Quiescent Power Consumption

The quiescent current of a component or circuit is the current that it uses internally rather than supplying on to the load; it is the difference between input and output current, and should be kept as low as possible to maximise efficiency and promote a green footprint, as increased quiescent current means increased quiescent power consumption.

In semiconductor circuits, quiescent current consists of bias current such as band-gap reference, sampling resistor, and error amplifier currents, and the gate drive current of the series pass element, which do not contribute to output power. The value of quiescent current is mostly determined by the series pass element, topologies, ambient temperature, etc.

For bipolar transistors, the quiescent current increases proportionally with the output current, because the series pass element is a current-driven device. In addition, in the dropout region the quiescent current can increase due to the additional parasitic current path between the emitter and the base of the bipolar transistor, which is caused by a lower base voltage than that of the output voltage.

For MOS transistors, the quiescent current has a near constant value with respect to the load current since the device is a voltage-driven device. The only things that contribute to the quiescent current for MOS transistors are the biasing currents of band–gap, sampling resistor, and error amplifier.