When choosing the input voltage range, the input common mode voltage range (VICR) must be taken in to account. If temperature operation is above or below 25°C the VICR can range from 0V to VCC – 1.5V. This limits the input voltage range to as high as VCC – 1.5V and as low as 0V. Operation outside of this range can yield incorrect comparisons.
Below is a list of input voltage situation and their outcomes:
- When both IN- and IN+ are both within the common mode range:
- If IN- is higher than IN+ and the offset voltage, the output is low and the output transistor is sinking current
- If IN- is lower than IN+ and the offset voltage, the output is high impedance and the output transistor is not conducting
- When IN- is higher than common mode and IN+ is within common mode, the output is low and the output transistor is sinking current
- When IN+ is higher than common mode and IN- is within common mode, the output is high impedance and the output transistor is not conducting
- When IN- and IN+ are both higher than common mode, please see the Both Inputs Above Input Range Behavior section of the LM339 Family Application Note (SNOAA35).