A DC/DC voltage regulator is operative to maintain a level output voltage despite variations in power supply voltage or current drawn by a load. As one example, a step down converter may take a relatively unstable input voltage and maintain a desired level output voltage that is nominally lower than the input voltage. Specifically, a step down converter may receive as an input a voltage in the range of 2-10 volts and output a level voltage of (for example) 1.8 volts.
Many portable devices require a steady voltage supply such as that provided by a DC/DC voltage regulator. Further, the advent of portable electronics and the need for longer battery life requires new types of voltage regulators. These DC/DC voltage regulators need to be efficient while operating in both low current and high current load conditions. For example, handheld electronics such as PDA's and cell phones now require high efficiency at varying loads (such as standby and active modes) to extend battery life. The standby mode requires a very low amount of current to operate. Only critical systems and volatile memory need to be powered to constantly refresh and maintain the data in the device. Because of these requirements, new voltage regulator schemes have been developed that are very efficient at all current levels.
For example, a pulse width modulated (PWM) switch-mode regulator is an efficient regulation scheme during heavy loads. It offers high efficiency, low output voltage ripple, good line and load regulation. However at light loads the PWM regulator has poor efficiency.
At light loads, a pulsed frequency modulation (PFM) switch-mode regulator is commonly used due to its high efficiency. However the large output voltage ripple, poor line and load regulation inherent to PFM precludes its use in many systems. Thus, a low quiescent current LDO regulator is desirable in these systems. The LDO regulator offers relatively good light load efficiency, low output voltage ripple, and good line and load regulation, but at heavy loads the efficiency is far below that of switching regulators.
Combining a PWM switch-mode regulator and a linear LDO regulator in parallel offers the high efficiency and good output voltage regulation required by many portable battery powered systems. Switch-mode and linear regulators have been paralleled in the prior art, but the regulation voltages have been slightly different, and the control scheme of the regulators are very basic. These types of systems do not have the optimal efficiency and the output voltage regulates at two different voltages making the load regulation poor.