Application Info

MONOSTABLE OPERATION

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

(

Figure 1). The external capacitor is initially held discharged

by internal circuitry. Upon application of a negative trigger

pulse of less than 1/3 V

is set which both releases the short circuit across the capaci-

tor and drives the output high.

The voltage across the capacitor then increases exponen-

tially for a period of t

the output stays high, at the end of which time the voltage

equals 2/3 V

in turn discharges the capacitor and drives the output to its

low state.

Figure 2 shows the waveforms generated in this

mode of operation. Since the charge and the threshold level

of the comparator are both directly proportional to supply

voltage, the timing internal is independent of supply.

Reset overrides Trigger, which can override threshold.

Therefore the trigger pulse must be shorter than the desired

t

400ns for the Reset. 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 cir-

cuit can be reset during this time by the application of a

negative pulse to the reset terminal. The output will then re-

main in the low state until a trigger pulse is again applied.

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

be connected to V

Figure 3 is a nomograph for easy determination of RC values

for various time delays.

Note: In monstable 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 (Trigger and

Threshold terminals connected together) it will trigger itself

and free run as a multivibrator. The external capacitor

charges through R

the duty cycle may be precisely 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 fre-

quency are independent of the supply voltage.

Figure 5 shows the waveform generated in this mode of

operation.

DS008669-4

FIGURE 1. Monostable (One-Shot)

DS008669-10

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

DS008669-11

FIGURE 3. Time Delay

DS008669-5

FIGURE 4. Astable (Variable Duty Cycle Oscillator)

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