8.2.2.3 Inductor Selection

The most critical parameters for the inductor are the inductance, peak current, and the DC resistance. The inductance is related to the peak-to-peak inductor ripple current, the input and the output voltages.

Equation 5.

A higher value of ripple current reduces inductance, but increases the conductance loss, core loss, and current stress for the inductor and switch devices. It also requires a bigger output capacitor for the same output voltage ripple requirement. A reasonable value is setting the ripple current to be 30% of the DC output current. Because the ripple current increases with the input voltage, the maximum input voltage is always used to determine the inductance. The DC resistance of the inductor is a key parameter for the efficiency. Lower DC resistance is available with a bigger winding area. A good tradeoff between the efficiency and the core size is letting the inductor copper loss equal 2% of the output power. See Selecting Inductors for Buck Converters for more information on selecting inductors. A good starting point for most applications is a 10 µH to 22 µH with 1.1 A or greater current rating for the LM2842-Q1 or a 0.7 A or greater current rating for the LM284x-Q1. Using such a rating enables the device to current limit without saturating the inductor. This is preferable to the device going into thermal shutdown mode and the possibility of damaging the inductor if the output is shorted to ground or other long-term overload.