6

FN2867.9

August 24, 2006

OUTPUT DRIVE CAPABILITY

The output driver consists of a CMOS inverter capable of

driving most logic families including CMOS and TTL. As

such, if driving CMOS, the output swing at all supply

voltages will equal the supply voltage. At a supply voltage of

4.5V or more, the ICM7555 and ICM7556 will drive at least

two standard TTL loads.

ASTABLE OPERATION

The circuit can be connected to trigger itself and free run as

a multivibrator, see Figure 2A. The output swings from rail to

rail, and is a true 50% duty cycle square wave. (Trip points

and output swings are symmetrical.) Less than a 1%

frequency variation is observed over a voltage range of +5V

to +15V.

The timer can also be connected as shown in Figure 2B. In this

circuit, the frequency is:

equation:

MONOSTABLE OPERATION

In this mode of operation, the timer functions as a one-shot.

See Figure 3. Initially the external capacitor (C) is held

discharged by a transistor inside the timer. Upon application of

a negative TRIGGER pulse to pin 2, the internal flip-flop is set

which releases the short circuit across the external capacitor

and drives the OUTPUT high. The voltage across the capacitor

comparator resets the flip-flop, which in turn discharges the

capacitor rapidly and also drives the OUTPUT to its low state.

TRIGGER must return to a high state before the OUTPUT can

return to a low state.

CONTROL VOLTAGE

The CONTROL VOLTAGE terminal permits the two trip

voltages for the THRESHOLD and TRIGGER internal

comparators to be controlled. This provides the possibility of

oscillation frequency modulation in the astable mode or even

inhibition of oscillation, depending on the applied voltage. In

the monostable mode, delay times can be changed by

varying the applied voltage to the CONTROL VOLTAGE pin.

RESET

The RESET terminal is designed to have essentially the

same trip voltage as the standard bipolar 555/6, i.e., 0.6V to

0.7V. At all supply voltages it represents an extremely high

input impedance. The mode of operation of the RESET

function is, however, much improved over the standard

bipolar NE/SE 555/6 in that it controls only the internal flip-

flop, which in turn controls simultaneously the state of the

OUTPUT and DISCHARGE pins. This avoids the multiple

threshold problems sometimes encountered with slow falling

edges in the bipolar devices.

OUTPUT

1

2

3

4

8

7

6

5

OPTIONAL

CAPACITOR

C

FIGURE 2B. ALTERNATE ASTABLE CONFIGURATION

f

1

1.4 RC

------------------

=

(EQ. 1)

f1.44 RA 2RB

+

()

⁄

C

=

(EQ. 2)

DRA RB

+

() RA 2RB

+

()

⁄

=

(EQ. 3)

TRIGGER

OUTPUT

RESET

1

2

3

4

8

7

6

5

DISCHARGE

THRESHOLD

CONTROL

VOLTAGE

OPTIONAL

CAPACITOR

C

ICM7555

FIGURE 3. MONOSTABLE OPERATION

ICM7555, ICM7556