TS555 Datasheet, PDF (9/12 Pages) STMicroelectronics – LOW POWER SINGLE CMOS TIMER

TS555
ASTABLE OPERATION
When the circuit is connected as shown in figure 4
(pin 2 and 6 connected) it triggers itself and free
runs as a multivibrator. The external capacitor
charges through R_A and R_B and discharges
through R_B only. Thus the duty cycle may be pre-
cisely set by the ratio of these two resistors.
In the astable mode of operation, C charges and
discharges between 1/3 V_CC and 2/3 V_CC. As in
the triggered mode, the charge and discharge
times and therefore frequency, are independent of
the supply voltage.
Figure 4:
V_CC
Reset
RA
4
8
Out
3
7
TS555
RB
Control
Voltage
0.01 µF
5
1
6
2
C
Figure 5 shows actual waveforms generated in
this mode of operation.
The charge time (output HIGH) is given by :
t1 = 0.693 (R_A + R_B) C
and the discharge time (output LOW) by :
t2 = 0.693 x R_B x C
Thus the total period T is given by :
T = t1 + t2 = 0.693 (R_A + 2R_B) C
The frequency of oscillation is then :
f = _T-1- = _(---R-----_A-----+-1---.-2-4---4_R-----_B-----₎--_C---
The duty cycle is given by :
D = -_R----_A-----^R-+----^B-2----_R----_B---
Figure 5:
t = 0.5 ms / div
OUTPUT VOLTAGE = 5.0V/div
CAPACITOR VOLTAGE = 1.0V/div
RA = RB = 4.8 kΩ, C = 0.1 µF , R _L = 1.0k Ω
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