Datasheet

LT1720/LT1721
16
17201fc
The circuit in Figure 9 shows a crystal oscillator circuit
that generates two nonoverlapping clocks by making full
use of the two independent comparators of the LT1720.
C1 oscillates as before, but with a lower reference level,
C2’s output will toggle at different times. The resistors set
the degree of separation between the output’s high pulses.
With the values shown, each output has a 44% high and
56% low duty cycle, sufficient to allow 2ns between the
high pulses. Figure 10 shows the two outputs.
APPLICATIONS INFORMATION
The optional A1 feedback network shown can be used to
force identical output duty cycles. The steady state duty
cycles of both outputs will be 44%. Note, though, that
the addition of this network only adjusts the percentage
of time each output is high to be the same, which can be
important in switching circuits requiring identical settling
times. It cannot adjust the relative phases between the two
outputs to be exactly 180° apart, because the signal at the
input node driven by the crystal is not a pure sinusoid.
Figure 8. Timing Skew of Figure 7’s Circuit
SUPPLY VOLTAGE (V)
2.5
OUTPUT SKEW (ps)
4.5 6.0
1000
800
600
400
200
0
1720/21 F08
3.5 5.53.0 4.0 5.0
Figure 10. Nonoverlapping Outputs of Figure 9’s Circuit
Figure 9. Crystal-Based Nonoverlapping 10MHz Clock Generator
–
+
–
+
–
+
C1
1/2 LT1720
C2
1/2 LT1720
A1
LT1636
V
CC
2.7V TO 6V
2k
620Ω
220Ω
10MHz
CRYSTAL (AT-CUT)
100k
100k
2.2k
1.3k
2k
1k
17201 F09
0.1μF
0.1μF
0.1μF
OUTPUT 0
OUTPUT 1
GROUND
CASE
OPTIONAL—
SEE TEXT
20ns/DIV
Q1
2V/DIV
Q0
2V/DIV
17201 F10