LT1719CS8 datasheet(16/20 Pages) LINER

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LT1719

APPLICATIONS INFORMATION

±VTRIP Test Circuit

The input trip points test circuit uses a 1kHz triangle wave

to repeatedly trip the comparator being tested. The LT1719

output is used to trigger switched capacitor sampling of

the triangle wave, with a sampler for each direction. Be-

cause the triangle wave is attenuated 1000:1 and fed to the

LT1719’s differential input, the sampled voltages are there-

fore 1000 times the input trip voltages. The hysteresis and

offset are computed from the trip points as shown.

Crystal Oscillator

A simple crystal oscillator using an LT1719 is shown on

the first page of this data sheet. The 2k-620

Ω resistor pair

set a bias point at the comparator’s noninverting input.

The 2k-1.8k-0.1

µF path sets the inverting input node at an

appropriate DC average level based on the output. The

crystal’s path provides resonant positive feedback and

stable oscillation occurs. Although the LT1719 will give

the correct logic output when one input is outside the

common mode range, additional delays may occur when

it is so operated, opening the possibility of spurious

operating modes. Therefore, the DC bias voltages at the

inputs are set near the center of the LT1719’s common

mode range and the 220

Ω resistor attenuates the feed-

back to the noninverting input. The circuit will operate with

any AT-cut crystal from 1MHz to 10MHz over a 2.7V to 6V

supply range. As the power is applied, the circuit remains

off until the LT1719 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 of 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 8 forces a 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 while A1 compares a band-limited ver-

sion of the output and biases C1’s negative input. C1’s only

degree of freedom to respond is variation of pulse width;

hence the output is forced to 50% duty cycle. Again, the

circuit operates from 2.7V to 6V. There is a slight duty

cycle dependence on comparator loading, so minimal

capacitive and resistive loading should be used in critical

applications.

–

LT1719

LT1636

VCC

2.7V TO 6V

620

Ω

220

Ω

1MHz TO 10MHz

CRYSTAL (AT-CUT)

100k

200k

1720 F07

1.8k

0.1

µF

0.1

µF

0.1

µF

OUTPUT

VCC

GROUND

CASE

Figure 8. Crystal Oscillator with a Forced 50% Duty Cycle

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