m74hc4538 STMicroelectronics, m74hc4538 Datasheet - Page 5

• Manufacturer Part Number: m74hc4538
• Description: Dual Retriggerable Monostable Multivibrator
• Manufacturer: STMicroelectronics

M74HC4538
2
Functional description
Standby state
The external capacitor Cx, is fully charged to V
triggering, transistor Qp and Qn (connected to the Rx/Cx node) are both turned-off. The two
comparators that control the timing and the two reference voltage sources stop operating.
The total supply current is therefore only leakage current.
Trigger operation
Triggering occurs when:
After the multivibrator has been retriggered, the comparator C1 and C2 start operating and
Qn is turned on. Cx then discharges through Qn. The voltage at the node Rx/Cx external
falls.
When it reaches V
flip-flop and Qn is turned off.
At this point C1 stops functioning but C2 continues to operate.
The voltage at R/C external begins to rise with a time constant set by the external
components Rx and Cx.
Triggering the multivibrator causes Q to go high after internal delay due to the flip-flop and
the gate. Q remains high until the voltage at R/C external rises again to V
C2 output goes low and G goes low. C2 stops operating. That means that after triggering
when the voltage R/C external returns to V
monostable state. In the case where Rx · Cx are large enough and the discharge time of the
capacitor and the delay time in the IC can be ignored, the width of the output pulse t
as follows:
Re-triggered operation
When a second triggered pulse follows the first, its effect will depend on the state of the
multivibrator. If the capacitor Cx is being charged, the voltage level of Rx/Cx external falls to
V
period Rx · Cx seconds, the capacitor charging time constant. If the second trigger pulse is
very close to the initial trigger pulse it is ineffective; i.e. the second trigger must arrive in the
capacitor discharge cycle to be ineffective; hence the minimum time for a second trigger to
be effective, t
Reset operation
CD is normally high. If CD is low, the trigger is not effective because Q output goes low and
trigger control flip-flop is reset. Also transistor Op is turned on and Cx is charged quickly to
V
operating state.
REFL
CC
. Then, if CD input goes low the IC becomes waiting state both in operating and non
–
–
again and Q remains high i.e. the retrigger pulse arrives in a time shorter than the
A is low and B has a falling edge
B is high and A has a rising edge
rr
(min.) depends on V
REFL
the output of comparator C1 becomes low. This in turn resets the
t
CC
W
(
OUT
and Cx.
)
REFH
=
. 0
72
CC
the multivibrator has returned to its
Cx
in the standby state. Hence, before
•
Rx
Functional description
REFH
. At this point
w(out)
5/18
is