SC4519 Semtech Corporation, SC4519 Datasheet - Page 8

• Manufacturer Part Number: SC4519
• Description: 3A Step-Down Switching Regulator
• Manufacturer: Semtech Corporation

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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 SC4519 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 allow large ripple currents,
poor efficiencies and require more output capacitance
for low output ripple. 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:
Where:
fs = switching frequency,
output load current and
V
POWER MANAGEMENT
Application Information (Cont.)
O
= ratio of the peak to peak inductor current to the
2007 Semtech Corp.
= output voltage.
2.900
2.880
2.860
2.840
2.820
2.800
2.780
2.760
2.740
2.720
2.700
Maximum Load Current vs Input Voltage
4
6
8
L
V
10
Vi (V)
V
I
L=10uH
O
f
s
12
(V
δ
I
I
OMAX
14
V
O
)
16
18
Vo=2.5V
Vo=3.3V
Vo=5V
8
The peak to peak inductor current is:
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
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
without saturation and produce low core loss during the
high frequency operation. The core loss can be found in
the manufacturer’s datasheet. The inductor’s copper loss
can be estimated as follows:
Where:
be calculated as follows:
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 SC4519
regulates the inductor current to a new value during a
load transient, the output capacitor delivers all the
additional current needed by the load. The ESR and ESL
of the output capacitor, the loop parasitic inductance
between the output capacitor and the load combined
with inductor ripple current are all major contributors to
the output voltage ripple. Surface mount ceramic
capacitors are recommended.
Input Capacitor Selection
The input capacitor selection is based on its ripple current
level, required capacitance and voltage rating. This
I
LRMS
is the RMS current in the inductor. This current can
P
I
COPPER
LRMS
I
I
PEAK
p
I
p
OMAX
I
2
LRMS
δ
I
OMAX
I
OMAX
1
R
WINDING
12
1
I
p
2
δ
2
p
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SC4519
PEAK
PEAK