8.2.2.3 Inductor Selection – L1

The inductor value is determined based upon the operating frequency, load current, ripple current, and the input and output voltages. See Figure 20 for details.

To keep the circuit in continuous conduction mode (CCM), the maximum ripple current IRIPPLE should be less than twice the minimum load current. For the specified minimum load of 0.6 A, the maximum ripple current is 1.2 Ap-p. Also, the minimum value of L must be calculated both for a buck and buck-boost configurations. The final value of inductance will generally be a compromise between the two modes. It is desirable to have a larger value inductor for buck mode, but the saturation current rating for the inductor must be large for buck-boost mode, resulting in a physically large inductor. Additionally, large value inductors present buck-boost mode loop compensation challenges which will be discussed in Error Amplifier Configuration . For the design example, the inductor values in both modes are calculated as:

Equation 10.

Equation 11.

where

• V_OUT is the output voltage
• V_IN(MAX) is the maximum input voltage
• f is the switching frequency
• I_RIPPLE is the selected inductor peak to peak ripple current (1.2 A selected for this example)
• V_IN(MIN) is the minimum input voltage

The resulting inductor values are:

A 10-µH inductor was selected which is a compromise between these values, while favoring the buck-boost mode. As illustrated in the compensation section, the inductor value should be as low as possible to move the buck-boost right-half-plane zero to a higher frequency. The ripple current is then rechecked with the selected inductor value using Equation 10 and Equation 11.

Because the inductor selected is lower than calculated for the Buck mode, the minimum load current for CCM in buck mode is 1.42 A at maximum VIN.

With a 10-µH inductor, the worst case peak inductor currents can be estimated for each case, assuming a 20% inductor value tolerance and 80% efficiency of the converter.

Equation 16.

Equation 17.

where

• η is efficiency
• L_TOL is the inductor tolerance

For this example, Equation 16 and Equation 17 yield:

An acceptable current limit setting would be 6.7 A for buck mode because the LM25118 automatically doubles the current limit threshold in buck-boost mode. The selected inductor must have a saturation current rating at least as high as the buck-boost mode cycle-by-cycle current limit threshold, in this case at least 13.5 A. A 10-µH, 15-A inductor was chosen for this application.