HCF4047B datasheet(8/12 Pages) STMICROELECTRONICS

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HCF4047B

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Where tM = monostable mode pulse width. Values

for tM are as follows :

Typ : VTR = 0.5 VDD tM = 2.48 RC

Min : VTR = 0.33 VDD tM = 2.71 RC

Max : VTR = 0.67 VDD tM = 2.48 RC

Thus if tM = 2.48 RC is used, the maximum

variation will be (+ 9.3%, - 0.0%).

Note : In the astable mode, the first positive half

cycle has a duration of TM ; succeeding durations

are tA/2.

In addition to variations from unit to unit, the

monostable pulse width may vary as a function of

frequency with respect to VDD and temperature.

4 - RETRIGGER MODE

The HCF4047B 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 terminals 8 and 12, and the

output is taken from terminal 10 or 11. As shown in

fig.A normal monostable action is obtained when

one retrigger pulse is applied. Extended pulse

duration is obtained when more than one pulse is

applied. For two input pulses, tRE = t1’ + t1 + 2t2.

For more than two pulses, tRE (Q OUTPUT)

terminates at some variable time tD after the

termination of the last retrigger pulse. tD is variable

because tRE (Q OUTPUT) terminates after the

second positive edge of the oscillator output

appears at flip-flop 4 (see logic diagram).

FIGURE A : Retrigger-mode waveforms

5 - EXTERNAL COUNTER OPTION

Time tM 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. B. The

pulse duration at the output is

text = (N - 1) (tA) + (tM + tA/2)

Where text = pulse duration of the circuitry, and N

is the number of counts used.

FIGURE B : Implementation of external counter option

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