Applications Information

MONOSTABLE OPERATION

In this mode of operation, the timer functions as a one-shot

(Figure 1). The external capacitor is initially held discharged

by a transistor inside the timer. Upon application of a nega-

tive trigger pulse of less than 1/3 V

set which both releases the short circuit across the capacitor

and drives the output high.

The voltage across the capacitor then increases exponen-

tially for a period of t = 1.1 R

voltage equals 2/3 V

flip-flop which in turn discharges the capacitor and drives the

output to its low state. Figure 2 shows the waveforms gen-

erated in this mode of operation. Since the charge and the

threshold level of the comparator are both directly propor-

tional to supply voltage, the timing interval is independent of

supply.

During the timing cycle when the output is high, the further

application of a trigger pulse will not effect the circuit so long

as the trigger input is returned high at least 10µs before the

end of the timing interval. However the circuit can be reset

during this time by the application of a negative pulse to the

reset terminal (pin 4). The output will then remain in the low

state until a trigger pulse is again applied.

When the reset function is not in use, it is recommended that

it be connected to V

triggering.

Figure 3 is a nomograph for easy determination of R, C

values for various time delays.

NOTE: In monostable operation, the trigger should be driven

high before the end of timing cycle.

ASTABLE OPERATION

If the circuit is connected as shown in Figure 4 (pins 2 and 6

connected) it will trigger itself and free run as a multivibrator.

The external capacitor charges through R

charges through R

set by the ratio of these two resistors.

In this mode of operation, the capacitor charges and dis-

charges between 1/3 V

mode, the charge and discharge times, and therefore the

frequency are independent of the supply voltage.

00785105

FIGURE 1. Monostable

00785106

Top Trace: Input 5V/Div.

TIME = 0.1 ms/DIV.

Middle Trace: Output 5V/Div.

Bottom Trace: Capacitor Voltage 2V/Div.

C = 0.01µF

FIGURE 2. Monostable Waveforms

00785107

FIGURE 3. Time Delay

00785108

FIGURE 4. Astable

www.national.com

7