5

FN2867.9

August 24, 2006

Schematic Diagram

Application Information

General

The ICM7555 and ICM7556 devices are, in most instances,

direct replacements for the NE/SE 555/6 devices. However,

it is possible to effect economies in the external component

count using the ICM7555 and ICM7556. Because the bipolar

NE/SE 555/6 devices produce large crowbar currents in the

output driver, it is necessary to decouple the power supply

lines with a good capacitor close to the device. The ICM7555

and ICM7556 devices produce no such transients. See

Figure 1.

The ICM7555 and ICM7556 produce supply current spikes

of only 2mA - 3mA instead of 300mA - 400mA and supply

decoupling is normally not necessary. Also, in most

instances, the CONTROL VOLTAGE decoupling capacitors

are not required since the input impedance of the CMOS

comparators on chip are very high. Thus, for many

applications, two capacitors can be saved using an ICM7555

and three capacitors with an ICM7556.

POWER SUPPLY CONSIDERATIONS

Although the supply current consumed by the ICM7555 and

ICM7556 devices is very low, the total system supply current

can be high unless the timing components are high

impedance. Therefore, use high values for R and low values

for C in Figures 2A, 2B, and 3.

RESET

DISCHARGE

TRIGGER

THRESHOLD

GND

OUTPUT

CONTROL

VOLTAGE

R

NN

NPN

P

R

R

N

N

N

N

N

P

P

N

N

P

P

P

R = 100k

Ω ±20% (TYP)

TIME (ns)

400

800

600

200

0

0

100

200

300

400

500

SE/NE555

ICM7555/56

FIGURE 1. SUPPLY CURRENT TRANSIENT COMPARED WITH

A STANDARD BIPOLAR 555 DURING AN OUTPUT

TRANSITION

GND

TRIGGER

RESET

1

2

3

4

8

7

6

5

DISCHARGE

THRESHOLD

CONTROL

VOLTAGE

10K

OPTIONAL

CAPACITOR

C

R

FIGURE 2A. ASTABLE OPERATION

ICM7555, ICM7556