LMC555CM datasheet(6/21 Pages) TI

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LMC555

SNAS558J – FEBRUARY 2000 – REVISED MARCH 2013

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APPLICATION INFORMATION

MONOSTABLE OPERATION

In this mode of operation, the timer functions as a one-shot (Figure 5). The external capacitor is initially held

discharged by internal circuitry. Upon application of a negative trigger pulse of less than 1/3 VS to the Trigger

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

Figure 5. Monostable (One-Shot)

The voltage across the capacitor then increases exponentially for a period of tH = 1.1 RAC, which is also the time

that the output stays high, at the end of which time the voltage equals 2/3 VS. The comparator then resets the

flip-flop which in turn discharges the capacitor and drives the output to its low state. Figure 6 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.

VCC = 5V

Top Trace: Input 5 V/Div.

TIME = 0.1 ms/Div.

Middle Trace: Output 5 V/Div.

RA = 9.1 kΩ

Bottom Trace: Capacitor Voltage 2 V/Div.

C = 0.01 µF

Figure 6. Monostable Waveforms

Reset overrides Trigger, which can override threshold. Therefore the trigger pulse must be shorter than the

desired tH. The minimum pulse width for the Trigger is 20ns, and it is 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 circuit can be reset during

this time by the application of a negative pulse to the reset terminal. The output will then remain 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+ to avoid any possibility of false

triggering. Figure 7 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.

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