SC4605IMSTRT datasheet(8/19 Pages) SEMTECH

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SC4605

POWER MANAGEMENT

The inductor value can be determined according to its

operating point and the switching frequency as follows:

OMAX

)

(

⋅

∆

⋅

−

⋅

Where:

fs = switching frequency and

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

maximum output load current.

The peak to peak inductor current is:

OMAX

•

∆

−

After the required inductor value is selected, the proper

selection of the core material is based on the peak in-

ductor current and efficiency requirements. The core

must be able to handle the peak inductor current IPEAK

without saturation and produce low core loss during the

high frequency operation.

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

loss can be found in the manufacturer’s datasheet. The

inductor’ copper loss can be estimated as follows:

WINDING

LRMS

COPPER

⋅

Where:

ILRMS 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 se-

lecting the type and quantity of the output capacitors.

The first one is the required ESR (Equivalent Series Re-

sistance) which should be low enough to reduce the volt-

age deviation from its nominal one during its load changes.

The second one is the required capacitance, which should

be high enough to hold up the output voltage. Before the

SC4605 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 speciality poly-

mer aluminum electrolytic chip capacitors in UE series

from Panasonic provide low ESR and reduce the total

capacitance required for a fast transient response.

POSCAP from Sanyo is a solid electrolytic chip capacitor

that has a low ESR and good performance for high fre-

quency with a low profile and high capacitance. Above

mentioned capacitors are recommended to use in

SC4605 applications.

Boost Capacitor Selection

The boost capacitor selection is based on its discharge

ripple voltage, worst case condition time and boost cur-

rent. The worst case conduction time T

can be estimated

as follows:

max

⋅

Where:

= the switching frequency and

Dmax = maximum duty ratio, 0.97 for the SC4605.

The required minimum capacitance for boost capacitor

will be:

boost

⋅

Where:

= the boost current and

= discharge ripple voltage

With f

= 300kH, V

= 0.3V and I

= 50mA, the required

capacitance for the boost capacitor is:

540

300

max

boost

⋅

Input Capacitor Selection

The input capacitor selection is based on its ripple cur-

rent level, required capacitance and voltage rating. This

capacitor must be able to provide the ripple current by

the switching actions. For the continuous conduction

Applications Information (Cont.)

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