Ashkan, I don't have any specific knowledge about what happens when the supply drops below 2V. However, if the device would reliably operate at 1.8V, we probably would have specified some limited operation. It's likely that the device at 1.8V may have problems with both I2C communications and operation. In either case, the result may be that the device will not operate (instead of degrading performance). There isn't an LDO that turns on and off between the supply and the internal circuitry. Joseph Wu
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