EL4584 データシートの表示（PDF） - Renesas Electronics
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EL4584 Datasheet PDF : 15 Pages
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EL4584
smaller, t (see “Timing Diagrams” on page 5) increases,
and if R2 increases, t decreases. For LDET to be low at
lock, |t| < 50 ns. C4 is used mainly to attenuate high
frequency noise from the charge pump.
C3 = -K----d-N--K----V----2nC----O--- = ---4---.--7---7-9---1e---0--–----5---5---0---6-0--.-0-0---5--2--e----6---- = 0.01µF
C4 = -C1---0-3- = 0.0001µF
(EQ. 9)
(EQ. 10)
R3 = K---2--d--N-K----V----C---n-O--- = ---4--2-.--7---7--9--e--1---–0----5-----1----6---.-5-0---05---0-e--0--6---- = 31.5k
(EQ. 11)
Lock Time
Let = R3C3. As t increases, damping increases, but so does
lock time. Decreasing t decreases damping and speeds up
loop response, but increases overshoot and thus increases
the number of hunting oscillations before lock. Critical
damping (= 1) occurs at minimum lock time. Because
decreased damping also decreases loop stability, it is
sometimes desirable to design slightly overdamped (> 1),
trading lock time for increased stability.
FIGURE 17. TYPICAL LOOP FILTER
TABLE 5. LC LOOP FILTER COMPONENTS (APPROXIMATE)
FREQUENCY
R2
R3
C3
C4
(MHz)
(k)
(k)
(µF)
(µF)
13.301
100
30
0.01
0.001
13.5
100
30
0.01
0.001
14.75
100
33
0.01
0.001
17.734
100
39
0.01
0.001
10.738
100
22
0.01
0.001
12.273
100
27
0.01
0.001
14.318
100
30
0.01
0.001
TABLE 6. XTAL LOOP FILTER COMPONENTS
(APPROXIMATE)
FREQUENCY
R2
R3
C3
C4
(MHz)
(k)
(M)
(pF)
(pF)
13.301
100
4.3
68
6.8
13.5
100
4.3
68
6.8
14.75
100
4.3
68
6.8
17.734
100
4.3
68
6.8
10.738
100
4.3
68
6.8
12.273
100
4.3
68
6.8
14.318
100
4.3
68
6.8
PCB Layout Considerations
It is highly recommended that power and ground planes be
used in layout. The oscillator and filter sections constitute a
feedback loop and thus care must be taken to avoid any
feedback signal influencing the oscillator except at the
control input. The entire oscillator/filter section should be
surrounded by copper ground to prevent unwanted
influences from nearby signals. Use separate paths for
analog and digital supplies, keeping the analog (oscillator
section) as short and free from spurious signals as possible.
Careful attention must be paid to correct bypassing. Keep
lead lengths short and place bypass capacitors as close to
the supply pins as possible. If laying out a PCB to use
discrete components for the VCO section, care must be
taken to avoid parasitic capacitance at the OSC pins 3 and
5, and FILTER out (pin 7). Remove ground and power plane
copper above and below these traces to avoid making a
capacitive connection to them. It is also recommended to
enclose the oscillator section within a shielded cage to
reduce external influences on the VCO, as they tend to be
very sensitive to “handwaving” influences, the LC variety
being more sensitive than crystal controlled oscillators. In
general, the higher the operating frequency, the more
important these considerations are. Self contained VCXO or
VCO modules are already mounted in a shielding cage and
therefore do not require as much consideration in layout.
Many crystal manufacturers publish informative literature
regarding use and layout of oscillators which should be
helpful.
The VCO and loop filter section of the EL4583, EL4584,
EL4585 demo board can be implemented in Figures 18, 19
and 20.
FN7174 Rev 3.00
May 9, 2008
Page 11 of 15
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