2 Light Load Efficiency and Low Quiescent Current

The POL regulators generating low voltages from an array of batteries must employ an energy-saving pulse-skipping technique now referred to as Eco-Mode. The inductor current in a synchronous-buck converter is a triangle wave. As the output current decreases from a heavy-load condition, the inductor current is reduced and the rippled valley of the triangle wave eventually touches the zero level at the boundary between continuous conduction-mode (CCM) and DCM. With Eco-Mode, the rectifying MOSFET is turned off when the converter detects zero current in the inductor. As the load current further decreases, the on-time is held nearly constant so that the off-time is extended, and the switching frequency is reduced to maintain regulation. As a result, the power MOSFETs and inductor are idle for longer time periods and conduction losses are greatly reduced. The point at which the DC/DC converter enters the light-load efficiency mode is shown in Equation 1.

DC/DC converters and controllers with low quiescent current will have improved light-load efficiency performance and extended battery run-time, since less power is dissipated within the IC. The TPS51285A features Ultra Low Quiescent current (ULQ) of 25-uA for longer battery life in system stand-by mode. Figure 1 shows 90% efficiency with 7.4-V input and 5-V output at 1-mA, illustrating the combined affects of Eco-Mode and ULQ.