Datasheet

LM3478
LM3478-Q1
SNVS085V –JULY 2000–REVISED FEBRUARY 2013 www.ti.com
Figure 31. 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 32 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)
(12)
Average I
L(max)
= I
out
/ (1-D) (13)
ΔI
L(max)
= D x V
in
/ (2 x f
s
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 above.
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