M74HC4538RM13TR データシートの表示（PDF） - STMicroelectronics

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M74HC4538RM13TR

Dual retriggerable monostable multivibrator

STMicroelectronics 

Pin connection and IEC logic symbols

Figure 4. Timing chart

M74HC4538

Figure 5. Block diagram

1. Cx, Rx, Dx are external components.

2. Dx is a clamping diode.

The external capacitor is charged to VCC in the standby state, i.e. no trigger. When the supply voltage is

turned off Cx is discharged mainly through an internal parasitic diode (see figures). If Cx is sufficiently large

and VCC decreases rapidly, there will be some possibility of damaging the IC with a surge current or latch-

up. If the voltage supply filter capacitor is large enough and VCC decreases slowly, the surge current is

automatically limited and damage to the IC is avoided. The maximum forward current of the parasitic diode

is approximately 20 mA. In cases where Cx is large the time taken for the supply voltage to fall to 0.4 VCC

can be calculated as follows: tf > (VCC - 0.7) x Cx/20 mA.

In cases where tf is too short an external clamping diode is required to protect the IC from the surge

current.

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