The LM53625/35-Q1 is optimized for a nominal inductance of 2.2 μH for the 5-V and 3.3-V versions. This gives a ripple current that is approximately 20% to 30% of the full load current of 3.5 A. 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.

The most important inductor parameters are saturation current and parasitic resistance. Inductors with a saturation current of between 7 A and 8 A are appropriate for most applications when using the LM53625/35-Q1. Of course, the inductor parasitic resistance must be as low as possible to reduce losses at heavy loads. Table 9-2 gives a list of several possible inductors that can be used with the LM53625/35-Q1.

The LM53625 and LM53635 devices run in current mode and with internal compensation. This compensation is stable with inductance between 1.5 μH and 10 μH. For most applications, use 2.2 μH with the fixed 5-V and 3.3-V versions of the LM53625 and LM53635 devices. Adjustable devices operate at the same frequency under high input-voltage conditions as devices set to deliver 3.3 V (see Figure 9-28). Inductor current ripple at high input voltages can become excessive when using a 2.2-μH inductor with an adjustable device that is delivering output voltage above 6 V. A 4.7-μH inductor might be necessary. Inductance that is too high is not recommended as it can result in poor load transient behavior and instability for extreme inductance choice. See Table 9-2 for typical recommended values.

The inductor must be rated to handle the peak load current plus the ripple current — take care when reviewing 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 LM53635, TI recommends a saturation current of 7.5 A or higher, and for the LM53625, a saturation current of 6.5 A or higher is recommended

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.