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M74HC4538 Folha de dados(PDF) 5 Page - STMicroelectronics

Nome de Peças M74HC4538
Descrição Electrónicos  Dual retriggerable monostable multivibrator
Download  18 Pages
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Fabricante Electrônico  STMICROELECTRONICS [STMicroelectronics]
Página de início  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

M74HC4538 Folha de dados(HTML) 5 Page - STMicroelectronics

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M74HC4538
Functional description
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2
Functional description
Standby state
The external capacitor Cx, is fully charged to VCC in the standby state. Hence, before
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:
A is low and B has a falling edge
B is high and A has a rising edge
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 VREFL the output of comparator C1 becomes low. This in turn resets the
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 VREFH . At this point
C2 output goes low and G goes low. C2 stops operating. That means that after triggering
when the voltage R/C external returns to VREFH the multivibrator has returned to its
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 tw(out) is
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
VREFL again and Q remains high i.e. the retrigger pulse arrives in a time shorter than the
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, trr (min.) depends on VCC and Cx.
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
VCC. Then, if CD input goes low the IC becomes waiting state both in operating and non
operating state.
Rx
Cx
72
.
0
=
t
)
OUT
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