Why does the LM2931 draw excessive ground current when the input drops, forcing the output to an out of regulation condition?
The LM2931 is one of a group of LDO regulators that has a characteristic in their ground pin current referred to as the “carrot”. The carrot is a point in the ground pin current that spikes up as the input voltage is reduced. The error amplifier in the regulator always tries to force the output to be the right voltage by adjusting the current through the pass device (in this case, the PNP transistor). As the input voltage is reduced (and the voltage across the pass transistor decreases) the current gain of the PNP begins to drop. To maintain the correct output voltage, the error amplifier has to drive the base of the PNP harder to supply the same load current. The PNP base drive current leaves the regulator as ground pin current. As the input voltage drops further the regulator will approach dropout, causing the error amplifier to drive the PNP base with maximum current (this is the top of the carrot). This value of current may be 3 or 4 times the maximum ground pin current that is req
