MC1455, MC1455B, NCV1455B

http://onsemi.com

5

Figure 13. Representative Circuit Schematic

100

Threshold

Comparator

Trigger

Comparator

Flip−Flop

Output

Threshold

Trigger

Reset

Discharge

Gnd

Discharge

Reset

100 k

5.0 k

5.0 k

e

4.7 k

c b

7.0 k

6.8 k

3.9 k

b

220

4.7 k

Output

Control Voltage

5.0 k

1.0 k

4.7 k

830

4.7k

10 k

c

GENERAL OPERATION

The MC1455 is a monolithic timing circuit which uses an

external resistor − capacitor network as its timing element. It

can be used in both the monostable (one−shot) and astable

modes with frequency and duty cycle controlled by the

capacitor and resistor values. While the timing is dependent

upon the external passive components, the monolithic circuit

provides the starting circuit, voltage comparison and other

functions needed for a complete timing circuit. Internal to the

integrated circuit are two comparators, one for the input

signal and the other for capacitor voltage; also a flip−flop and

digital output are included. The comparator reference

voltages are always a fixed ratio of the supply voltage thus

providing output timing independent of supply voltage.

Monostable Mode

In the monostable mode, a capacitor and a single resistor

are used for the timing network. Both the threshold terminal

and the discharge transistor terminal are connected together

in this mode (refer to circuit in Figure 14). When the input

comparator output triggers the flip−flop so that its output

sets low. This turns the capacitor discharge transistor “off”

and drives the digital output to the high state. This condition

allows the capacitor to charge at an exponential rate which

is set by the RC time constant. When the capacitor voltage

flip−flop. This action discharges the timing capacitor and

returns the digital output to the low state. Once the flip−flop

has been triggered by an input signal, it cannot be retriggered

until the present timing period has been completed. The time

Various combinations of R and C and their associated times

are shown in Figure 16. The trigger pulse width must be less

than the timing period.

A reset pin is provided to discharge the capacitor, thus

interrupting the timing cycle. As long as the reset pin is low,

the capacitor discharge transistor is turned “on” and

prevents the capacitor from charging. While the reset

voltage is applied the digital output will remain the same.

The reset pin should be tied to the supply voltage when not

in use.

Figure 14. Monostable Circuit

Reset

8

Discharge

7

6

5

Threshold

Control

Voltage

0.01

mF

1

2

Trigger

Output

3

4

MC1455

C