Application Note: Output Inductor Selection for the AAT115X Series Buck Converter

This Application Note summarizes the method for selecting the output inductor for AnalogicTech's AAT115X series of DC/DC inductor-based buck converters. It presents the suggested method for determining the proper inductor value and confirming that the selected inductor ratings are not exceeded. AnalogicTech's AAT115X switch mode buck converters require that the output inductor ripple current be limited to 30% to 40% of the rated current at the maximum input voltage (Eq. 1). This limits the peak switch current while still providing a good ramp slope for the PWM peak current mode comparator input. The AAT115X peak current mode control loop senses the current flowing through the high-side P-channel MOSFET and compares it to the output of the voltage loop error amplifier. Sufficient sensed current slope is important for good noise immunity and a wide duty cycle dynamic range. Moreover, this value guarantees that there is sufficient internal slope compensation which assures stability of the current mode loop for applications requiring greater than 50% duty cycle. With the inductor ripple current set to 40% of the rated current at the maximum input voltage, the converter is guaranteed to operate in continuous conduction mode (CCM) above 20% of full load for the full input voltage range.