Aug 1998 4.5ns Dual-Comparator-Based Crystal Oscillator has 50% Duty Cycle and Complementary Outputs

DESIGN IDEAS
ing spikes ruin an otherwise clean
output.
Efficiency of the circuit is detailed
in Figure 5. Efficiency reaches 73%
at a 50mA load, and is above 70% at
a 200mA load. Larger inductors with
less copper resistance can be used to
increase efficiency, although such
inductors are more expensive than
the Murata units specified.
90
80
EFFICIENCY (%)
low noise that can be achieved with a
ceramic capacitor may be corrupted
by noise spikes if proper layout practice is not followed. To illustrate this
point, output and switch waveforms
from Figure 1’s circuit, with a 10µ F
ceramic output capacitor and 200mA
load, but with D1’s cathode arbitrarily connected to the ground plane,
are shown in Figure 4. 60mV switch-
70
60
50
40
10
30
100
LOAD CURRENT (mA)
3
300
1610 TA02
Figure 5. 5V to –5V converter efficiency
reaches 73%.
4.5ns Dual-Comparator-Based Crystal
Oscillator has 50% Duty Cycle
and Complementary Outputs
by Joseph Petrofsky and Jim Williams
Figure 1’s circuit uses the LT1720
dual comparator in a 50% duty cycle
crystal oscillator. Output frequencies
of up to 10MHz are practical.
Resistors at C1’s positive input set
a DC bias point. The 2k–0.068µ F
path furnishes phase-shifted feed-
back and C1 acts like a wideband,
unity-gain follower at DC. The crystal’s
path provides resonant positive feedback and stable oscillation occurs.
C2, sensing C1’s input, provides a
low skew, complementary output. A1
compares band-limited versions of
2.7V–6V
2k
1MHz–10MHz
CRYSTAL (AT-CUT)
220Ω
620Ω
+
GROUND
CASE
C1
1/2 LT1720
the outputs and biases C1’s negative
input. C1’s only degree of freedom to
respond is variation of pulse width;
hence, the outputs are forced to 50%
duty cycle.
The circuit operates with AT-cut
fundamental crystals from 1MHz to
10MHz, over a 2.7V–6V power supply
range. 50% duty cycle is maintained
at all supply voltages, with output
skew below 800 picoseconds. Figure
2 plots skew, which is seen to vary by
about 800ps over a 2.7V–6V supply
excursion.
OUTPUT
–
1000
100k
2k
0.1µF
–
680Ω
0.1µF
100k
OUTPUT SKEW (ps)
0.068µF
800
+
A1
LT1636
600
400
200
+
C2
1/2 LT1720
OUTPUT
–
0
2.5
3.0
3.5 4.0 4.5 5.0
SUPPLY VOLTAGE (V)
5.5
6.0
AN70 F52
Figure 1. Crystal oscillator has complementary outputs and 50% duty cycle. A1’s feedback
maintains output duty cycle despite supply variations.
36
Figure 2. Output skew varies only 800ps over
a 2.7V–6V supply excursion.
Linear Technology Magazine • August 1998
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