LMV331 datasheet(10/21 Pages) NSC

10 / 21 page

Application Circuits (Continued)

Pulse generator with variable duty cycle:

The pulse generator with variable duty cycle is just a minor

modification of the basic square wave generator. Providing a

separate charge and discharge path for capacitor C

1 gener-

ates a variable duty cycle. One path, through R

2 and D2 will

charge the capacitor and set the pulse width (t

1). The other

path, R

1 and D1 will discharge the capacitor and set the time

between pulses (t

2).

By varying resistor R

1, the time between pulses of the gen-

erator can be changed without changing the pulse width.

Similarly, by varying R

2, the pulse width will be altered with-

out affecting the time between pulses. Both controls will

change the frequency of the generator. The pulse width and

time between pulses can be found from:

Solving these equations for t

1 and t2

1 =R4C1ln2

2 =R5C1ln2

These terms will have a slight error due to the fact that V

max

is not exactly equal to 2/3 V

CC but is actually reduced by the

diode drop to:

Positive Peak Detector:

Positive peak detector is basically the comparator operated

as a unit gain follower with a large holding capacitor from the

output to ground. Additional transistor is added to the output

to provide a low impedance current source. When the output

of the comparator goes high, current is passed through the

transistor to charge up the capacitor. The only discharge

path will be the 1M ohm resistor shunting C1 and any load

that is connected to the output. The decay time can be al-

tered simply by changing the 1M ohm resistor. The output

should be used through a high impedance follower to a avoid

loading the output of the peak detector.

Negative Peak Detector:

For the negative detector, the output transistor of the com-

parator acts as a low impedance current sink. The only dis-

charge path will be the 1 M

Ω resistor and any load imped-

ance used. Decay time is changed by varying the 1 M

Ω

resistor

DS100080-9

FIGURE 7. Pulse Generator

DS100080-17

FIGURE 8. Positive Peak Detector

DS100080-18

FIGURE 9. Negative Peak Detector

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