8.2.1.2.1 Inductor Selection

The inductor is the most important component in switching power regulator design because it affects power supply steady state operation, transient behavior, and loop stability. The inductor value, DC resistance and saturation current are important specifications to be considered for better performance. Although the boost power stage can be designed to operate in discontinuous conduction mode (DCM) at maximum load, where the inductor current ramps down to zero during each switching cycle, most applications are more efficient if the power stage operates in continuous conduction mode (CCM), where a DC current flows through the inductor. Therefore, the Equation 7 and Equation 8 are for CCM operation only. The TPS61197 device is designed to work with inductor values from 4.7 µH and 470 µH, depending on the switching frequency. Running the controller at higher switching frequencies allows the use of smaller and/or lower profile inductors in the 4.7-µH range. Running the controller at slower switching frequencies requires the use of larger inductors, near 470 µH, to maintain the same inductor current ripple but may improve overall efficiency due to smaller switching losses. Inductor values can have ±20% tolerance with no current bias. When the inductor current approaches saturation level, its inductance can decrease 20% to 35% from the value measured at near 0 A, depending on how the inductor vendor defines saturation.

In a boost regulator, the inductor DC current can be calculated with Equation 6.

Equation 6.

where

• V_OUT = boost output voltage
• I_OUT = boost output current
• V_IN = boost input voltage
• η = power conversion efficiency, use 95% for TPS61197 applications

The inductor peak-to-peak ripple current can be calculated with Equation 7.

Equation 7.

where

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

Therefore, the inductor peak current is calculated with Equation 8.

Equation 8.

Select an inductor, which saturation current is higher than calculated peak current. To calculate the worst case inductor peak current, use the minimum input voltage, maximum output voltage and maximum load current.

Regulator efficiency is dependent on the resistance of its high current path and switching losses associated with the switch FET and power diode. Besides the external switch FET, the overall efficiency is also affected by the inductor DC resistance (DCR). Usually the lower DC resistance shows higher efficiency. However, there is a tradeoff between DCR and inductor footprint; furthermore, shielded inductors typically have higher DCR than unshielded ones.