CMOS Low Power Timer – LMC555

Rev 1.1

21/01/18

 

Application Notes Continued

 

Astable Mode

 

 

 

The circuit can be connected to trigger itself & free-run as a multivibrator (Figure 3). The output swings from rail-to-rail

and is a true 50% duty cycle square wave. Less than a 1% frequency variation is observed over a voltage range of

+5V to +15V. Duty Cycle is configurable by setting the ratio of resistors RA + RB (Figure 4), the external capacitor

charges through RA + RB and discharges through RB.

 

Monostable Mode

 

 

 

The timer functions as a one-shot. Initially the external capacitor (C) is held discharged by a transistor inside the

timer. Upon application of a negative TRIGGER pulse to pin 2 the internal flip-flop is set which releases the short

circuit across the external capacitor and drives the OUTPUT high. The voltage across the capacitor now increases

exponentially with a time constant t=RAC. When the voltage across the capacitor equals 2/3 VDD, the comparator

resets the flip-flop, which in turn discharges the capacitor rapidly & drives the OUTPUT to its low state. TRIGGER

must return to a high state before the OUTPUT can return to a low state (Figure 2).

 

Control Voltage

 

 

 

The CONTROL VOLTAGE terminal permits the two trip voltages for the THRESHOLD and TRIGGER internal

comparators to be controlled. This provides the possibility of oscillation frequency modulation in the astable mode or

even inhibition of oscillation, depending on the applied voltage. In the monostable mode, delay times can be changed

by varying the applied voltage to the CONTROL VOLTAGE pin.

RESET

 

 

 

 

The RESET terminal is designed to have essentially the same trip voltages as the standard bipolar 555 i.e. 0.6V to

0.7V. At all supply voltages it represents an extremely high input impedance. The mode of operation of the RESET

function is much improved over the standard bipolar 555 in that it controls only the internal flip-flop, which in turn

controls simultaneously the state of the OUTPUT and DISCHARGE pins. This avoids the multiple threshold problems

sometimes encountered with slow falling edges in the legacy bipolar 555 devices. If RESET is not used tie to VDD.

 

 

 

 

 

 

 

Figure 1 – Switching Waveform

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