Specification Sheet

L B
H A B
t R
Low-to-high ratio
t R R
= =
+
H B
H L A B
t R
Output waveform duty cycle 1
t t R 2R
= = -
+ +
L B
H L A B
t R
Output driver duty cycle
t t R 2R
= =
+ +
( )
A B
1.44
frequency
R 2R C
»
+
( )
H L A B
period t t 0.693 R 2R C= + = +
( )
L B
t 0.693 R C=
( )
H A B
t 0.693 R R C= +
GND
OUT
V
CC
CONT
RESET
DISCH
THRES
TRIG
C
R
B
R
A
Output
R
L
0.01 µF
V
CC
(5 V to 15 V)
(see Note A)
NOTE A: Decoupling CONT voltage to ground with a capacitor can
improve operation. This should be evaluated for individual
applications.
Open
5 8
4
7
6
2
3
1
Pin numbers shown are for the D, JG, P, PS, and PW packages.
Voltage − 1 V/div
Time − 0.5 ms/div
t
H
Capacitor Voltage
Output Voltage
t
L
R
A
= 5 kW R
L
= 1 kW
R
B
= 3 kW See Figure 12
C = 0.15 µF
NA555 , NE555 , SA555 , SE555
www.ti.com SLFS022I –SEPTEMBER 1973–REVISED SEPTEMBER 2014
Feature Description (continued)
8.3.2 A-stable Operation
As shown in Figure 12, adding a second resistor, R
B
, to the circuit of Figure 9 and connecting the trigger input to
the threshold input causes the timer to self-trigger and run as a multi-vibrator. The capacitor C charges through
R
A
and R
B
and then discharges through R
B
only. Therefore, the duty cycle is controlled by the values of R
A
and
R
B
.
This astable connection results in capacitor C charging and discharging between the threshold-voltage level (≈
0.67 × V
CC
) and the trigger-voltage level (≈ 0.33 × V
CC
). As in the mono-stable circuit, charge and discharge
times (and, therefore, the frequency and duty cycle) are independent of the supply voltage.
Figure 12. Circuit for Astable Operation Figure 13. Typical Astable Waveforms
Figure 12 shows typical waveforms generated during astable operation. The output high-level duration t
H
and
low-level duration t
L
can be calculated as follows:
(1)
(2)
Other useful relationships are shown below:
(3)
(4)
(5)
(6)
(7)
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