8.2.1.2 Inductor Selection

The duty cycle (D) can be approximated quickly using the ratio of output voltage (V_OUT) to input voltage (V_IN):

Equation 3.

The catch diode (D1) forward voltage drop and the voltage drop across the internal PMOS must be included to calculate a more accurate duty cycle. Calculate D by using the following formula:

Equation 4.

V_SW can be approximated by:

The diode forward drop (V_D) can range from 0.3 V to 0.7 V depending on the quality of the diode. The lower the V_D, the higher the operating efficiency of the converter.

The inductor value determines the output ripple current (Δi_L, as defined in ). Lower inductor values decrease the size of the inductor, but increase the output ripple current. An increase in the inductor value decreases the output ripple current. In general, the ratio of ripple current to the output current is optimized when it is set between 0.2 and 0.4 for output currents above 2 A. This ratio r is defined as:

Equation 6.

One must ensure that the minimum current limit (3.4 A) is not exceeded, so the peak current in the inductor must be calculated. The peak current (I_LPK) in the inductor is calculated by:

When the designed maximum output current is reduced, the ratio r can be increased. At a current of 0.1 A, r can be made as high as 0.9. The ripple ratio can be increased at lighter loads because the net ripple is actually quite low, and if r remains constant the inductor value can be made quite large. An equation empirically developed for the maximum ripple ratio at any current below 2 A is:

Note that this is just a guideline, and it needs to be combined with two important factors for proper selection of inductance values at any operating condition. The first consideration is at output voltage above 2.5 V, one needs to ensure that the inductance given by the above guideline should not be less than 1 µH for the LMR10530X or 0.5 µH for the LMR10530Y. Because the LMR10530 has a fixed internal corrective ramp signal, a very low inductance value at high output voltage generates a very steep down slope of inductor current, which results in an insufficient slope compensation, and cause instability known as sub-harmonic oscillation. Another consideration is at low load current, one needs to ensure that the inductance value given by the guideline should not exceed 10 µH for the LMR10530X and 4.7 µH for the LMR10530Y, since too much inductance effectively flattens the down slope of the inductor current, and may significantly limit the system bandwidth and phase margin resulting in instability.

The LMR10530 operates at frequencies allowing the use of ceramic output capacitors without compromising transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple. See the Output Capacitor section for more details on calculating output voltage ripple.

Now that the ripple current is determined, the inductance is calculated by:

Equation 9.

where

• f_SW is the switching frequency.

When selecting an inductor, make sure that it is capable of supporting the peak output current without saturating. Inductor saturation will result in a sudden reduction in inductance and prevent the regulator from operating properly. Because of the operating frequency of the LMR10530, ferrite based inductors are preferred to minimize core losses. This presents little restriction since the variety and availability of ferrite-based inductors is large. Lastly, inductors with lower series resistance (DCR) will provide better operating efficiency. For recommended inductor selection, refer to Other System Examples.