LM3481 Datasheet, PDF (14/22 Pages) National Semiconductor (TI) – High Efficiency Low-Side N-Channel Controller for Switching Regulators

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FIGURE 12. Simplified Boost Converter Diagram (a) First cycle of operation. (b) Second cycle of operation
POWER INDUCTOR SELECTION
The inductor is one of the two energy storage elements in a
boost converter. Figure 13 shows how the inductor current
varies during a switching cycle. The current through an in-
ductor is quantified as:
the converter will operate in discontinuous conduction mode.
If Δi_L is smaller than I_L, the inductor current will stay above
zero and the converter will operate in continuous conduction
mode. All the analysis in this datasheet assumes operation in
continuous conduction mode. To operate in continuous con-
duction mode, the following conditions must be met:
I_L > Δi_L
Choose the minimum I_OUT to determine the minimum L. A
common choice is to set (2 x 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,
I_{L_peak} = I_L(max) + Δi_L(max)
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FIGURE 13. a. Inductor current b. Diode current c. Switch
current
If V_L(t) is constant, di_L(t)/dt must be constant. Hence, for a
given input voltage and output voltage, the current in the in-
ductor changes at a constant rate.
The important quantities in determining a proper inductance
value are I_L (the average inductor current) and Δi_L (the induc-
tor current ripple difference between the peak inductor current
and the average inductor current). If Δi_L is larger than I_L, the
inductor current will drop to zero for a portion of the cycle and
A core size with ratings higher than these values should be
chosen. If the core is not properly rated, saturation will dra-
matically reduce overall efficiency.
The LM3481 can be set to switch at very high frequencies.
When the switching frequency is high, the converter can op-
erate with very small inductor values. With a small inductor
value, the peak inductor current can be extremely higher than
the output currents, especially under light load conditions.
The LM3481 senses the peak current through the switch. The
peak current through the switch is the same as the peak cur-
rent calculated above.
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