HCF4047B datasheet(9/12 Pages) STMICROELECTRONICS

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HCF4047B

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6 - POWER CONSUMPTION

In the standby mode (Monostable or Astable),

power dissipation will be a function of leakage

current 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 = ——————————

(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

voltage

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

the

desired

period

(within

the

limitations discussed above).

7 - 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). 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

the system (unless this capacitance can be

measured and taken into account). R must be

much larger than the COS/MOS "ON" resistance

in series with it, which typically is hundreds of

ohms. In addition, with very large values of R,

some short-term instability with respect to time

may be noted.

The recommended values for these components

to maintain agreement with previously calculated

formulas without trimming should be :

C > 100pF, up to any practical value, for astable

modes ;

C > 1000pF, up to any practical value, for

monostable modes.

10K

Ω< R < 1MΩ.

TEST CIRCUIT

CL = 50pF or equivalent (includes jig and probe capacitance)

RL = 200KΩ

RT = ZOUT of pulse generator (typically 50Ω)

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