MC3456

6

MOTOROLA ANALOG IC DEVICE DATA

Figure 15. Monostable Circuit

Figure 16. Monostable Waveforms

Figure 17. Astable Circuit

(RA = 10 kΩ, C = 0.01 µF, RL = 1.0 kΩ, VCC = 15 V)

t = 20

µs/cm

(RA = 5.1 kΩ, C = 0.0 1 µF, RL = 1.0 kΩ, RB = 3.9 kΩ, VCC = 15 V)

t = 50

µs/cm

Figure 18. Astable Waveforms

Control

Voltage

3 (11)

RL

+VCC (5.0 to 15 V)

Reset

VCC

14

Discharge

1 (13)

2 (12)

Threshold

0.01

µF

7

6 (8)

Trigger

Output

5 (9)

4 (10)

RA

RL

C

Gnd

1/2

MC3456

RB

RL

+VCC (5.0 V to 15 V)

Reset

VCC

14

Discharge

1 (13)

2 (12)

Threshold

Control

Voltage

0.01

µF

7

6 (8)

Trigger

Output

5 (9)

4 (10)

RA

RL

C

3 (11)

Gnd

1/2

MC3456

Pin numbers in parenthesis ( ) indicate B–Channel

Astable Mode

In the astable mode the timer is connected so that it will

retrigger itself and cause the capacitor voltage to oscillate

between 1/3 VCC and 2/3 VCC (see Figure 17).

The external capacitor charges to 2/3 VCC through RA and

RB and discharges to 1/3 VCC through RB. By varying the

ratio of these resistors the duty cycle can be varied. The

charge and discharge times are independent of the supply

voltage.

The charge time (output high) is given by:

t1 = 0.695 (RA+RB) C

The discharge time (output low) by:

t2 = 0.695 (RB) C

Thus the total period is given by:

T = t1 + t2 = 0.695 (RA + 2RB) C

The frequency of oscillation is then: f =

1

T

=

(RA +2RB) C

1.44

and may be easily found as shown in Figure 19.

RA +2RB

RB

The duty cycle is given by: DC =

To obtain the maximum duty cycle, RA must be as small as

possible; but it must also be large enough to limit the

discharge current (Pin 7 current) within the maximum rating

of the discharge transistor (200 mA).

The minimum value of RA is given by:

RA ≥

VCC (Vdc)

I7 (A)

≥

VCC (Vdc)

0.2

Figure 19. Free Running Frequency

100

10

1.0

0.1

0.01

0.001

(RA + 2 RB)

0.1

1.0

10

100

1.0 k

10 k

100 k

f, FREE RUNNING FREQUENCY (Hz)