LMV7291 datasheet(13/14 Pages) NSC

13 / 14 page

Typical Applications (Continued)

SQUARE WAVE GENERATOR

A typical application for a comparator is as a square wave

oscillator. The circuit below generates a square wave whose

period is set by the RC time constant of the capacitor C

1and

resistor R

4. The maximum frequency is limited by the large

signal propagation delay of the comparator, and by the ca-

pacitive loading at the output, which limits the output slew

rate.

To analyze the circuit, consider it when the output is high.

That implies that the inverted input (V

C) is lower than the

non-inverting input (V

A). This causes the C1 to get charged

through R

4, and the voltage VC increases till it is equal to the

non-inverting input. The value of V

A at this point is

If R

1 =R2 =R3 then VA1 =2VCC/3

At this point the comparator switches pulling down the output

to the negative rail. The value of V

A at this point is

If R

1 =R2 =R3 then VA2 =VCC/3

The capacitor C

1 now discharges through R4, and the volt-

age V

C decreases till it is equal to VA2, at which point the

comparator switches again, bringing it back to the initial

stage. The time period is equal to twice the time it takes to

discharge C

from 2V

CC/3

to V

CC/3,

which is given by

4C1.ln2. Hence the formula for the frequency is:

F = 1/(2.R

4.C1.ln2)

20080055

FIGURE 6. Negative Peak Detector

20080056

20080057

FIGURE 7. Squarewave Oscillator

LMV7291 데이터시트(PDF) 13 Page - National Semiconductor (TI)

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