XRK4991A Datasheet, PDF (4/13 Pages) Exar Corporation – 3.3V HIGH-SPEED (85 MHZ) PROGRAMMABLE SKEW CLOCK BUFFER

XRK4991A
PRELIMINARY
3.3V HIGH-SPEED (85 MHZ) PROGRAMMABLE SKEW CLOCK BUFFER
xr
REV. P1.0.2
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.
TABLE 1: PLL PROGRAMMABLE SKEW RANGE AND RESOLUTION TABLE
Timing Unit Calculation (t_U)
VCO Frequency Range (F_NOM) ^(1,2)
Skew Adjustment Range ⁽³⁾
Max Adjustment:
FSEL = LOW
1/(44 x F_NOM)
15 to 35MHz
FSEL = MID FSEL = HIGH
1/(26 x F_NOM) 1/(16 x F_NOM)
25 to 60MHz 40 to 100MHz
+9.09ns
+49°
+9.23ns
+83°
+9.38ns
+135°
COMMENTS
ns
Phase Degrees
+14%
+23%
+37%
% of Cycle Time
Example 1, F_NOM = 15MHz
Example 2, F_NOM = 25MHz
Example 3, F_NOM = 30MHz
Example 4, F_NOM = 40MHz
Example 5, F_NOM = 50MHz
Example 6, F_NOM = 80MHz
t_U = 1.52ns
t_U = 0.91ns
t_U = 0.76ns
t_U = 1.54ns
t_U = 1.28ns
t_U = 0.96ns
t_U = 0.77ns
t_U = 1.56ns
t_U = 1.25ns
t_U = 0.78ns
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 4t_U skewed output is used for feedback, all other
outputs will be skewed –4t_U in addition to whatever skew value is programmed for those outputs. ‘Max adjustment’
range applies to output pairs 3 and 4 where ± 6t_U skew adjustment is possible and at the lowest FNOM value.
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