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M74HC4538TTR 数据表(PDF) 5 Page - STMicroelectronics |
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M74HC4538TTR 数据表(HTML) 5 Page - STMicroelectronics |
5 / 18 page M74HC4538 Functional description 5/18 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 ( W |
类似零件编号 - M74HC4538TTR |
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类似说明 - M74HC4538TTR |
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