ICM7555, ICM7556
6
FN2867.10
June 28, 2016
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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 4. 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 5
. In this
circuit, the frequency is as shown by Equation 2
:
The duty cycle is controlled by the values of R
A
and R
B
, by
Equation 3:
MONOSTABLE OPERATION
In this mode of operation, the timer functions as a one-shot (see
Figure 6
). 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 now increases
exponentially with a time constant t = R
A
C. When the voltage
across the capacitor equals
2
/
3
V+, the 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 555/556, 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
SE/NE 555/556 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
V
DD
OPTIONAL
CAPACITOR
C
V
DD
R
A
R
B
FIGURE 5. ALTERNATE ASTABLE CONFIGURATION
f
1
1.4 RC
------------------
=
(EQ. 1)
f1.44R
A
2R
B
+ C=
(EQ. 2)
DR
A
R
B
+R
A
2R
B
+=
(EQ. 3)
TRIGGER
OUTPUT
RESET
1
2
3
4
8
7
6
5
V
DD
DISCHARGE
THRESHOLD
CONTROL
VOLTAGE
OPTIONAL
CAPACITOR
C
V
DD
≤18V
R
A
ICM7555
t
OUTPUT
= -ln
(1/3) R
A
C = 1.1R
A
C
FIGURE 6. MONOSTABLE OPERATION
