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How to read datasheets

Which correctly describes what can happen when a 3.3 V signal from another circuit is applied to an input pin of an IC that is not powered?

  1. ACurrent flows into the supply rail through the input protection diode, which can lead to malfunction or pin damage
  2. BWith no power applied, the input pin is the same as an open circuit, so nothing in particular happens
  3. CPower supplied through the input pin causes the entire IC to start up properly and begin operating normally
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Answer: A. Current flows into the supply rail through the input protection diode, which can lead to malfunction or pin damage

In many CMOS ICs, the absolute maximum rating of an input pin is defined relative to the supply voltage, as in "−0.3 V to VDD + 0.3 V." If 3.3 V is applied while the supply is at 0 V, the protection diode from the input to VDD conducts, and current flows from the signal line into the supply rail (back-powering). As a result, the supply rail may be pulled up to an intermediate voltage, causing malfunction, or the input clamp current rating may be exceeded, damaging the pin. Some products have protection that permits inputs while powered off, so check the datasheet.

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