This dual monostable multivibrator may be triggered by either the positive or the negative edge of an input pulse, and produces a precision output pulse over a wide range of pulse widths. Because the device has conditioned trigger inputs, there are no trigger—input rise and fall time restrictions. The output pulse width is determined by the external timing components, Rx and Cx. The device has a reset function which forces the Q output low and the Q output high, regardless of the state of the output pulse circuitry. Absolute maximum continuous ratings are those values beyond which damage to the device may occur.
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My opinions now out of the way, do we know anything about the protection diodes in this IC at all? From blowing up my share of CMOS ICs in the past, during which I was trusting the datasheets a little too much, I found that the protection diodes are generally very very very close to the edge of adequacy.
Especially in higher power level parts. But these diodes are almost always reversed diodes, not forward diodes that you mention Recently we were working with an RF CMOS switch that supposedly had a degree of protection built in.
The switches would randomly die strange deaths but always with the voltage rails partially shorted. Initially it was thought that the parts may have been faulty because there is also external protection diodes. Testing showed that both the external and internal diodes were blown but still showed that the diode was intact. Somehow the substrate had been compromised and some kind of current path was created.
Changing the external protection diodes seemed to fix the main problem that we were seeing. Generally when you want to use a crowbar, you already see the need to recycle power.
I used one on a robot for overvoltage protection, a picofuse, a zener shunt and a SCR. It worked really well in instances where the cable to the robot would get cut, shorting higher voltages in the cable to the lesser voltaged circuits. But I digress. Quote would there be any long-term degradation effects from repetitive forward conduction?
Drops of. I guess in closing my excited rant which has spanned about 2 hours in the making due to this pesky thing they call "work" I would sum it up as such: If you can afford to use external diodes, do it. Personally I would rather plan to use them but run as many tests as I could to see if I could get away without them.
SPICE model index
MC74HC4538A: Dual Precision Monostable Multivibrator
74HC4538 PDF Datasheet浏览和下载