SLVSCN9B December 2014 – June 2020 TPS61175-Q1
The selection of the inductor affects steady state operation as well as transient behavior and loop stability. These factors make it the most important component in power regulator design. There are three important inductor specifications, inductor value, DC resistance and saturation current. Considering inductor value alone is not enough.
Inductor values can have ±20% tolerance with no current bias. When the inductor current approaches saturation level, its inductance can fall to some percentage of its 0-A value depending on how the inductor vendor defines saturation current. For CCM operation, the rule of thumb is to choose the inductor so that its inductor ripple current (ΔIL) is no more than a certain percentage (RPL% = 20–40%) of its average DC value (IIN(AVG) = IL(AVG))
Rearranging and solving for L gives
Choosing the inductor ripple current to closer to 20% of the average inductor current results in a larger inductance value, maximizes the converter’s potential output current and minimizes EMI. Choosing the inductor ripple current closer to 40% of IL(AVG) results in a smaller inductance value, and a physically smaller inductor, improves transient response but results in potentially higher EMI and lower efficiency if the DCR of the smaller packaged inductor is significantly higher. Using an inductor with a smaller inductance value than computed above may result in the converter operating in DCM. This reduces the boost converter’s maximum output current, causes larger input voltage and output ripple and typically reduces efficiency. Table 4 lists the recommended inductor for the TPS61175-Q1.
(L × W × H mm)
The device has built-in slope compensation to avoid subharmonic oscillation associated with current mode control. If the inductor value is lower than 4.7μH, the slope compensation may not be adequate, and the loop can be unstable. Applications requiring inductors above 47μH have not been evaluated. Therefore, the user is responsible for verifying operation if they select an inductor that is outside the 4.7μH–47μH recommended range.