SC4620 datasheet(13/19 Pages) SEMTECH

13 / 19 page

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SC4620

POWER MANAGEMENT

Application information (Cont.)

550kHz, the hiccup period is around 238ms. (refer to

Figure 7).

A poor layout will make OCP trip point shift and is not

easily to calculate by R

ISET. This is because it is affected

by ground bounce, spiker voltage between Vin pin and PH

pin, and internal parameter tolerance. Users can refer to

Figure 14, it shows how to set maximum output current

by R

ISET.

Iout

Imax

Vout

0.6 * Vout

Figure 16. Over Current Protection Characteristic

Inductor Selection

For a typical SC4620 application, the inductor selection

is mainly based on its value, saturation current and DC

resistance. 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.

The inductor value can be determined according to its

operating point and the switching frequency as follows:

where

fs = switching frequency.

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

maximum output load current.

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 requirements. 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 and is given as follows:

The power loss for the inductor includes its core loss and

copper loss. If possible, the winding resistance should be

minimized to reduce any copper loss of the inductor, (the

core loss can be found in the manufacturer’s datasheet).

The inductor’s copper loss can be estimated as follows:

where

LRMS is the RMS current in the inductor.

This current can be calculated as follows:

Output Capacitor Selection

Basically there are two major factors to consider in select-

ing the type and quantity of the output capacitors. The

first one is the required ESR (Equivalent Series Resis-

tance) which should be low enough to reduce the voltage

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

SC4620 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 para-

sitic inductance between the output capacitor and the

load combined with inductor ripple current are all major

contributors to the output voltage ripple.

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

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