LMC7221 Operation at Startup

Hello Donald,

Operation below the minimum supply voltage is not specified. Start-up requires transitioning through the sub-minimum supply region (0V to 2.7V) where the device is not fully functional ("waking up").

The LMC7221 has a Rail to Rail input, which means there are two input stages and the required crossover logic circuitry, all of which needs to be functional to ensure the output to be correct. Simpler devices such as the LM339 types can be somewhat predicted (but not guaranteed), but due to the more complicated R-R input stage of the 7221, there is more circuitry to "wake up" and operation cannot be assured.

Even if we measured a few on the bench and found some correlation, we still could not state if ALL current and future devices will follow the trend. There are too many variables.

My experience is that any "flops" tend to occur between 0.6V up to about a volt below minimum supply voltage (as thresholds are being crossed and bias voltages are being established). Also, the supply ramp rate has a large affect on the start-up behavior, as well as the initial applied input voltages (and their ramp rate) and temperature. Again...lots of variables.

I am not aware of any TI comparators with known startup behavior. Hopefully the PA or SLL guys can correct me if I am wrong...

You may need a way to delay power to the pull-up resistor, or, place a JFET/MOSFET on the output to hold it low until a certain time (I know you wanted it high...but that is more complicated and would have it's own start-up time).

Regards,