Datasheet

January 1995 12
Philips Semiconductors Product specification
Monostable/astable multivibrator
HEF4047B
MSI
3. Retrigger mode operation
The HEF4047B can be used in the retrigger mode to extend the output pulse duration, or to compare the frequency of
an input signal with that of the internal oscillator. In the retrigger mode the input pulse is applied to pins 8 and 12, and
the output is taken from pin 10 or 11. Normal monostable action is obtained when one retrigger pulse is applied (Fig.8).
Extended pulse duration is obtained when more than one pulse is applied. For two input pulses, t
RE
=t
1
’+t
1
+2t
2
.
For more than two pulses, t
RE
(output O), terminates at some variable time, t
D
, after the termination of the last retrigger
pulse; t
D
is variable because t
RE
(output O) terminates after the second positive edge of the oscillator output appears at
flip-flop 4.
4. 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 capacitors for long time periods, and extremely fast recovery time. A typical
implementation is shown in Fig.9.
The pulse duration at the output is:
Where t
ext
= pulse duration of the circuitry, and N is the number of counts used.
Fig.8 Retrigger mode waveforms.
t
ext
N1–()t
A
() t
M
12t
A
⁄+()+=
Fig.9 Implementation of external counter option.