Use Equation 2 to calculate the recommended value of the output inductor L.

Equation 2.

where

• K_IND is a coefficient that represents the amount of inductor ripple current relative to the maximum LED current.
• I_LED is the maximum LED current.
• V_OUT is the sum of the voltage across the LED load and the voltage across the sense resistor.

In general, the value of K_IND is suggested between 0.2 and 0.4. For the application that can tolerate higher LED current ripple or use larger output capacitors, one can choose 0.4 for K_IND, otherwise, smaller K_IND like 0.2 can be chosen to get smaller LED current ripple.

With the chosen inductor value, the user can calculate the actual inductor current ripple using Equation 3.

Equation 3.

For TPS92200, TI suggests that the inductor current ripple be larger than 300 mA to assure loop stability. If the calculated inductor current ripple is less than 300 mA, TI suggests a smaller inductor.

The inductor RMS current and saturation-current ratings must be greater than those seen in the application. These ratings ensure that the inductor does not overheat or saturate. During power up, transient conditions, or fault conditions, the inductor current can exceed its normal operating current. For this reason, the most conservative approach is to specify an inductor with a saturation current rating equal to or greater than the converter current limit. This action is not always possible due to application size limitations. The peak-inductor-current and RMS current equations are shown in Equation 4 and Equation 5.

Equation 4.

Equation 5.

In this design, V_IN(max) = 13.2 V, V_OUT = 3.6 V, I_LED = 1.5 A, choose K_IND = 0.3, the calculated inductance is 5.8-µH. A 4.7-µH inductor is chosen. With this inductor, the ripple, peak, and RMS currents of the inductor are 0.56 A, 1.78 A and 1.51 A respectively. The chosen inductor has ample margin.