sa555d-dt STMicroelectronics, sa555d-dt Datasheet - Page 9

• Manufacturer Part Number: sa555d-dt
• Description: General Purpose Single Bipolar Timers
• Manufacturer: STMicroelectronics

NE555 - SA555 - SE555
4
4.1
Application information
Monostable operation
In the monostable mode, the timer generates a single pulse. As shown in
external capacitor is initially held discharged by a transistor inside the timer.
Figure 12. Typical schematics in monostable operation
The circuit triggers on a negative-going input signal when the level reaches 1/3 V
triggered, the circuit remains in this state until the set time has elapsed, even if it is triggered
again during this interval. The duration of the output HIGH state is given by t = 1.1 R
is easily determined by
Note that because the charge rate and the threshold level of the comparator are both
directly proportional to supply voltage, the timing interval is independent of supply. Applying
a negative pulse simultaneously to the reset terminal (pin 4) and the trigger terminal (pin 2)
during the timing cycle discharges the external capacitor and causes the cycle to start over.
The timing cycle now starts on the positive edge of the reset pulse. During the time the reset
pulse is applied, the output is driven to its LOW state.
When a negative trigger pulse is applied to pin 2, the flip-flop is set, releasing the short-
circuit across the external capacitor and driving the output HIGH. The voltage across the
capacitor increases exponentially with the time constant t = R
the capacitor equals 2/3 V
capacitor rapidly and drives the output to its LOW state.
Figure 13
When Reset is not used, it should be tied high to avoid any possibility of unwanted
triggering.
shows the actual waveforms generated in this mode of operation.
Trigger
Output
V
CC
= 5 to 15V
Figure
CC
, the comparator resets the flip-flop which then discharges the
14.
2
3
Reset
4
NE555
1
8
7
5
6
1
Control Voltage
C
1
. When the voltage across
Application information
0.01
R1
μ
C1
Figure
F
12, the
CC
1
. Once
C
1
and
9/20