Philips Semiconductors Linear Products

Product specification

NE/SA/SE555/SE555C

Timer

August 31, 1994

352

TYPICAL APPLICATIONS

DURATION OF

TRIGGER PULSE AS

SEEN BY THE TIMER

VCC

VCC

10k

2

555

.001

µF

NOTE: All resistor values are in

Ω

Figure 1. AC Coupling of the Trigger Pulse

1

SWITCH GROUNDED

AT THIS POINT

OVOLTS

1/3 VCC

VCC

Trigger Pulse Width Requirements and Time

Delays

Due to the nature of the trigger circuitry, the timer will trigger on the

negative going edge of the input pulse. For the device to time out

properly, it is necessary that the trigger voltage level be returned to

some voltage greater than one third of the supply before the time out

period. This can be achieved by making either the trigger pulse

sufficiently short or by AC coupling into the trigger. By AC coupling

the trigger, see Figure 1, a short negative going pulse is achieved

when the trigger signal goes to ground. AC coupling is most

frequently used in conjunction with a switch or a signal that goes to

ground which initiates the timing cycle. Should the trigger be held

low, without AC coupling, for a longer duration than the timing cycle

the output will remain in a high state for the duration of the low

trigger signal, without regard to the threshold comparator state. This

the state of the bi-stable flip-flop. When the trigger signal then

returns to a high level, the output will fall immediately. Thus, the

output signal will follow the trigger signal in this case.

Another consideration is the “turn-off time”. This is the measurement

low. These steps cause the 2

µs max. delay as stated in the data

sheet.

Also, a delay comparable to the turn-off time is the trigger release

and the circuit then follows the same path and time delay explained

as “turn off time”. This trigger release time is very important in

designing the trigger pulse width so as not to interfere with the

output signal as explained previously.