SC4518HEVB datasheet(8/15 Pages) SEMTECH

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POWER MANAGEMENT

SC4518H

Maximum Load Current vs Input Voltage

L=10uH

1.700

1.720

1.740

1.760

1.780

1.800

1.820

1.840

1.860

1.880

1.900

1214

1618

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 SC4518H

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:

OMAX

)

(

⋅

∆

⋅

−

⋅

Where:

fs = switching frequency,

∆I = ratio of the peak to peak inductor current to the

output load current and

O = output voltage.

The peak to peak inductor current is:

Application Information (Cont.)

OMAX

•

∆

−

OMAX

PEAK

−

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:

WINDING

LRMS

COPPER

⋅

Where:

LRMS is the RMS current in the inductor. This current can

be calculated as follows:

OMAX

LRMS

∆

⋅

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

SC4518H 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.

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