 82 MHz in four separate
ranges. The bandwidth of the frequency spread at FSOUT is
determined by the values of the loop filter components. The
modulation rate is determined internally by the input frequency
and the selected input frequency range.
The Bandwidth of these products can be programmed from as
little as 1.0% up to as much as 4.0% by selecting the proper
loop filter value. Refer to the Loop Filter Selection chart in
Table 2 and Table 3 for the recommended values. Due to a
wide range of application requirements, an external loop filter
(LF) is used on the FS78x products. The user can select the
exact amount of frequency modulation suitable for the application. Using a fixed internal loop filter would severely l imit
the use of a wide range of modulation bandwidths (Spread %)
to a few discrete values. Refer to FS791/2/4 products for applications requiring 80- to 140-MHz frequency range.
• Programmable EMI reduction
• Fast time to market
• Lower cost of compliance
• No degradation in rise/fall times
• Lower component and PCB layer count
Cypress Semiconductor Corporation•198 Champion Court•San Jose, CA 95134-1709•408-943-2600
Document #: 38-07029 Rev. *F Revised January 2, 2005
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FS781/82/84
Block Diagram
250 K
1(3)
Xin
Xout
VDD
8 pF
2(4)
Power Contol
8(2)
Logic
VSS
Loop Filter
4(6)
Reference
Divider
Modulation
Control
8 pF
Input Control Logic
VDD
5(7)
3(5)
Phase
Detector
10 pF.
VCO / N
7(1)
S0S1
VSS
Pin Configuration
VCO
Output
Divider
and
Mux
(TSSOP Pin #)
6(8)
Pin Description
PinNameI/OTypeDescription
1/2 (SOIC)
3/4 (TSSOP)
7/3 (SOIC)
1/5 (TSSOP)
4 (SOIC)
6 (TSSOP)
6 (SOIC)
8 (TSSOP)
8 (SOIC)
2 (TSSOP)
5 (SOIC)
7 (TSSOP)
X
IN/XOUT
I/OAnalogPins form an on-chip reference oscillator when connected to terminals
of an external parallel resonant crystal. X
TTL/CMOS external clock source. If X
other than crystal, leave X
OUT
S0 / S1ICMOS/TTL Digital control inputs to select input frequency range and output
frequency scaling. Refer to Table 2 and Table 3 for selection. S0 has internal
pull-down. S1 has internal pull-up.
LFIAnalogLoop Filter. Single ended three-state output of the phase detector. A two-pole
passive loop filter is connected to LF.
FSOUTOCMOS/TTL Modulated Clock Frequency Outp ut. The center frequency is the same as
the input reference frequency for FS781. Input frequency is multiplied by 2×
and 4× for FS782 and FS784, respectively.
FS7811×1× modulated frequency of input clock
FS7822×2× modulated frequency of input clock
FS7844×4× modulated frequency of input clock
Loop Filter Selection Chart
The following table provides a list of recommended loop filter
values for the FS781/82/84. The FS78X is divided into four
ranges and operated at both 3.3V and 5.5 VDC. The loop filter
at the right is representative of the loop filter components in
Table 2.
S1S0Input Frequency Range (MHz)Modulation Divider Number
006 to 16120
0116 to 32240
1032 to 66480
1166 to 82720
FS781/82/84
SSCG Modulation Profile
The digital control inputs S0 and S1 determine the modulation
frequency of FS781/2/4 products. The input frequency is
divided by a fixed number, depending on the operating range
that is selected. The modulation frequency of the FS78x can
be determined from Table 4. To compute the modulation
frequency, determine the values of S0 and S1, and find the
modulation divider number in Table 4.
Theory of Operation
The FS781/82/84 devices are phase-locked loop-(PLL)-type
clock generators using Direct Digital Synthesis (DDS). ‘By
precisely controlling the bandwidth of the output clock, the
FS781/2/4 products become a low-EMI clock generator. The
theory and detailed operation of these products will be
discussed in the following sections.
EMI
All clocks generate unwanted energy in their harmonics.
Conventional digital clocks are square waves with a duty cycle
that is very close to 50%. Because of the 50/50 duty cycle,
digital clocks generate most of their harmonic energy in the
odd harmonics (e.g., third, fifth, seventh). It is possible to
reduce the amount of energy contained in the fundamental
and harmonics by increasing the bandwidth of the fundamental clock frequency. Conventional digital clocks have a
very high Q factor, which means that all of the energy at that
frequency is concentrated in a very narrow bandwidth, conse-
quently, higher energy peaks. Regulatory agencies test
electronic equipment by the amount of peak energy radiated
from the equipment. By reducing the peak energy at the fundamental and harmonic frequencies, the equipment under test is
able to satisfy agency requirements for EMI. Conventional
methods of reducing EMI have been to use shielding, filtering,
multi-layer PCBs, etc. These FS781/2 and 4 reduce the peak
energy in the clock by increasing the clock bandwidth and
lowering the Q of the clock.
SSCG
The FS781/82/84 products use a unique method of modulating
the clock over a very narrow bandwidth and controlled rate of
change, both peak to peak and cycle to cycle. The FS78x
products take a narrow band digital reference clock in the
range of 6–82 MHz and produce a clock that sweeps between
a controlled start and stop frequency and precise rate of
change. To understand what happens to an SSCG clock,
consider that we have a 20-MHz clock with a 50% duty cycle.
From a 20-MHz clock we know the following:
Clock Frequency = Fc = 20 MHz.
Clock Period = Tc = 1/20 MHz = 50 ns.
Consider that this 20-MHz clock is applied to the X
the FS78x as either an externally driven clock or the result of
a parallel resonant crystal connected to pins 1 and 2 of the
FS78x. Also consider that the products are operating from a
5V DC power supply and the loop filter is set for a total
bandwidth spread of 2%. Refer toFigure 2.
input of
IN
+ .5%
1.0%
Xin
Total
- .5%
TIME (microseconds)
Note:
5. With the correct loop filter connected to Pin 4, the following profile will provide the best EMI reduction. This profile can be seen on a Time Domain Analyzer.
Document #: 38-07029 Rev. *FPage 5 of 12
Figure 1. Frequency Profile in Time Domain
[5]
[+] Feedback
50%50%
n
Tc = 50 ns.
Figure 2. 20-MHz Unmodulated Clock
From the above parameters, the output clock at FSOUT will be
sweeping symmetrically around a center frequency of 20 MHz.
The minimum and maximum extremes of this clock will be
+200 kHz and –200 kHz. So we have a clock that is sweeping
from 19.8 MHz to 20.2 MHz and back again. If we were to look
at this clock on a spectrum analyzer we would see the picture
in Figure 3. Keep in mind that this is a drawing of a perfect
clock with no noise.
Fc = 20 MHz
Fmin =
19.8 MHz
Figure 3. Spectrum Analysis of 19.8–20.2 MHz Clock
We see that the original 20-MHz reference clock is at the
center frequency (Cf), and the min. and max. extremes are
positioned symmetrically about the center frequency. This type
of modulation is called Center-Spread. Figure 4 shows a
20-MHz clock as it would be seen on an oscilloscope. The top
trace is the non-modulated reference clock. The bottom trace
is the modulated clock at pin 6. From this comparison chart
you can see that the frequency is decreasing and the period
of each successive clock is increasing. The Tc mea surement s
on the left and right of the bottom trace indicate the max. and
min. extremes of the clock. Intermediate clock changes are
small and accumulate to achieve the total period deviation.
The reverse of this figure would show the clock going fro m
minimum extreme back to the high extreme.
Fmax =
20.2 MHz
FS781/82/84
Tc =49.50 ns.
Figure 4. Period Comparison Chart
Looking at Figure 3, you will note that the peak amplitude of
the 20-MHz non-modulated clock is higher than the wideband
modulated clock. This difference in peak amplitudes between
modulated and unmodulated clocks is the reason why SSCG
clocks are so effective in digital systems. This figure refers to
the fundamental frequency of a clock. A very important characteristic of the SSCG clock is that the bandwidth of the fundamental frequency is multiplied by the harmonic number. In
other words, if the bandwidth of a 20-MHz clock is 200 kHz,
the bandwidth of the third harmonic will be 3 × 200, or 600 kHz.
The amount of bandwidth is relative to the amount of energy
in the clock. Consequently, the wider the bandwidth, the
greater the energy reduction of the clock.
Most applications will not have a problem meeting agency
specifications at the fundamental frequency. It is the higher
harmonics that usually cause the most problems. With an
SSCG clock, the bandwidth and peak energy reduction
increases with the harmonic number. Consider that the
eleventh harmonic of a 20-MHz clock is 220 MHz. With a total
spread of 200 kHz at 20 MHz, the spread at the eleventh
harmonic would be 2.20 MHz, which greatly reduces the peak
energy content. It is typical to see as much as 12- to 18-dB
reduction at the higher harmonics, due to a modulated clock.
The difference in the peak energy of the modulated clock and
the non-modulated clock in typical applications will see a
2–3dB reduction at the fundamental and as much as 8 – 10
dB reduction at the intermediate harmonics: third, fifth,
seventh, etc. At the higher harmonics, it is quite possible to
reduce the peak harmonic energy, compared to the unmodulated clock, by as much as 12 to 18 dB.
Application Notes and Schematic
Figure 5 is configured for the following parameters:
Package selected = FS781.
X
= 20-MHz crystal
IN
FSOUT = 20 MHz (S0 = 1 and S1 = 0).
Bandwidth of the FSOUT clock is determined by the values of
the loop filter connected to pin 4.
Tc = 50.50
Document #: 38-07029 Rev. *FPage 6 of 12
[+] Feedback
FS781/82/84
Crystal is 20 MHz is 1st Order
with 18 pF load capacitance.
If Crystal load capacitance is
different than 18 pF, C1 and C2
must be re-calculated.
For third overtone crystals, a
parallel or series resonant tr ap
is required.
Mount loop filter components as
close to LF pin as possible.
C2
27 pF
C3
27 pF
20 MHz
Y1
R6
** Occasionally, C8 is used to
create a second pole for this loop
filter. Refer to Loop Filter Selection
table.
Figure 5. FS781 Schematic
1
Xin
2
Xout
VDD
8
7
S0
C1
0.1 uF
VDD
FS781
(SOIC)
3
S1
4
LF
C8C7
FSOUT
VSS
6
5
FSOUT
**
.
Document #: 38-07029 Rev. *FPage 7 of 12
[+] Feedback
FS781/82/84
Absolute Maximum Ratings
This device contains circuitry to protect the input against
damage due to high static voltages or electric fields; however,
precautions should be taken to avoid application of any
voltage higher than the absolute maximum rated voltages to
[6]
this circuit. For proper operation, VIN and V
constrained to the range, V
inputs are tied high or low internally. Refers to electrical speci-
< (VIN or V
SS
OUT
should be
OUT
) < VDD. All digital
fications for operating supply range.
Table 5. Absolute Maximum Ratings
ParameterDescriptionMin.Max.Unit
V
DD
VIRvssInput, relative to V
VORvssOutput, relative to V
Operating Voltage3.06.0VDC
SS
SS
–0.3VDD + 0.3VDC
–0.3VDD + 0.3VDC
TOPTemperature, Operating0+70°C
TSTTemperature, Storage–65+150°C
T
J
Table 6. DC Electrical Characteristics V
Temperature, Junction–+125°C
= 3.3V and 5.0V ±10%, XIN = 48 MHz, TA = 0°C to 70°C
DD
ParameterDescriptionMin.Typ.Max.Unit
V
IL
V
IH
I
IL
I
IH
V
OL
V
OH
V
OL
V
OH
Input Low Voltage–0.3 * V
Input High Voltage0.7 * V
DD
DD
VDC
VDC
Input Low Current100µA
Input High Current100µA
Output Low Voltage IOL= 10 mA, VDD = 5V 0.4VDC
Output High Voltage IOH = 10 mA, VDD = 5VVDD – 1.0VDC
Output Low Voltage IOL= 6 mA, VDD = 3.3V0.4VDC
Output High Voltage IOH = 5 mA, VDD = 3.3V2.4VDC
RpdResistor, Pull-down (Pin 7)60K125K200KΩ
RpuResistor, Pull-up (Pin 3)60K125K200KΩ
C
C
I
I
xin
xout
CC
CC
Input Capacitance (Pin 1) 8pF
Output Capacitance (Pin 2)10pF
5V Dynamic Supply Current (CL = No Load)38mA
3.3V Dynamic Supply Current (CL = No Load)20mA
ISCShort Circuit Current (FSOUT)25mA
BWBW% Variations, 3.30V
BWBW% Variations, 5.00V
[7]
[7]
–200+20%
–300+30%
Table 7. Timing Electrical Characteristics VDD = 3.3V and 5.0V ±10%, TA = 0°C to 70°C, CL = 15 pF, XIN = 48 MHz
ParameterDescriptionMin.Typ.Max.Unit
tTLHOutput Rise Time Measured at 10%–90% @ 5 VDC1.82.22.7ns
tTHLOutput Fall Time Measured at 10%–90% @ 5 VDC1.52.02.5ns
tTLHOutput Rise Time Measured at 0.8V–2.0V @ 5 VDC0.50.650.8ns
tTHLOutput Fall Time Measured at 0.8V–2.0 V @ 5 VDC0.50.650.8ns
tTLHOutput Rise Time Measured at 10%–90% @ 3. 3 VD C2.12.653.2ns
tTHLOutput Fall Time Measured at 10%–90% @ 3.3 VDC1.72.12.6ns
tTLHOutput Rise Time Measured at 0.8V–2.0V @ 3.3 VDC0.70.951.2ns
tTHLOutput Fall Time Measured at 0.8V–2.0 V @ 3.3 VDC0.60.851.1ns
TsymF1Output Duty Cycle455055%
Notes:
6. Single Pow e r Supply: The Voltage on any input or /O pin cannot exceed the power pin during power-up.
7. Percentage variations from the bandwidth % values given in
Table 2 and Table 3.
Document #: 38-07029 Rev. *FPage 8 of 12
[+] Feedback
FS781/82/84
Table 7. Timing Electrical Characteristics V
= 3.3V and 5.0V ±10%, TA = 0°C to 70°C, CL = 15 pF , XIN = 48 MHz (continued)
IMIFS781BZB8-pin 150-mil SOICCommercial, 0 to 70°C
IMIFS781BZBT8-pin 150-mil SOIC – Tape and ReelCommercial, 0 to 70°C
IMIFS782BZB8-pin 150-mil SOICCommercial, 0 to 70°C
IMIFS782BZBT8-pin 150-mil SOIC – Tape and ReelCommercial, 0 to 70°C
IMIFS784BZB8-pin 150-mil SOICCommercial, 0 to 70°C
IMIFS784BZBT8-pin 150-mil SOIC – Tape and ReelCommercial, 0 to 70°C
IMIFS781BT8-pin (4.4 mm body) TSSOPCommercial, 0 to 70°C
IMIFS781BTT8-pin (4.4 mm body) TSSOP – Tape and ReelCommercial, 0 to 70°C
IMIFS784BT8-pin (4.4 mm body) TSSOPCommercial, 0 to 70°C
IMIFS784BTT8-pin (4.4 mm body) TSSOP – Tape and ReelCommercial, 0 to 70°C
Lead-free
CYIFS781BSXC8-pin 150-mil SOICCommercial, 0 to 70°C
CYIFS781BSXCT8-pin 150-mil SOIC – Tape and ReelCommercial, 0 to 70°C
CYIFS782BSXC8-pin 150-mil SOICCommercial, 0 to 70°C
CYIFS782BSXCT8-pin 150-mil SOIC – Tape and ReelCommercial, 0 to 70°C
CYIFS784BSXC8-pin 150-mil SOICCommercial, 0 to 70°C
CYIFS784BSXCT8-pin 150-mil SOIC – Tape and ReelCommercial, 0 to 70°C
CYIFS781BZXC8-pin (4.4 mm body) TSSOPCommercial, 0 to 70°C
CYIFS781BZXCT8-pin (4.4 mm body) TSSOP – Tape and ReelCommercial, 0 to 70°C
CYIFS782BZXC8-pin (4.4 mm body) TSSOPCommercial, 0 to 70°C
CYIFS782BZXCT8-pin (4.4 mm body) TSSOP – Tape and ReelCommercial, 0 to 70°C
CYIFS784BZXC8-pin (4.4 mm body) TSSOPCommercial, 0 to 70°C
CYIFS784BZXCT8-pin (4.4 mm body) TSSOP – Tape and ReelCommercial, 0 to 70°C
Note:
8. The ordering part number differs from the marking on the actual device.
Document #: 38-07029 Rev. *FPage 9 of 12
[+] Feedback
Marking Example
8Lead(150Mil)SOICS08
FS781/82/84
Cypress
FS781BS
Date Code, Lot #
Package Drawing and Dimensions
14
58
FS781 B S
8-lead (150-Mil) SOIC S8
PIN1ID
0.150[3.810]
0.157[3.987]
0.230[5.842]
0.244[6.197]
Cypress
FS781BT
Date Code, Lot #
Package
S = SOIC
T = TSSOP
Revision
Cypress Device Driver
1. DIMENSIONS IN INCHES[MM] MIN.
2. PIN1IDISOPTIONAL,
ROUND ON SINGLE LEADFRAME
RECTANGULAR ON MATRIX LEADFRAME
3. REFERENCE JEDEC MS-012
4. PACKAGE WEIGHT 0.07gms
S08.15 STANDARD PKG.
SZ08.15 LEAD FREE PKG.
PART #
MAX.
0.189[4.800]
0.196[4.978]
0.050[1.270]
BSC
0.0138[0.350]
0.0192[0.487]
0.061[1.549]
0.068[1.727]
0.004[0.102]
0.0098[0.249]
0.004[0.102]
SEATING PLANE
0°~8°
0.016[0.406]
0.035[0.889]
0.010[0.254]
0.016[0.406]
X 45°
0.0075[0.190]
0.0098[0.249]
51-85066-*C
Document #: 38-07029 Rev. *FPage 10 of 12
[+] Feedback
Package Drawing and Dimensions (continued)
8-Lead Thin Shrunk Small Outline Package (4.40 MM Body) Z8
FS781/82/84
0.19[0.007]
0.30[0.012]
0.85[0.033]
0.95[0.037]
2.90[0.114]
3.10[0.122]
1
4.30[0.169]
4.50[0.177]
8
0.65[0.025]
BSC.
0.05[0.002]
0.15[0.006]
PIN1ID
6.25[0.246]
6.50[0.256]
1.10[0.043] MAX.
0.076[0.003]
SEATING
PLANE
DIMENSIONS IN MM[INCHES] MIN.
0.25[0.010]
BSC
GAUGE
PLANE
0°-8°
0.50[0.020]
0.70[0.027]
MAX.
0.09[[0.003]
0.20[0.008]
51-85093-*A
All product and company names mentioned in this document may be the trademarks of their respective holders.
**10694806/07/01IKAConvert from IMI to Cypress
*A11165402/27/02IKLAdd new marking suffix for SOIC packages. Converted to FrameMaker.
*B11835508/30/02RGLSwap the location of S0 and S1 in tables 2 and 3 in pages 2,3 and 4.
*C12267912/14/02RBIAdd power up requirements to operating conditions information.
*D277189See ECNRGLAdded Lead-free Devices
*E314274See ECNRGLFixed the Ordering Information to match the DevMaster
*F417662See ECNRGLAdded Maximum Junction Temperature in Absolute Maximum Ratings table
Orig. of
ChangeDescription of Change
Document #: 38-07029 Rev. *FPage 12 of 12
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