FS6377-01G-XTP データシートの表示（PDF） - ON Semiconductor
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FS6377-01G-XTP Datasheet PDF : 24 Pages
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FS6377
For example, a fixed divide-by-eight could be used in the feedback divider. Unfortunately, a divide-by-eight would limit the effective
modulus of the entire feedback divider to multiples of eight. This limitation would restrict the ability of the PLL to achieve a desired input-
frequency-to-output frequency ratio without making both the reference and feedback divider values comparatively large.
A large feedback modulus means that the divided VCO frequency is relatively low, requiring a wide loop bandwidth to permit the low
frequencies. A narrow loop bandwidth tuned to high frequencies is essential to minimizing jitter; therefore, divider moduli should always
be as small as possible.
To understand the operation, refer to Figure 4. The M-counter (with a modulus always equal to M) is cascaded with the dual-modulus
prescaler. The A-counter controls the modulus of the prescaler. If the value programmed into the A-counter is A, the prescaler will be
set to divide by N+1 for A prescaler outputs. Thereafter, the prescaler divides by N until the M-counter output resets the A-counter, and
the cycle begins again. Note that N=8 and A and M are binary numbers.
Suppose that the A-counter is programmed to zero. The modulus of the prescaler will always be fixed at N; and the entire modulus of
the feedback divider becomes MxN.
Next, suppose that the A-counter is programmed to a one. This causes the prescaler to switch to a divide-by-N+1 for its first divide
cycle and then revert to a divide-by-N. In effect, the A-counter absorbs (or "swallows") one extra clock during the entire cycle of the
feedback divider. The overall modulus is now seen to be equal to MxN+1.
This example can be extended to show that the feedback divider modulus is equal to MxN+A, where A<M.
Figure 4: Feedback Divider
3.1.3. Feedback Divider Programming
For proper operation of the feedback divider, the A-counter must be programmed only for values that are less than or equal to the M-
counter. Therefore, not all divider moduli below 56 are available for use. The selection of divider values is listed in Table 2.
Above a modulus of 56, the feedback divider can be programmed to any value up to 2047.
Table 2: Feedback Divider Modulus Under 56
M-Counter:
FBKDIV[10:3]
A-Counter: FBKDIV[2:0]
000
001
010
011
100
101
110
111
00000001
8
9
00000010
16
17
18
00000011
24
25
26
27
00000100
32
33
34
35
36
00000101
40
41
42
43
44
45
00000110
48
49
50
51
52
53
54
00000111
56
57
58
59
60
61
62
63
Feedback Divider Modulus
Rev. 4 | Page 4 of 24 | www.onsemi.com
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