Keeping power under control

The increasing number of electronic devices in a car, which need to maintain basic functionality even in standby mode, can drain a battery if the car is left idle for a long period. The systems usually have separate voltage rails for standby and for fully operational modes. Very low quiescent current (lq) linear low dropout regulators (LDOs) have traditionally been used for an always-on 5V rail. These rails are decreasing to 3.3V. The low efficiency and high power dissipation at higher load currents of LDOs makes them less usable. Most automotive applications that use switching power supplies use the pulse width modulation (PWM) control schemes that include a fixed switching frequency. Three alternatives are the pulse frequency mode (PFM) control scheme, which varies the switching frequency with the load current, the hysteretic control scheme and a pure burst mode switching regulator. A combination of these techniques could be used to improve the efficiency over a wide load range. The operation of such a device is discussed.