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LM3478

SNVS085X – JULY 2000 – REVISED DECEMBER 2017

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Typical Applications (continued)

5. Get more information about WEBENCH tools at www.ti.com/webench.

8.2.1.2.2

Power Inductor Selection

The inductor is one of the two energy storage elements in a boost converter. Figure 29 shows how the inductor

current varies during a switching cycle. The current through an inductor is quantified using Equation 10, which

shows the relationship of L, IL and VL.

(10)

The important quantities in determining a proper inductance value are IL (the average inductor current) and ΔIL

(the inductor current ripple). If ΔIL is larger than IL, the inductor current will drop to zero for a portion of the cycle

and the converter will operate in the DCM. All the analysis in this datasheet assumes operation in the CCM. To

operate in the CCM, the following condition must be met by using Equation 11.

(11)

Choose the minimum IOUT to determine the minimum inductance value. A common choice is to set ΔIL to 30% of

IL. Choosing an appropriate core size for the inductor involves calculating the average and peak currents

expected through the inductor. Use Equation 12, Equation 13, and Equation 14 to the peak inductor current in a

boost converter.

ILPEAK = Average IL(max) + ΔIL(max)

(12)

Average IL(max) = Iout / (1-D)

(13)

ΔIL(max) = D x Vin / (2 x fs x L)

(14)

An inductor size with ratings higher than these values has to be selected. If the inductor is not properly rated,

saturation will occur and may cause the circuit to malfunction.

The LM3478 can be set to switch at very high frequencies. When the switching frequency is high, the converter

can be operated with very small inductor values. The LM3478 senses the peak current through the switch which

is the same as the peak inductor current as calculated in the previous equation.