LM3478 MDC National Semiconductor, LM3478 MDC Datasheet - Page 14

• Manufacturer Part Number: LM3478 MDC
• Description: LM3478/LM3478Q High Efficiency Low-Side N-Channel Controller for Switching Regulator; ; Container: Tray
• Manufacturer: National Semiconductor

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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
and
L
V
:
L
The important quantities in determining a proper inductance
(the average inductor current) and ΔI
value are I
(the in-
L
L
ductor current ripple). If ΔI
is larger than I
, the inductor
L
L
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
to 30% of I
.
L
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:
+ ΔI
I
= Average I
LPEAK
L(max)
L(max)
Average I
= I
/ (1-D)
L(max)
out
ΔI
= D x V
/ (2 x f
x L)
L(max)
in
s
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
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the peak current through the switch which is the same as the
peak inductor current as calculated above.
FIGURE 9. Inductor Current and Diode Current
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