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XRK4991A Datasheet(PDF) 4 Page - Exar Corporation

Part No. XRK4991A
Description  3.3V HIGH-SPEED (85 MHZ) PROGRAMMABLE SKEW CLOCK BUFFER
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Maker  EXAR [Exar Corporation]
Homepage  http://www.exar.com
Logo EXAR - Exar Corporation

XRK4991A Datasheet(HTML) 4 Page - Exar Corporation

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XRK4991A
PRELIMINARY
xr
3.3V HIGH-SPEED (85 MHZ) PROGRAMMABLE SKEW CLOCK BUFFER
REV. P1.0.2
4
EXTERNAL FEEDBACK
By providing external feedback, the XRK4991A gives users flexibility with regard to skew adjustment. The
FB_IN signal is compared with the input CLKIN signal at the phase detector in order to drive the VCO. Phase
differences cause the VCO to adjust upwards or downwards accordingly. An internal loop filter moderates the
response of the VCO to the phase detector. The loop filter transfer function has been chosen to provide
minimal jitter (or frequency variation) while still providing accurate responses to input frequency changes.
NOTES:
1. The device may be operated outside recommended frequency ranges without damage, but functional operation is
not guaranteed. Selecting the appropriate FSEL value based on input frequency range allows the PLL to operate
in its ‘sweet spot’ where jitter is lowest.
2. The level to be set on FSEL is determined by the nominal operating frequency of the VCO and Time Unit
Generator. The VCO frequency always appears at QA[1:0], QB[1:0] and the higher outputs when they are
operated in their undivided modes. The frequency appearing at the CLKIN and FB_IN inputs will be the same as
the VCO when the output connected to FB_IN is undivided. The frequency of the CLKIN and FB_IN inputs will be
1/2 or 1/4 the VCO frequency when the part is configured for a frequency multiplication by using a divided output
as the FB_IN input.
3. Skew adjustment range assumes that a zero skew output is used for feedback. If a skewed Q output is used for
feedback, then adjustment range will be greater. For example if a 4tU skewed output is used for feedback, all other
outputs will be skewed –4tU in addition to whatever skew value is programmed for those outputs. ‘Max adjustment’
range applies to output pairs 3 and 4 where ± 6tU skew adjustment is possible and at the lowest FNOM value.
TABLE 1: PLL PROGRAMMABLE SKEW RANGE AND RESOLUTION TABLE
FSEL = LOW
FSEL = MID
FSEL = HIGH
COMMENTS
Timing Unit Calculation (tU)
1/(44 x FNOM)
1/(26 x FNOM)
1/(16 x FNOM)
VCO Frequency Range (FNOM) (1,2)
15 to 35MHz
25 to 60MHz
40 to 100MHz
Skew Adjustment Range (3)
Max Adjustment:
+9.09ns
+9.23ns
+9.38ns
ns
+49°
+83°
+135°
Phase Degrees
+14%
+23%
+37%
% of Cycle Time
Example 1, FNOM = 15MHz
tU = 1.52ns
Example 2, FNOM = 25MHz
tU = 0.91ns
tU = 1.54ns
Example 3, FNOM = 30MHz
tU = 0.76ns
tU = 1.28ns
Example 4, FNOM = 40MHz
tU = 0.96ns
tU = 1.56ns
Example 5, FNOM = 50MHz
tU = 0.77ns
tU = 1.25ns
Example 6, FNOM = 80MHz
tU = 0.78ns


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