SC4624SETRT datasheet(13/20 Pages) SEMTECH

13 / 20 page

 2007 Semtech Corp.

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SC4624

POWER MANAGEMENT

Application information (Cont.)

After the high side PMOS turn on around 30ns, the OCP

comparator will compare between V2 and V1. When the

converter detects an over current condition (V2 > V1) as

shown in Figure 16, the SC4624 proceeds into the cycle by

cycle protection mode (Point B to Point C), which responds

to minor over current cases and the output voltage is

monitored.

If the over current and low output voltage (set at 60% of

nominal output voltage) occur at the same time, the SS

pin is pull low by an internal switch and the comp pin is

pulled low and the devices stops switching. Assume start

from FB = 0V, FB and SS voltage rise forward 0.5V. Once

SS voltage exceeds 0.4V, the hiccup comparator becomes

enabled. The hiccup period is around 217/F

OSC. (Point C to

Point D).

For example, with a switching frequency application of

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 SC4624 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

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