An inductive converter normally requires two main passive components for storing energy during the conversion. Therefore, each converter requires an inductor and a storage capacitor. In selecting the right inductor, TI recommends keeping the possible peak inductor current below the current limit threshold of the power switch in the chosen configuration. For example, the current limit threshold of the switch for the boost converter and for the inverting converters is nominally 800 mA for the TPS65130 device and 1950 mA for TPS65131 device. The highest peak current through the switches and the inductor depend on the output load, the input voltage (V_IN), and the output voltages (V_POS, V_NEG). Use Equation 3 to estimate the peak inductor current in the boost converter, I_{L_P}. Equation 4 shows the corresponding formula for the inverting converter, I_{L_N}.

Equation 3.

Equation 4.

The second parameter for choosing the inductor is the desired current ripple in the inductor. Normally, it is advisable to work with a ripple of less than 20% of the average inductor current. A smaller ripple reduces the losses in the inductor, as well as output voltage ripple and EMI. But in the same way, output voltage regulation gets slower, causing greater voltage changes at fast load changes. In addition, a larger inductor usually increases the total system cost. Keep those parameters in mind and calculate the possible inductor value with Equation 5 for the boost converter and Equation 6 for the inverting converter.

Equation 5.

Equation 6.

Parameter f is the switching frequency. For the boost converter, ΔI_L-P is the ripple current in the inductor, that is, 20% of I_L-P. Accordingly, for the inverting converter, ΔI_L-N is the ripple current in the inductor, that is, 20% of I_L-N. V_I is the input voltage, which is 3.3 V in this example. So, the calculated inductance value for the boost inductor is 5.1 μH and for the inverting converter inductor is 5.1 μH. With these calculated values and the calculated currents, it is possible to choose a suitable inductor.

In typical applications, the recommendation is to choose a 4.7-μH inductor. The device is optimized to work with inductance values from 3.3 μH to 6.8 μH. Nevertheless, operation with greater inductance values may be possible in some applications. Perform detailed stability analysis in this case. Be aware of the possibility that load transients and losses in the circuit can lead to higher currents than estimated in Equation 3 and Equation 4. Also, the losses caused by magnetic hysteresis and conductor resistance are a major parameter for total circuit efficiency.

Table 8-3 shows inductors from different suppliers used with the TPS65130/1 converter: