monostable multivibrator. 74HC4538 Datasheet

74HC4538 multivibrator. Datasheet pdf. Equivalent

74HC4538 Datasheet
Recommendation 74HC4538 Datasheet
Part 74HC4538
Description Dual retriggerable precision monostable multivibrator
Feature 74HC4538; MM74HC4538 Dual Retriggerable Monostable Multivibrator February 1984 Revised August 2000 MM74HC453.
Manufacture Fairchild
Download 74HC4538 Datasheet

Fairchild 74HC4538
February 1984
Revised August 2000
Dual Retriggerable Monostable Multivibrator
General Description
The MM74HC4538 high speed monostable multivibrator
(one shots) is implemented in advanced silicon-gate
CMOS technology. They feature speeds comparable to low
power Schottky TTL circuitry while retaining the low power
and high noise immunity characteristic of CMOS circuits.
Each multivibrator features both a negative, A, and a posi-
tive, B, transition triggered input, either of which can be
used as an inhibit input. Also included is a clear input that
when taken low resets the one shot. The MM74HC4538 is
retriggerable. That is, it may be triggered repeatedly while
their outputs are generating a pulse and the pulse will be
Pulse width stability over a wide range of temperature and
supply is achieved using linear CMOS techniques. The out-
put pulse equation is simply: PW = 0.7(R)(C) where PW is
in seconds, R is in ohms, and C is in farads. This device is
pin compatible with the CD4528, and the CD4538 one
shots. All inputs are protected from damage due to static
discharge by diodes to VCC and ground.
s Schmitt trigger on A and B inputs
s Wide power supply range: 2–6V
s Typical trigger propagation delay: 32 ns
s Fanout of 10 LS-TTL loads
s Low input current: 1 µA max
Ordering Code:
Order Number Package Number
Package Description
16-Lead Small Outline Integrated Circuit (SOIC), JEDEC MS-012, 0.150 Narrow
16-Lead Small Outline Package (SOP), EIAJ TYPE II, 5.3mm Wide
16-Lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300 Wide
Devices also available in Tape and Reel. Specify by appending the suffix letter Xto the ordering code.
Connection Diagram
Truth Table
H = HIGH Level
L = LOW Level
↑ = Transition from LOW-to-HIGH
↓ = Transition from HIGH-to-LOW
= One HIGH Level Pulse
= One LOW Level Pulse
X = Irrelevant
© 2000 Fairchild Semiconductor Corporation DS005217

Fairchild 74HC4538
Block Diagrams
Note: Pin 1 and Pin 15 must be hard-wired to GND.
Logic Diagram

Fairchild 74HC4538
Timing Diagram
Circuit Operation
The MM74HC4538 operates as follows (refer to logic dia-
gram). In the quiescent state, the external timing capacitor,
CX, is charged to VCC. When a trigger occurs, the Q output
goes HIGH and CX discharges quickly to the lower refer-
ence voltage (VREF Lower = 1/3 VCC). CX then charges,
through RX, back up to the upper reference voltage (VREF
Upper = 2/3 VCC), at which point the one-shot has timed out
and the Q output goes LOW.
The following, more detailed description of the circuit oper-
ation refers to both the logic diagram and the timing dia-
In the quiescent state, before an input trigger appears, the
output latch is HIGH and the reset latch is HIGH (#1 in logic
Thus the Q output (pin 6 or 10) of the monostable multivi-
brator is LOW (#2, timing diagram).
The output of the trigger-control circuit is LOW (#3), and
transistors M1, M2, and M3 are turned off. The external
timing capacitor, CX, is charged to VCC (#4), and the upper
reference circuit has a LOW output (#5). Transistor M4 is
turned ON and transmission gate T1 is turned OFF. Thus
the lower reference circuit has VCC at the noninverting
input and a resulting LOW output (#6).
In addition, the output of the trigger-control reset circuit is
The MM74HC4538 is triggered by either a rising-edge sig-
nal at input A (#7) or a falling-edge signal at input B (#8),
with the unused trigger input and the Reset input held at
the voltage levels shown in the Truth Table. Either trigger
signal will cause the output of the trigger-control circuit to
go HIGH (#9).
The trigger-control circuit going HIGH simultaneously ini-
tiates three events. First, the output latch goes LOW, thus
taking the Q output of the HC4538 to a HIGH State (#10).
Second, transistor M3 is turned on, which allows the exter-
nal timing capacitor, CX, to rapidly discharge toward
ground (#11). (Note that the voltage across CXappears at
the input of the upper reference circuit comparator.) Third,
transistor M4 is turned off and transmission gate T1 is
turned ON, thus allowing the voltage across CX to also
appear at the input of the lower reference circuit compara-
When CX discharges to the reference voltage of the lower
reference circuit (#12), the outputs of both reference cir-
cuits will be HIGH (#13). The trigger-control reset circuit
goes HIGH, resetting the trigger-control circuit flip-flop to a
LOW State (#14). This turns transistor M3 OFF again,
allowing CX to begin to charge back up toward VCC, with a
time constant t = RXCX (#15). In addition, transistor M4 is
turned ON and transmission gate T1 is turned OFF. Thus a
high voltage level is applied to the input of the lower refer-
ence circuit comparator, causing its output to go LOW
(#16). The monostable multivibrator may be retriggered at
any time after the trigger-control circuit goes LOW.
When CX charges up to the reference voltage of the upper
reference circuit (#17), the output of the upper reference
circuit goes LOW (#18). This causes the output latch to

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