比较器的坑

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比较器的坑 [复制链接]

板上用6颗LM393，OC输出端用于控制P-MOS对12V进行通断控制（不考虑速率），P-MOS的GS间有并联10K电阻。
为了降低系统休眠电流，LM393的供电受软件控制，休眠时LM393无供电。
结果杯具了：休眠断电后，LM393的OC对地呈低阻，VCC对地接并电阻消掉虚电压，OC输出还是0.0几伏，说好的NPN集电极开路呢？
除了OC端，所有脚短路也是如此，我去年买了个表。。。
内部电路图有假？还是我看不懂？不说了，改电路

topwon

lesson learn:
"Protecting Off-Amps" by John Ardizzoni
Strangely enough, most of the off-amp questions I receive do involve a voltage—but not the supply voltage. The voltage of interest is usually a voltage applied to either an input or output of the off-amp. To save power, users will selectively shut down certain parts of their systems. This often results in voltages being applied to un-powered parts of the system. Users are typically interested in knowing how the off-amp will react to this errant voltage, and what precautions they must take to ensure the safety of the off-amp.
Getting to the bottom of off-amp questions can be challenging, with many factors that must be taken into account. The amplifier type, peripheral circuitry, and input and output voltages are all important. The best way to protect an off-amp is to ensure that the power to any circuit that touches the off-amp is off as well. If that’s not possible, then make sure to limit the current entering the off-amp. Always check with the applications engineer if you need additional help.
芯片规格书中的原理框图应该是不会把完整的电路画出来，所以出现这种不可预知的结果也就成为一种可能，看来我们只能惹不起就躲得起了。

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topwon 发表于 2018-11-7 14:47
lesson learn:
"Protecting Off-Amps" by John Ardizzoni
Strangely enough, most of the off-amp questi ...

除OC之外的所有引脚全短接，说明比较器OC输出并不仅仅是一个三极管，可能还有别的隐去的电路。
它只有在具备正常供电的条件下，才能保证在正相端电位高于负相端的情况下，输出级的三极管处于截止状态，达到电平转换的逻辑控制目的。
真是吃了大亏，不过，我稍稍改了一些电路参数，暂时可以不用割铜皮或改板，样机交付暂时无碍。这是一个血的教训

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此贴的目的：提醒电工们，在chip无供电的情况下，不要认为OC, OD还成立。