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How do I avoid power supply sequencing issues? Does it matter if I apply the inputs first or the supply voltage first?
Having input pins dependent on the voltage supply can result in part damage and lead to undesirable circuit results. TI's comparators with integrated Fail Safe input pins can solve this problem by making the input pins’ absolute maximum ratings independent of the power supply. The input pins’ voltage can be greater than the positive supply rail and the output pin as long as it is within the range specified in the absolute maximum table (includes the instance when the supply voltage is 0V).
The fail safe feature is crucial for scenarios such as the power supply ramping down faster than the input supply or during startup if the input supply ramps faster than the output supply (power supply sequencing issue). This is also helpful for lab testing. In case the lab user accidentally supplies power to the input before supply power to the rails, no damage will occur to the part because the input stage limits have no dependence on the supply range.
Make sure to check the datasheet Absolute Maximum Ratings specification table to see whether or not the input has dependence on the supply voltage. In the figure below, notice the difference in the ratings. The top table has fail safe inputs while the bottom table does not. Many of our devices have fail-safe inputs, including the industry standard LM393B and LM2903B devices.
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