LM3478MA_15 Datasheet, PDF (14/26 Pages) Texas Instruments – High-Efficiency Low-Side N-Channel Controller for Switching Regulator

LM3478MA
SNVS705B – FEBRUARY 2011 – REVISED FEBRUARY 2013
www.ti.com
Figure 30. Simplified Boost Converter
(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 31 shows how the inductor
current varies during a switching cycle. The current through an inductor is quantified by the following relationship
of L, I_L and V_L:
(10)
The important quantities in determining a proper inductance value are I_L (the average inductor current) and ΔI_L
(the inductor 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 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:
(11)
Choose the minimum Iout to determine the minimum inductance 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)
(12)
(13)
(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 above.
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