The TPS62851x is designed for a nominal 0.47-µH inductor with a switching frequency of typically 2.25 MHz. Larger values can be used to achieve a lower inductor current ripple but they can have a negative impact on efficiency and transient response. Smaller values than 0.47 µH cause a larger inductor current ripple which causes larger negative inductor current in forced PWM mode at low or no output current. For a higher or lower nominal switching frequency, the inductance must be changed accordingly. See Section 7.3 for details.
The inductor selection is affected by several effects like inductor ripple current, output ripple voltage, PWM-to-PFM transition point, and efficiency. In addition, the inductor selected has to be rated for appropriate saturation current and DC resistance (DCR). Equation 5 calculates the maximum inductor current.
|TYPE||INDUCTANCE [µH]||CURRENT [A](1)||FOR DEVICE||NOMINAL SWITCHING FREQUENCY||DIMENSIONS [LxBxH] mm||MANUFACTURER(2)|
|0.47 µH, ±20%||see data sheet||TPS628510/511 / 512||2.25 MHz||2.0 x 1.2 x 1.0||Murata|
|DFE201210U-1R0M||1 µH, ±20%||see data sheet||TPS628510/511 / 512||2.25 MHz||2.0x 1.2 x 1.0||
|0.68 µH, ±20%||see data sheet||TPS628510/511 / 512||2.25 MHz||2.0x 1.2 x 1.0||Murata|
|XEL3515-561ME||0.56 µH, ±20%||4.5||TPS628510/511 / 512||2.25 MHz||3.5 x 3.2 x 1.5||Coilcraft|
|XFL4015-701ME||0.70 µH, ±20%||
|TPS628510/511 / 512||2.25 MHz||4.0 x 4.0 x 1.6||Coilcraft|
|XFL4015-471ME||0.47 µH, ±20%||3.5||TPS628510/511 / 512||2.25 MHz||4.0 x 4.0 x 1.6||Coilcraft|
Calculating the maximum inductor current using the actual operating conditions gives the minimum saturation current of the inductor needed. A margin of about 20% is recommended to add. A larger inductor value is also useful to get lower ripple current, but increases the transient response time and size as well.