SE555-SP Datasheet, PDF (8/18 Pages) Texas Instruments – QML CLASS V PRECISION TIMER

SE555-SP
SGLS401 – FEBRUARY 2010
www.ti.com
APPLICATION INFORMATION
Monostable Operation
For monostable operation, any of these timers can be connected as shown in Figure 10. If the output is low,
application of a negative-going pulse to the trigger (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 the threshold (THRES) input. If TRIG has returned to a high level, the output of the threshold
comparator resets the flip-flop (Q goes high), drives the output low, and discharges C through Q1.
VCC
(5 V to 15 V)
RA
Input
5
CONT
4 _RESET
7 _DISCH
6 _THRES
2 _TRIG
8
VCC
RL
3
OUT
Output
GND
1
Figure 10. Circuit for Monostable Operation
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_AC. Figure 12 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 reinitiates 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.
8
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