Step down power converter with pre-dropout control

A method of controlling a step-down converter configured to receive an input voltage and provide an output voltage, the method comprising

• providing a driver configured to drive a power stage of the converter in a synchronous mode or in an asynchronous mode of operation with a minimum off-time;
• wherein when the output voltage approaches the input voltage, a duty cycle of the converter increases up to a value limited by the minimum off-time;
• generating a first clock signal having a predefined pulse width;
• generating a first stretchable clock signal having an adjustable pulse width and an adjustable period;
• and
• upon transition from the synchronous mode to the asynchronous mode, increasing the adjustable pulse width and the adjustable period of the stretchable clock signal.

A controller for use with a step-down converter, the controller comprising:

• a driver configured to drive a power stage of the converter in a synchronous mode or in an asynchronous mode of operation with a minimum off-time;
• wherein when the output voltage approaches the input voltage, a duty cycle of the converter increases up to a value limited by the minimum off-time;
• and
• a clock source configured to generate a first clock signal having a predefined pulse width;
• wherein the driver is configured to generate a first stretchable clock signal having an adjustable pulse width and an adjustable period;
• and
• wherein upon transition from the synchronous mode to the asynchronous mode the driver is configured to increase the adjustable pulse width and the period of the first stretchable clock signal.