se555fe NXP Semiconductors, se555fe Datasheet - Page 7

• Manufacturer Part Number: se555fe
• Description: Timer
• Manufacturer: NXP Semiconductors

Philips Semiconductors Linear Products
TYPICAL APPLICATIONS
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
is due to the predominance of Q
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.
August 31, 1994
Timer
V
CC
NOTE: All resistor values are in
15
on the base of Q
.001 F
V
CC
10k
2
Figure 1. AC Coupling of the Trigger Pulse
555
16
, controlling
1/3 V
O
VOLTS
CC
SWITCH GROUNDED
AT THIS POINT
V
CC
352
Another consideration is the “turn-off time”. This is the measurement
of the amount of time required after the threshold reaches 2/3 V
to turn the output low. To explain further, Q
turns on after reaching 2/3 V
on Q
allows current from Q
low. These steps cause the 2 s max. delay as stated in the data
sheet.
Also, a delay comparable to the turn-off time is the trigger release
time. When the trigger is low, Q
on Q
current to Q
trigger is released, Q
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.
6
15
. Current from Q
. Q
15
20
turns off Q
and Q
DURATION OF
TRIGGER PULSE AS
SEEN BY THE TIMER
24
10
19
6
, which results in output high. When the
16
and Q
turns on Q
to turn on Q
NE/SA/SE555/SE555C
and allows Q
1
CC
11
, which then turns on Q
10
shut off, Q
is on and turns on Q
16
20
which turns Q
and Q
17
to turn on. This turns off
1
15
at the threshold input
24
turns off, Q
to given an output
Product specification
17
off. This
5
11
, which turns
16
which turns
turns on
CC