LMV331 datasheet(9/21 Pages) NSC

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Application Circuits (Continued)

Square Wave Oscillator

Comparators are ideal for oscillator applications. This square

wave generator uses the minimum number of components.

The output frequency is set by the RC time constant of the

capacitor C

1 and the resistor in the negative feedback R4.

The maximum frequency is limited only by the large signal

propagation delay of the comparator in addition to any ca-

pacitive loading at the output, which would degrade the out-

put slew rate.

To analyze the circuit, assume that the output is initially high.

For this to be true, the voltage at the inverting input V

c has to

be less than the voltage at the non-inverting input V

a. For Vc

to be low, the capacitor C

1 has to be discharged and will

charge up through the negative feedback resistor R

4. When

it has charged up to value equal to the voltage at the positive

input V

a1, the comparator output will switch.

a1 will be given by:

If:

1 = R2 = R3

Then:

a1 = 2Vcc/3

When the output switches to ground, the value of V

a is re-

duced by the hysteresis network to a value given by:

a2 = Vcc/3

Capacitor C

must now discharge through R

towards

ground. The output will return to its high state when the volt-

age across the capacitor has discharged to a value equal to

a2.

For the circuit shown, the period for one cycle of oscillation

will be twice the time it takes for a single RC circuit to charge

up to one half of its final value. The time to charge the ca-

pacitor can be calculated from

Where V

max is the max applied potential across the capaci-

tor = (2V

cc/3)

and V

C = Vmax/2 = VCC/3

One period will be given by:

1/freq = 2t

or calculating the exponential gives:

1/freq = 2(0.694) R

4 C1

Resistors R

3 and R4 must be at least two times larger than

5 to insure that Vo will go all the way up to Vcc in the high

state. The frequency stability of this circuit should strictly be

a function of the external components.

Free Running Multivibrator

A simple yet very stable oscillator that generates a clock for

slower digital systems can be obtained by using a resonator

as the feedback element. It is similar to the free running mul-

tivibrator, except that the positive feedback is obtained

through a quartz crystal. The circuit oscillates when the

transmission through the crystal is at a maximum, so the

crystal in its series-resonant mode.

The value of R

1 and R2 are equal so that the comparator will

switch symmetrically about +V

cc/2. The RC constant of R3

and C

1 is set to be several times greater than the period of

the oscillating frequency, insuring a 50% duty cycle by main-

taining a DC voltage at the inverting input equal to the abso-

lute average of the output waveform.

When specifying the crystal, be sure to order series resonant

with the desired temperature coefficient

DS100080-8

DS100080-24

FIGURE 5. Squarewave Oscillator

DS100080-7

FIGURE 6. Crystal controlled Oscillator

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