HCC4047B Datasheet, PDF (9/15 Pages) STMicroelectronics – LOW-POWELOW-POWER MONOSTABLE/ASTABLE MULTIVIBRATOR

Figure A : Retrigger-mode Waveforms.
HCC/HCF4047B
5 - EXTERNAL COUNTER OPTION
Time t_M can be extended by any amount with the use
of external counting circuitry. Advantages include
digitally controlled pulse duration, small timing capa-
citors for long time periods, and extremely fast re-
covery time.
A typical implementation is shown in fig. B. The
pulse duration at the output is
t_ext = (N – 1) (t_A) + (t_M + t_A/2)
Where text = pulse duration of the circuitry, and N is
the number of counts used.
Figure B : Implementation of External Counter Option.
6 - POWER CONSUMPTION
In the standby mode (Monostable or Astable),
power dissipation will be a function of leakage cur-
rent in the circuit, as shown in the static electrical
characteristics. For dynamic operation, the power
needed to charge the external timing capacitor C is
given by the following formula :
Astable Mode : P = 2CV²f. (Output at Pin 13)
P = 4CV²f. (Output at Pin 10 and 11)
(2.9CV²) (Duty Cycle)
Monostable Mode : P =
T
(Output at Pin 10 and 11)
The circuit is designed so that most of the total
power is consumed in the external components. In
practice, the lower the values of frequency and volt-
age used, the closer the actual power dissipation will
be to the calculated value.
Because the power dissipation does not depend on
R, a design for minimum power dissipation would be
a small value of C. The value of R would depend on
the desired period (within the limitations discussed
above).
7 - TIMING-COMPONENT LIMITATIONS
The capacitor used in the circuit should be non-po-
larized and have low leakage (i.e. the parallel resist-
ance of the capacitor should be an order of
magnitude greater than the external resistor used).
Three is no upper or lower limit for either R or C value
to maintain oscillation.
However, in consideration of accuracy, C must be
much larger than the inherent stray capacitance in
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