other digital video and imaging applications. The P278xA
reduces electromagnetic interference (EMI) at the clock
source, which provides system wide reduction of EMI of
all clock dependent signals. The P278xA allows
significant system cost savings by reducing the number of
circuit board layers and shielding that are traditionally
required to pass EMI regulations.
The P278xA uses the most efficient and optimized
modulation profile approved by the FCC. The P278xA
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
Product Description
The P278xA is a versatile spread spectrum frequency
modulator designed specifically for digital camera and
The P278xA is targeted towards MFP, xDSL, fax modem,
set-top box, USB controller, DSC, and embedded systems.
Block Diagram
FS0
FS1
LF
VDD
PLL
Modulation
XIN /CLKIN
XOUT
Crystal
Oscillator
Frequency
Divider
Feedback
Divider
Phase
Detector
VCO
Output
Divider
ModOUT
VSS
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.
July 2005
rev 1.5
Pin Configuration
XIN / CLKIN
XOUT
FS1
LF
1
2
3
4
P278XA-08S
P278XA-08T
P2781/82/84
8
7
6
5
VDD
FS0
ModOUT
VSS
Standard pin Configuration offered in both
8 pin SOIC and TSSOP Packages
Pin Description (P278XA)
Pin#
1
2
3
4
5
6
7
8
Pin Name
XIN/CLKIN
XOUT
FS1
LF
VSS
ModOUT
FS0
VDD
Type
I
O
I
I
I
O
I
P
Description
Connect to crystal or clock input. This pin has dual functions. It can be connected
either to an external crystal or an external reference clock.
Crystal connection. If using an external reference, this pin must be left
unconnected.
Digital logic input used to select input frequency range
(see the Input Frequency Selection Table). This pin has an internal pull-up
resistor.
External Loop Filter for the PLL. By changing the value of the CRC circuit, the
percentage spread can be adjusted accordingly. See the Loop Filter Selection
Table for detail value.
Ground Connection. Connect to system ground.
Spread Spectrum Clock Output.
Digital logic input used to select input frequency range
(see the Input Frequency Selection Table). This pin has an internal pull-up
resistor.
Connect to +3.3 V
Input Frequency Selection Table
FS1
0
0
1
1
FS0
0
1
0
1
Input (MHz)
3 to 9
10 to 19
20 to 38
39 to 78
Output Frequency Scaling (MHz)
P2781A
P2782A
P2784A
3 to 9
10 to 19
20 to 38
39 to 78
6 to 18
20 to 38
40 to 76
78 to 156
12 to 36
40 to 76
80 to 152
156 to 312
Modulation Rate (KHz)
Fin / 128
Fin / 256
Fin / 512
Fin / 1024
General Purpose EMI Reduction IC
Notice: The information in this document is subject to change without notice.
2 of 11
July 2005
rev 1.5
Loop Filter Selection Table VDD 3.3V
P2781/82/84
Contact Alliance for loop values that are not listed in the table and for component
selection values for industrial and automotive temperatures.
BW = ±0.50%
Input
MHz
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21-22
23-24
25-26
27-28
29-30
31-32
33-34
35-36
37-38
39-42
43-46
47-50
51-54
55-58
59-62
63-66
67-70
71-74
75-78
FS1 FS0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
C1
(pF)
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
C2
(pF)
330,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
1
BW = ±0.75%
C1
(pF)
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
C2
(pF)
330,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
100,000
1
BW = ±1.00%
C1
(pF)
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
270
C2
(pF)
1
BW = ±1.25%
C1
(pF)
560
560
560
680
330
680
270
560
560
680
470
330
330
680
390
270
270
560
560
680
470
330
330
680
390
270
270
560
560
680
470
330
330
680
390
270
270
C2
(pF)
100,000
100,000
100,000
6,800
3,300
6,800
2,700
100,000
100,000
6,800
4,700
3,300
3,300
6,800
3,900
2,700
2,700
100,000
100,000
6,800
4,700
3,300
3,300
6,800
3,900
2,700
2,700
100,000
100,000
6,800
4,700
3,300
3,300
6,800
3,900
2,700
2,700
1
R1
(ohm)
220
270
390
510
620
820
1,000
330
390
510
560
620
750
820
910
1,000
1,200
330
390
510
560
620
750
820
910
1,000
1,200
330
390
510
560
620
750
820
910
1,000
1,200
R1
(ohm)
300
390
560
750
1,000
1,200
1,500
510
560
750
820
1,000
1,100
1,200
1,300
1,500
1,600
560
620
750
820
1,000
1,100
1,200
1,300
1,500
1,600
560
620
750
820
1,000
1,100
1,200
1,300
1,600
1,800
R1
(ohm)
R1
(ohm)
510
680
910
1,200
1,200
2,200
2,200
910
1,100
1,200
1,200
1,200
1,500
2,200
2,200
2,200
2,700
910
1,100
1,200
1,200
1,200
1,500
2,200
2,200
2,200
2,700
910
1,100
1,200
1,200
1,200
1,500
2,200
2,200
2,200
2,700
100,000
390
100,000
560
100,000
750
10,000
1,000
5,600
1,200
12,000
2,200
5,600
2,200
100,000
750
100,000 866(1%)
10,000
1,000
12,000
1,200
5,600
1,200
3,900
1,200
12,000
2,200
10,000
2,200
5,600
2,200
3,300
2,200
100,000
750
100,000 866 (1%)
10,000
1,000
12,000
1,200
6,800
1,200
3,900
1,200
12,000
2,200
10,000
2,200
5,600
2,200
3,300
2,200
100,000
750
100,000 866 (1%)
10,000
1,000
12,000
1,200
6,800
1,200
3,900
1,200
12,000
2,200
8,200
2,200
5,600
2,200
3,300
2,200
1 The BW value is representative of typical conditions
General Purpose EMI Reduction IC
Notice: The information in this document is subject to change without notice.
3 of 11
July 2005
rev 1.5
Spread Spectrum Selection
P2781/82/84
The P278xA performs Zero Cycle Slip when set at low percentage spreading. This allows no occurrence of system timing
error. The optimal setting should minimize system EMI to the fullest without affecting system performance. The spreading is
described as a percentage deviation of the center frequency.
(Note: the center frequency is the frequency of the external reference input on CLKIN, Pin 1.)
The P2781A is designed for PC peripheral, networking, notebook PC, and LCD monitor applications. It is optimized for
operation between 3 to 78MHz range. In the following application schematic example, the P2781A spread percentage
selection is determined by the external LF value specified in the Loop Filter Selection Table. The Input Frequency Selection
Table specifies the input frequency range. The external LF allows the user to fine tune the spread percentage to optimize the
EMI reduction benefits of the spread spectrum.
16 MHz
1 XIN/CLKIN VDD 8
2 XOUT
3 FS1
FS0 7
ModOUT 6
VSS 5
P2781A
2.2 K
0.1µF
+3.3V
Modulated 16MHz Clock
To target Chip
C2
R1
C1
4 LF
0 Ohm
Note: Both logic input pins FS1 and FS0 have to be connected to either VDD or VSS. Do not leave them floating.
General Purpose EMI Reduction IC
Notice: The information in this document is subject to change without notice.
4 of 11
July 2005
rev 1.5
Absolute Maximum Ratings
Symbol
VDD
V
I
V
O
I
IK
I
OK
T
S
T
A
T
J
Input voltage, DC
Output voltage, DC
Input clamp current (V
I
<0 or V
I
>VDD)
Output clamp current (V
I
<0 or V
I
>VDD)
Storage temperature
Ambient temperature range, under bias
Junction temperature
Lead temperature (soldering 10 sec)
Input static discharge voltage protection
(As per JEDEC STD22- A114-B)
P2781/82/84
Parameter
Supply voltage, DC
Rating
(VSS – 0.5) to 7
(VSS-0.5) to (VDD+0.5)
(VSS-0.5) to (VDD + 0.5)
-50 to +50
-50 to +50
-65 to +125
-55 to 125
150
260
2
Unit
V
V
V
mA
mA
°C
°C
°C
°C
kV
Note: These are stress ratings only and functional operation is not implied. Exposure to absolute maximum ratings for extended periods may affect device
reliability.
DC Electrical Characteristics
(Test condition: All parameters are measured at room temperature (+25°C) unless otherwise stated)
Symbol
V
IL
V
IH
I
IL
I
IH
I
XOL
I
XOH
V
OL
V
OH
I
DD
I
CC
VDD
Input low voltage
Parameter
Input high voltage
Input low current
(internal input pull-up resistor on FS0 and FS1)
Input high current
(internal input pull-up resistor on FS0 and FS1)
XOUT output low current
XOUT output high current
Output low voltage (VDD = 3.3V, I
OL
= 20mA)
Output high voltage (VDD = 3.3V, I
OH
= 20mA)
Static supply current
Typical dynamic supply current
(25pF scope probe loading)
Operating voltage
Min
VSS – 0.3
2.0
-
-
-
-
-
2.5
-
5.2 at 3MHz
3.0
Typ
-
-
60
60
10
10
-
-
3
-
3.3
Max
0.8
VDD +0.3
-
-
-
-
0.4
-
-
21.2 at 82MHz
3.6
Unit
V
V
µA
µA
mA
mA
V
V
mA
mA
V
General Purpose EMI Reduction IC
Notice: The information in this document is subject to change without notice.
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