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LT1721IGN Datasheet(PDF) 15 Page - Linear Technology

Part # LT1721IGN
Description  UltraFast:4.5ns at 20mV Overdrive 7ns at 5mV Overdrive
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LT1721IGN Datasheet(HTML) 15 Page - Linear Technology

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LT1720/LT1721
15
17201fc
The gain and hysteresis stage of the LT1720/LT1721 is
simple, short and high speed to help prevent parasitic
oscillations while adding minimum dispersion. This
internal “self-latch” can be usefully exploited in many
applications because it occurs early in the signal chain, in
a low power, fully differential stage. It is therefore highly
immune to disturbances from other parts of the circuit,
either in the same comparator, on the supply lines, or from
the other comparator(s) in the same package. Once a high
speed signal trips the hysteresis, the output will respond,
after a fixed propagation delay, without regard to these
external influences that can cause trouble in nonhysteretic
comparators.
±VTRIP Test Circuit
The input trip points are tested using the circuit shown in
the Test Circuits section that precedes this Applications
Information section. The test circuit uses a 1kHz triangle
wave to repeatedly trip the comparator being tested. The
LT1720/LT1721 output is used to trigger switched capaci-
tor sampling of the triangle wave, with a sampler for each
direction. Because the triangle wave is attenuated 1000:1
and fed to the LT1720/LT1721’s differential input, the
sampled voltages are therefore 1000 times the input trip
voltages. The hysteresis and offset are computed from
the trip points as shown.
Crystal Oscillators
A simple crystal oscillator using one comparator of an
LT1720/LT1721 is shown on the first page of this data
sheet. The 2k-620Ω resistor pair set a bias point at the
comparator’s noninverting input. The 2k-1.8k-0.1μF path
sets the inverting input node at an appropriate DC aver-
age level based on the output. The crystal’s path provides
resonant positive feedback and stable oscillation occurs.
Although the LT1720/LT1721 will give the correct logic
output when one input is outside the common mode range,
additional delays may occur when it is so operated, open-
ing the possibility of spurious operating modes. Therefore,
the DC bias voltages at the inputs are set near the center
of the LT1720/LT1721’s common mode range and the
220Ω resistor attenuates the feedback to the noninvert-
ing input. The circuit will operate with any AT-cut crystal
from 1MHz to 10MHz over a 2.7V to 6V supply range.
APPLICATIONS INFORMATION
As the power is applied, the circuit remains off until the
LT1720/LT1721 bias circuits activate, at a typical VCC of
2V to 2.2V (25°C), at which point the desired frequency
output is generated.
The output duty cycle for this circuit is roughly 50%, but
it is affected by resistor tolerances and, to a lesser extent,
by comparator offsets and timings. If a 50% duty cycle is
required, the circuit of Figure 7 creates a pair of comple-
mentary outputs with a forced 50% duty cycle. Crystals are
narrow-band elements, so the feedback to the noninverting
input is a filtered analog version of the square wave output.
Changing the noninverting reference level can therefore
vary the duty cycle. C1 operates as in the previous example,
whereas C2 creates a complementary output by compar-
ing the same two nodes with the opposite input polarity.
A1 compares band-limited versions of 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. Again, the circuit operates from
2.7V to 6V, and the skew between the edges of the two
outputs are shown in Figure 8. There is a slight duty cycle
dependence on comparator loading, so equal capacitive
and resistive loading should be used in critical applications.
This circuit works well because of the two matched delays
and rail-to-rail style outputs of the LT1720.
Figure 7. Crystal Oscillator with Complementary
Outputs and 50% Duty Cycle
+
+
+
C1
1/2 LT1720
C2
1/2 LT1720
A1
LT1636
VCC
2.7V TO 6V
2k
620Ω
220Ω
1MHz TO 10MHz
CRYSTAL (AT-CUT)
100k
100k
17201 F07
1.8k
2k
1k
0.1μF
0.1μF
0.1μF
OUTPUT
OUTPUT
GROUND
CASE


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