LM555CMMSLASHNOPB datasheet(7/21 Pages) TI

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LM555

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SNAS548C – FEBRUARY 2000 – REVISED MARCH 2013

APPLICATIONS INFORMATION

MONOSTABLE OPERATION

In this mode of operation, the timer functions as a one-shot (Figure 12). The external capacitor is initially held

discharged by a transistor inside the timer. Upon application of a negative trigger pulse of less than 1/3 VCC to

pin 2, the flip-flop is set which both releases the short circuit across the capacitor and drives the output high.

Figure 12. Monostable

The voltage across the capacitor then increases exponentially for a period of t = 1.1 RA C, at the end of which

time the voltage equals 2/3 VCC. The comparator then resets the flip-flop which in turn discharges the capacitor

and drives the output to its low state. Figure 13 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

interval is independent of supply.

VCC = 5V

Top Trace: Input 5V/Div.

TIME = 0.1 ms/DIV.

Middle Trace: Output 5V/Div.

RA = 9.1kΩ

Bottom Trace: Capacitor Voltage 2V/Div.

C = 0.01

μF

Figure 13. Monostable Waveforms

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 10

μs 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 VCC to avoid any possibility of

false triggering.

Figure 14 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.

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