9.2.1.2.1.2.1 Inductor Selection

The LMS3655 devices run in current mode and with internal compensation. The compensation of the adjustable 5-V and 3.3-V configurations is stable with inductance between 6.5 µH and 20 µH. For most applications, the adjustable 5-V and 3.3-V configurations of the LMS3655 devices are optimized for a nominal inductance of 10 μH. This gives a ripple current that is approximately 20% to 30% of the full load current of 5.5 A. If applying a synchronization clock signal, the designer should appropriately size the inductor for the converter's operating switching frequency. For output voltages greater than 5 V, a proportionally larger inductor can be used, thus keeping the ratio of inductor current slope to internal compensating slope constant. Inductance that is too high is not recommended because it can result in poor load transient behavior and instability.

The inductor must be rated to handle the peak load current plus the ripple current—carefully review the different saturation current ratings specified by different manufacturers. Saturation current ratings are typically specified at 25°C, so ratings at maximum ambient temperature of the application should be requested from the manufacturer. For the LMS3655, TI recommends a saturation current of 10 A or higher. Carefully review the inductor parasitic resistance; the inductor parasitic resistance must be as low as possible to minimize losses at heavy loads. The best way to obtain an optimum design is to use the Texas Instruments WEBENCH Design Tool.

Table 5 gives a list of several possible inductors that can be used with the LMS3655.

The designer should choose the inductors that best match the system requirements. A very wide range of inductors are available as regarding physical size, height, maximum current (thermally limited, and inductance loss limited), series resistance, maximum operating frequency, losses, and so forth. In general, inductors of smaller physical size have higher series resistance (DCR) and implicitly lower overall efficiency is achieved. Very low-profile inductors may have even higher series resistance. TI recommends finding the best compromise between system performance and cost.