Since the selection of the inductor affects steady state operation, transient behavior, and loop stability, the inductor is the most important component in power regulator design. There are three important inductor specifications: inductance, saturation current, and DC resistance.

The TPS552872-Q1 is designed to work with inductor values between 1 μH and 10 μH. The inductor selection is based on consideration of both buck and boost modes of operation. For 2 MHz switching frequency, a 1uH or 2.2uH inductor is recommended. For 400kHz switching frequency, a 4.7uH inductor is recommended.

For buck mode, the inductor selection is based on limiting the peak-to-peak current ripple to the maximum inductor current at the maximum input voltage. In CCM, Equation 9 shows the relationship between the inductance and the inductor ripple current.

Equation 9.

where

• V_IN(MAX) is the maximum input voltage
• V_OUT is the output voltage
• ΔI_L(P-P) is the peak to peak ripple current of the inductor
• f_SW is the switching frequency

For a certain inductor, the inductor ripple current achieves maximum value when VOUT equals half of the maximum input voltage. Choosing higher inductance gets smaller inductor current ripple while smaller inductance gets larger inductor current ripple.

For boost mode, the inductor selection is based on limiting the peak-to-peak current ripple to the maximum inductor current at the maximum output voltage. In CCM, Equation 10 shows the relationship between the inductance and the inductor ripple current.

Equation 10.

where

• V_IN is the input voltage
• V_OUT(MAX) is the maximum output voltage
• ΔI_L(P-P) is the peak to peak ripple current of the inductor
• f_SW is the switching frequency

For a certain inductor, the inductor ripple current achieves maximum value when V_IN equals to the half of the maximum output voltage. Choosing higher inductance gets smaller inductor current ripple while smaller inductance gets larger inductor current ripple.

In buck mode, the inductor DC current equals to the output current. In boost mode, the inductor DC current can be calculated with Equation 11.

Equation 11.

where

• V_OUT is the output voltage
• I_OUT is the output current
• V_IN is the input voltage
• η is the power conversion efficiency

For a given maximum output current of the buck-boost converter TPS552872-Q1, the maximum inductor DC current happens at the minimum input voltage and maximum output voltage. Set the inductor current limit of the TPS552872-Q1 higher than the calculated maximum inductor DC current to make sure the TPS552872-Q1 has the desired output current capability.

In boost mode, the inductor ripple current is calculated with Equation 12.

Equation 12.

where

• ΔI_L(P-P) is the inductor ripple current
• L is the inductor value
• f_SW is the switching frequency
• V_OUT is the output voltage
• V_IN is the input voltage

Therefore, the inductor peak current is calculated with Equation 13.

Equation 13.

Normally, it is advisable to work with an inductor peak-to-peak current of less than 40% of the average inductor current for maximum output current. A smaller ripple from a larger valued inductor reduces the magnetic hysteresis losses in the inductor and EMI, but in the same way, load transient response time is increased. The selected inductor must have higher saturation current than the calculated peak current.

The conversion efficiency is dependent on the resistance of its current path. The switching loss associated with the switching MOSFETs, and the inductor core loss. Therefore, the overall efficiency is affected by the inductor DC resistance (DCR), equivalent series resistance (ESR) at the switching frequency, and the core loss. Table 8-2 lists recommended inductors for the TPS552872-Q1. In this application example, switching frequency is 2 MHz and the Coilcraft inductor XGL5030-102 is selected for its small size, high saturation current, and small DCR.