Datasheet

NE555, NE555Y, SA555, SE555, SE555C
PRECISION TIMERS
SLFS022A – SEPTEMBER 1973 – REVISED SEPTEMBER 2000
9
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
monostable operation
For monostable operation, any of these timers can be connected as shown in Figure 9. If the output is low,
application of a negative-going pulse to TRIG sets the flip-flop (Q goes low), drives the output high, and turns
off Q1. Capacitor C then is charged through R
A
until the voltage across the capacitor reaches the threshold
voltage of THRES input. If TRIG has returned to a high level, the output of the threshold comparator will reset
the flip-flop (Q goes high), drive the output low, and discharge C through Q1.
V
CC
(5 V to 15 V)
R
A
R
L
Output
GND
OUT
V
CC
CONT
RESET
DISCH
THRES
TRIGInput
5
8
4
7
6
2
3
1
Pin numbers shown are for the D, JG, and P packages.
Figure 9. Circuit for Monostable Operation
Voltage – 2 V/div
Time – 0.1 ms/div
Capacitor Voltage
Output Voltage
Input Voltage
R
A
= 9.1 kΩ
C
L
= 0.01 µF
R
L
= 1 kΩ
See Figure 9
Figure 10. Typical Monostable Waveforms
Monostable operation is initiated when TRIG
voltage falls below the trigger threshold. Once
initiated, the sequence ends only if TRIG is high
at the end of the timing interval. Because of the
threshold level and saturation voltage of Q1,
the output pulse duration is approximately
t
w
= 1.1R
A
C. Figure 11 is a plot of the time
constant for various values of R
A
and C. The
threshold levels and charge rates both are directly
proportional to the supply voltage, V
CC.
The timing
interval is, therefore, independent of the supply
voltage, so long as the supply voltage is constant
during the time interval.
Applying a negative-going trigger pulse
simultaneously to RESET and TRIG during the
timing interval discharges C and re-initiates the
cycle, commencing on the positive edge of the
reset pulse. The output is held low as long as the
reset pulse is low. To prevent false triggering,
when RESET is not used, it should be connected
to V
CC
.
– Output Pulse Duration – s
C – Capacitance – µF
10
1
10
–1
10
–2
10
–3
10
–4
1001010.10.01
10
–5
0.001
t
w
R
A
= 10 MΩ
R
A
= 10 kΩ
R
A
= 1 kΩ
R
A
= 100 kΩ
R
A
= 1 MΩ
Figure 11. Output Pulse Duration vs Capacitance