When the reset function is not used, connect the RESET pin to V+ to avoid any possibility of false triggering.

Figure 7-4 is a nomograph for easy determination of RC values for various time delays.

Note

In monostable operation, drive the trigger high before the end of timing cycle.

Figure 7-4. Time Delay

7.4.2 Astable Operation

If the circuit is connected as shown in Figure 7-5 (TRIGGER and THRESHOLD pins connected together), the

circuit triggers and free runs as a multivibrator. The external capacitor charges through RA + RB and discharges

through RB. Thus, the duty cycle can be precisely set by the ratio of these two resistors.

Figure 7-5. Astable (Variable Duty Cycle Oscillator)

In this mode of operation, the capacitor charges and discharges between 1/3 VS and 2/3 VS. As in the triggered

mode, the charge and discharge times, and therefore, the frequency are independent of the supply voltage.

Figure 7-6 shows the waveform generated in this mode of operation.

LMC555

SNAS558N – JANUARY 2000 – REVISED MARCH 2024

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