LTC3530 datasheet(10/16 Pages) LINER

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LTC3530

3530fb

Soft-Start

The soft-start function is combined with shutdown.

When the SHDN/SS pin is brought above 1V typical, the

IC is enabled but the EA duty cycle is clamped from VC.

A detailed diagram of this function is shown in Figure 3.

The components RSS and CSS provide a slow ramping

voltage on SHDN/SS to provide a soft-start function. To

ensure that VC is not being clamped, SHDN/SS must be

raised above 1.6V.

COMPONENT SELECTION

Inductor Selection

The high frequency operation of the LTC3530 allows the

use of small surface mount inductors. The inductor ripple

current is typically set to 20% to 40% of the maximum

inductor current. For a given ripple the inductance terms

are given as follows:

LBOOST >

VIN(MIN) •(VOUT –VIN(MIN))

f• IL •VOUT

LBUCK >

VOUT •(VIN(MAX) –VOUT)

f• IL •VIN(MAX)

where f = operating frequency, Hz

ΔIL = maximum allowable inductor ripple current, A

VIN(MIN) = minimum input voltage, V

VIN(MAX) = maximum input voltage, V

VOUT = output voltage, V

IOUT(MAX) = maximum output load current

For high efﬁciency, choose a ferrite inductor with a high

frequency core material to reduce core loses. The induc-

tor should have low ESR (equivalent series resistance) to

reduce the I2R losses, and must be able to handle the peak

inductor current without saturating. Molded chokes or chip

inductors usually do not have enough core to support the

peak inductor currents in the 1A to 2A region. To minimize

radiated noise, use a shielded inductor. See Table 1 for a

suggested list of inductor suppliers.

Output Capacitor Selection

The bulk value of the output ﬁlter capacitor is set to reduce

the ripple due to charge into the capacitor each cycle. The

steady state ripple due to charge is given by:

% Ripple_Boost =

IOUT(MAX) •(VOUT –VIN(MIN))•100

COUT •VOUT

2 • f

% Ripple_Buck =

8LCf2

(VIN(MAX) –VOUT)•100

VIN(MAX)

where COUT = output ﬁlter capacitor in Farads and

f = switching frequency in Hz.

VIN

VCI

SHDN/SS

3530 F05

Figure 3.

Table 1. Inductor Vendor Information

SUPPLIER

PHONE

FAX

WEB SITE

Coilcraft

(847) 639-6400

(847) 639-1469

www.coilcraft.com

CoEv Magnetics

(800) 227-7040

(650) 361-2508

www.circuitprotection.com/magnetics.asp

Murata

(814) 237-1431

(800) 831-9172

(814) 238-0409

www.murata.com

Sumida

USA: (847) 956-0666

Japan: 81(3) 3607-5111

USA: (847) 956-0702

Japan: 81(3) 3607-5144

www.sumida.com

TDK

(847) 803-6100

(847) 803-6296

www.component.tdk.com

TOKO

(847) 297-0070

(847) 699-7864

www.tokoam.com

OPERATION

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