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 2007 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC4521

Figure 2 shows the theoretical maximum load current

for the specific cases. In a real application, however, the

allowed maximum load current also depends on the layout

and the air cooling condition. Therefore, the maximum

load current may need to be degraded according to the

thermal situation of the application.

Maximum Load Current vs Input Voltage

L=10uH

2.700

2.720

2.740

2.760

2.780

2.800

2.820

2.840

2.860

2.880

2.900

46

8

10

12

14

16

18

Vi (V)

Vo=2.5V

Vo=3.3V

Vo=5V

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 SC4521 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:

Application Information (Cont.)

OMAX

s

I

O

I

O

I

δ

f

V

)

V

(V

V

L

⋅

⋅

⋅

−

⋅

=

Where:

fs = switching frequency,

δ = ratio of the peak to peak inductor current to the

output load current and

V

The peak to peak inductor current is:

OMAX

p

p

I

δ

I

•

=

−

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.

2

I

I

I

p

p

OMAX

PEAK

−

+

=

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 core loss can be found in

the manufacturer’s datasheet. The inductor’s copper loss

can be estimated as follows:

WINDING

LRMS

2

COPPER

R

I

P

⋅

=

Where:

I

be calculated as follows:

2

OMAX

LRMS

δ

12

1

1

I

I

⋅

+

⋅

=

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 SC4521

regulates the inductor current to a new value during a