Datasheet

January 1995 13
Philips Semiconductors Product specification
Monostable/astable multivibrator
HEF4047B
MSI
5. Timing component limitations
The capacitor used in the circuit should be non-polarized and have low leakage (i.e. the parallel resistance of the
capacitor should be an order of magnitude greater than the external resistor used).
There is no upper or lower limit for either R
t
or C
t
value to maintain oscillation.
However, in consideration of accuracy, C
t
must be much larger than the inherent stray capacitance in the system (unless
this capacitance can be measured and taken into account).
R
t
must be much larger than the LOCMOS ‘ON’ resistance in series with it, which typically is hundreds of ohms.
The recommended values for R
t
and C
t
to maintain agreement with previously calculated formulae without trimming
should be:
C
t
≥ 100 pF, up to any practical value,
10 kΩ≤R
t
≤1 MΩ.
6. Power consumption
In the standby mode (monostable or astable), power dissipation will be a function of leakage current in the circuit.
For dynamic operation, the power needed to charge the external timing capacitor C
t
is given by the following formulae:
Because the power dissipation does not depend on R
t
, a design for minimum power dissipation would be a small value
of C
t
. The value of R would depend on the desired period (within the limitations discussed previously).
Typical power consumption in astable mode is shown in Figs 10, 11 and 12.
Astable mode: P = 2 C
t
V
2
f (f at output pin 13)
P = 4 C
t
V
2
f (f at output pins 10 and 11)
Monostable mode:
P=
29C
t
V
2
,


duty cycle()
T
---------------------------------------------------------------------
f at output pins 10 and 11()