COMPARATOR

TRIGGER

FLIP FLOP

COMPARATOR

RESET

+Vcc

DISCHARGE

THRESHOLD

OUTPUT

STAGE

CONTROL

VOLTAGE

OUTPUT

LM555

SNAS548D – FEBRUARY 2000 – REVISED JANUARY 2015

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7 Detailed Description

7.1 Overview

The LM555 is a highly stable device for generating accurate time delays or oscillation. Additional terminals are

provided for triggering or resetting if desired. In the time delay mode of operation, the time is precisely controlled

by one external resistor and capacitor. For astable operation as an oscillator, the free running frequency and duty

cycle are accurately controlled with two external resistors and one capacitor. The circuit may be triggered and

reset on falling waveforms, and the output circuit can source or sink up to 200mA or driver TTL circuits. The

LM555 are available in 8-pin PDIP, SOIC, and VSSOP packages and is a direct replacement for SE555/NE555.

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 Direct Replacement for SE555/NE555

The LM555 timer is a direct replacement for SE555 and NE555. It is pin-to-pin compatible so that no schematic

or layout changes are necessary. The LM555 come in an 8-pin PDIP, SOIC, and VSSOP package.

7.3.2 Timing From Microseconds Through Hours

The LM555 has the ability to have timing parameters from the microseconds range to hours. The time delay of

the system can be determined by the time constant of the R and C value used for either the monostable or

astable configuration. A nomograph is available for easy determination of R and C values for various time delays.

7.3.3 Operates in Both Astable and Monostable Mode

The LM555 can operate in both astable and monostable mode depending on the application requirements.

•

Monostable mode: The LM555 timer acts as a “one-shot” pulse generator. The pulse beings when the LM555

timer receives a signal at the trigger input that falls below a 1/3 of the voltage supply. The width of the output

pulse is determined by the time constant of an RC network. The output pulse ends when the voltage on the

8

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