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FS781/82/84
Document #: 38-07029 Rev. *F Page 2 of 12
Output Frequency Selection
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.
Pin Description
Pin Name I/O Type Description
1/2 (SOIC)
3/4 (TSSOP)
X
IN
/X
OUT
I/O Analog Pins form an on-chip reference oscillator when connected to terminals
of an external parallel resonant crystal. X
IN
may be connected to
TTL/CMOS external clock source. If X
IN
is connected to an external clock
other than crystal, leave X
OUT
(pin 2) unconnected.
7/3 (SOIC)
1/5 (TSSOP)
S0 / S1 I CMOS/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.
4 (SOIC)
6 (TSSOP)
LF I Analog Loop Filter. Single ended three-state output of the phase detector. A two-pole
passive loop filter is connected to LF.
6 (SOIC)
8 (TSSOP)
FSOUT O CMOS/TTL Modulated Clock Frequency Output. 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.
8 (SOIC)
2 (TSSOP)
V
DD
P Power Positive Power Supply.
5 (SOIC)
7 (TSSOP)
V
SS
P Power Power Supply Ground.
Table 1. FSOUT SSCG (Modulated Output Clock) Product Selection
Product Number FSOUT Frequency Scaling Description
FS781 1× 1× modulated frequency of input clock
FS782 2× 2× modulated frequency of input clock
FS784 4× 4× modulated frequency of input clock
Block Diagram
Pin Configuration
Phase
Detector
VCO
1(3)
2(4)
Xin
Xout
10 pF.
Reference
Divider
8 pF
8 pF
250 K
VCO / N
Modulation
Control
Input Control Logic
Output
Divider
and
Mux
Power Contol
Logic
3(5)
7(1)
8(2)
5(7)
4(6)
VDD
S0S1
FSOUT
Loop Filter
VSS
6(8)
VSS
VDD
(TSSOP Pin #)
1
2
3
4
8
7
6
5
Xin
Xout
S1
LF
VDD
S0
FSOUT
VSS
FS78x
8 Pin SOIC Package
1
2
3
4
8
7
6
5
S0
VDD
Xin
Xout
FSOUT
VSS
LF
S1
FS78x
8 Pin TSSOP Package
C8
R6
C7
LF (pin 4)
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