SC4517A
POWER MANAGEMENT
Application Information (Cont.)
thermal situation of the application. For example, the
SC4517A with EDP package is able to handle higher
current than the SC4517A with MSOP package if their
working conditions are same.
Maximum Load Current vs Input Voltage
L=4.7uH
1.400
1.350
Iomax (A)
Vo=2.5V
1.300
1.250
1.200
4
6
8
10
12
14
16
18
Vi (V)
Vo=3.3V
Vo=5V
Where:
fs = switching frequency,
δ
= ratio of the peak to peak inductor current to the
output load current and
V
O
= output voltage.
The peak to peak inductor current is:
I
p
−
p
= δ •
I
OMAX
I
p
−
p
2
I
PEAK
=
I
OMAX
+
After the required inductor value is selected, the proper
selection of the core material is based on the peak
inductor current and efficiency specifications. The core
must be able to handle the peak inductor current I
PEAK
without saturation and produce low core loss during the
high frequency operation.
The power loss for the inductor includes its core loss and
copper loss. If possible, the winding resistance should
be minimized to reduce inductor’s copper loss. The core
must be able to handle the peak inductor current I
PEAK
without saturation and produce low core loss during the
high frequency operation. The power loss for the inductor
includes its core loss and copper loss. If possible, the
winding resistance should be minimized to reduce
inductor’s copper loss. The core loss can be found in the
manufacturer’s datasheet. The inductor’s copper loss
can be estimated as follows:
P
COPPER
=
I
2LRMS
⋅
R
WINDING
Figure 2. Theoretical maximum load current curves.
Inductor Selection
The factors for selecting the inductor include its cost,
efficiency, size and EMI. For a typical SC4517A
application, the inductor selection is mainly based on its
value, saturation current and DC resistance. Increasing
the inductor value will decrease the ripple level of the
output voltage while the output transient response will
be degraded. Low value inductors offer small size and
fast transient responses while they cause large ripple
currents, poor efficiencies and more output capacitance
to filter out the large ripple currents. The inductor should
be able to handle the peak current without saturating
and its copper resistance in the winding should be as low
as possible to minimize its resistive power loss. A good
trade-off among its size, loss and cost is to set the
inductor ripple current to be within 15% to 30% of the
maximum output current.
The inductor value can be determined according to its
operating point under its continuous mode and the
switching frequency as follows:
L
=
V
O
⋅
( V
I
−
V
O
)
V
I
⋅
f
s
⋅ δ ⋅
I
OMAX
Where:
I
LRMS
is the RMS current in the inductor. This current can
be calculated as follows:
I
LRMS
=
I
OMAX
⋅
1
+
1
2
⋅δ
12
Output Capacitor Selection
Basically there are two major factors to consider in
selecting the type and quantity of the output capacitors.
The first one is the required ESR (Equivalent Series
Resistance) which should be low enough to reduce the
output voltage deviation during load changes. The second
one is the required capacitance, which should be high
enough to hold up the output voltage. Before the
SC4517A regulates the inductor current to a new value
during a load transient, the output capacitor delivers all
8
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