LT1721IGN datasheet(16/28 Pages) LINER

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LT1720/LT1721

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

4.5

6.0

1000

800

600

400

200

1720/21 F08

3.5

5.5

3.0

4.0

5.0

Figure 10. Nonoverlapping Outputs of Figure 9’s Circuit

Figure 9. Crystal-Based Nonoverlapping 10MHz Clock Generator

–

1/2 LT1720

LT1636

VCC

2.7V TO 6V

620Ω

220Ω

10MHz

CRYSTAL (AT-CUT)

100k

2.2k

1.3k

17201 F09

0.1μF

OUTPUT 0

OUTPUT 1

GROUND

CASE

OPTIONAL—

SEE TEXT

20ns/DIV

2V/DIV

17201 F10

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