FS6377-01G-XTP ON Semiconductor, FS6377-01G-XTP Datasheet - Page 4

• Manufacturer Part Number: FS6377-01G-XTP
• Description: IC CLOCK GEN PLL PROG 16SOIC
• Manufacturer: ON Semiconductor
• Type: PLL Clock Generatorr

Specifications of FS6377-01G-XTP

• Pll: Yes
• Input: Crystal
• Output: CMOS
• Number Of Circuits: 1
• Ratio - Input:output: 1:4
• Differential - Input:output: No/No
• Frequency - Max: 230MHz
• Divider/multiplier: Yes/No
• Voltage - Supply: 3 V ~ 5.5 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Frequency-max: *
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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.
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]
00000001
00000010
00000011
00000100
00000101
00000110
00000111
000
8
16
24
32
40
48
56
001
9
17
25
33
41
49
57
010
18
26
34
42
50
58
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Figure 4: Feedback Divider
Feedback Divider Modulus
011
27
35
43
51
59
A-Counter: FBKDIV[2:0]
100
36
44
52
60
101
45
53
61
110
54
62
111
63