Diodes PAM2842 - Manual (Page 8)

PAM2842
Document number: DSxxxxx Rev. 1 - 2
8 of 17
www.diodes.com
October 2012
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PAM2842
A Product Line of
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Application Information
Topology Selection
When maximum power supply voltage is below than minimum load voltage, select the boost topology. When minimum power supply voltage is
high than maximum load voltage, select buck topology. When load voltage range is small and between the power supply voltage, select sepic
topology.
Table 1: Voltage Condition vs. Topology
Condition Topology
V
INMAX
< V
OMIN
Boost
V
INMIN
> V
OMAX
Buck
V
O IN
Sepic
Inductor Selection
The inductance, peak current rating, series resistance, and physical size should all be considered when selecting an inductor. These factors
affect the converter's operating mode, efficiency, maximum output load capability, transient response time, output voltage ripple, and cost.
The maximum output current, input voltage, output voltage, and switching frequency determine the inductor value. Large inductance can
minimizes the current ripple, and therefore reduces the peak current, which decreases core losses in the inductor and I2R losses in the entire
power path. However, large inductor values also require more energy storage and more turns of wire, which increases physical size and I2R
copper losses in the inductor. Low inductor values decrease the physical size, but increase the current ripple and peak current. Finding the best
inductor involves the compromises among circuit efficiency, inductor size, and cost.
When choosing an inductor, the first step is to determine the operating mode: continuous conduction mode (CCM) or discontinuous conduction
mode (DCM). When CCM mode is chosen, the ripple current and the peak current of the inductor can be minimized. If a small-size inductor is
required, DCM mode can be chosen. In DCM mode, the inductor value and size can be minimized but the inductor ripple current and peak
current are higher than those in CCM.
For the large power application, if chose DCM, the peak current will be very large, it will have great electrical stress on the components, so we
chose CCM.
When work in CCM mode, a reasonable ripple current is chosen to
ΔI
L
= 0.4I
L
For the boost topology,
D 1
I
I
O
L
−
= ,
V
V V
D
O
IN O
−
= ,
V LF
) V V ( V
I
O
IN O IN
L
−
= Δ
D: duty cycle, Io: output current, F: switching frequency.
From above equation we can get the inductance:
2
O O
IN O
2
IN
V FI
) V V ( V 5 . 2
L
−
=
The inductor's current rating should be higher
than
2
I
I
O
L
+
For the buck topology, I
L
= I
O
V
V
D
IN
O =
V LFV
V ) V V (
I
IN IN
O O IN
L
−
= Δ
so
2
O O
O IN O
V FI
) V V ( V 5 . 2
L
−
=