mm74hc4538 Fairchild Semiconductor, mm74hc4538 Datasheet - Page 4

• Manufacturer Part Number: mm74hc4538
• Description: Dual Retriggerable Monostable Multivibrator
• Manufacturer: Fairchild Semiconductor

www.fairchildsemi.com
Circuit Operation
toggle, taking the Q output of the HC4538 to a LOW State
(#19), and completing the time-out cycle.
RESET OPERATION
A low voltage applied to the Reset pin always forces the Q
output of the HC4538 to a LOW State.
The timing diagram illustrates the case in which reset
occurs (#20) while C
voltage of the upper reference circuit (#21). When a reset
occurs, the output of the reset latch goes LOW (#22), turn-
ing ON transistor M1. Thus C
up to V
Recovery time is the required delay after reset goes inac-
tive to a new trigger rising edge. On the diagram it is shown
as (#26) to (#27).
RETRIGGER OPERATION
In the retriggerable mode, the HC4538 may be retriggered
during timing out of the output pulse at any time after the
trigger-control circuit flip-flop has been reset (#24).
Because the trigger-control circuit flip-flop resets shortly
after C
lower reference circuit (#25), the minimum retrigger time, t
is a function of internal propagation delays and the dis-
charge time of C
at room temperature
POWER-DOWN CONSIDERATIONS
Large values of C
down the MM74HC4538 because of the amount of energy
stored in the capacitor. When a system containing this
CC
X
has discharged to the reference voltage of the
(#23) to await the next trigger signal.
X
:
X
X
may cause problems when powering
is charging up toward the reference
X
(Continued)
is allowed to quickly charge
rr
4
device is powered down, the capacitor may discharge from
V
Current through the protection diodes must be limited to 30
mA; therefore, the turn-off time of the V
must not be faster than t
V
no faster than t
usually not a problem because power supplies are heavily
filtered and cannot discharge at this rate.
When a more rapid decrease of V
the MM74HC4538 may sustain damage. To avoid this pos-
sibility, use an external clamping diode, D
from V
SET UP RECOMMENDATIONS
Minimum R
Minimum C
CC
CC
through the input protection diodes at pin 2 or pin 14.
CC
5V and C
to the C
X
X
X
1 k
0 pF.
X
(15V)•(15 F)/30 mA
pin.
15 F, the V
V
CC
•C
X
CC
/(30 mA). For example, if
CC
supply must turn OFF
to zero volts occurs,
CC
2.5 ms. This is
power supply
X
, connected