LM3478_09 Datasheet, PDF (14/22 Pages) National Semiconductor (TI) – High Efficiency Low-Side N-Channel Controller for Switching Regulator

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FIGURE 8. Simplified Boost Convertert
A. First Cycle Operation B. Second Cycle of Operation
POWER INDUCTOR SELECTION
The inductor is one of the two energy storage elements in a
boost converter. Figure 9 shows how the inductor current
varies during a switching cycle. The current through an in-
ductor is quantified by the following relationship of L, I_L and
V_L:
The important quantities in determining a proper inductance
value are I_L (the average inductor current) and ΔI_L (the in-
ductor current ripple). If ΔI_L is larger than I_L, the inductor
current will drop to zero for a portion of the cycle and the con-
verter 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:
Choose the minimum Iout to determine the minimum induc-
tance value. A common choice is to set ΔI_L to 30% of I_L.
Choosing an appropriate core size for the inductor involves
calculating the average and peak currents expected through
the inductor. In a boost converter the peak inductor current is:
I_LPEAK = Average I_L(max) + ΔI_L(max)
Average I_L(max) = I_out / (1-D)
ΔI_L(max) = D x V_in / (2 x f_s x L)
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 above.
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FIGURE 9. Inductor Current and Diode Current
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