Voltage regulator dropout voltage is defined as the input-output voltage differential at which the circuit ceases to regulate against further reduction in input voltage. Quiescent current is defined as that part of the positive input current that does not contribute to the positive load current or it can be defined as the regulator ground lead current. The LM2931 series of voltage regulators are characterized as low dropout low quiescent current devices. The typical dropout voltage is 0.3 volt at 150 ma. The maximum quiescent current is listed at 15 ma which is 10% of rating. These devices are characterized in the National Semiconductor Corporation LINEAR DATABOOK .COPYRGT. 1982. A higher current version, the LM2935 series, is characterized in the LINEAR SUPPLEMENT DATABOOK .COPYRGT. 1984. This device has a 750 ma rating with a typical dropout of 0.82 volt at rated current. Its quiescent current maximum is 120 ma.
Low dropout voltage is important in battery powered voltage regulator applications where it is desirable, from a battery life standpoint, to provide the rated output at the lowest possible battery voltage. Since the quiescent current does not contribute useful output, the importance of keeping it low is self evident. One thing that most low dropout IC voltage regulators have in common is a PNP pass transistor. One such device is discussed in U.S. Pat. No. 4,417,265 which issued Nov. 22, 1983, to Judd R. Murkland and James S. Congdon and is assigned to the assignee of the present invention. This patent discloses a plural-emitter base ballasted PNP lateral transistor and its teaching is incorporated herein by reference.