LTC3454 Просмотр технического описания (PDF) - Linear Technology

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LTC3454

1A Synchronous Buck-Boost High Current LED Driver

Linear Technology 

LTC3454

APPLICATIONS INFORMATION

COMPONENT SELECTION

Inductor Selection

The high frequency operation of the LTC3454 allows the

use of small surface mount inductors. The inductor cur-

rent ripple is typically set to 20% to 40% of the maximum

average inductor current. For a given ripple the inductance

term in boost mode is:

( ) L

>

VIN(MIN)2 • VOUT – VIN(MIN) • 100%

f •IOUT(MAX) • %Ripple • VOUT2

and in buck mode is:

( ) L > VIN(MAX) – VOUT • VOUT • 100%

f • VIN(MAX) • %Ripple •IOUT

where f = operating frequency, Hz

%Ripple = allowable inductor current ripple, %

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 an inductor with a high fre-

quency core material, such as ferrite, to reduce core loses.

The inductor 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 peak inductor currents >1A. To

minimize radiated noise, use a toroid, pot core or shielded

bobbin inductor. For white LED application, a 4.7μH/5μH

inductor value is recommended. See Table 1 for a list of

component suppliers.

Table 1. Inductor Vendor Information

SUPPLIER

WEB SITE

Coilcraft

www.coilcraft.com

Cooper/Coiltronics www.cooperet.com

Murata

www.murata.com

Sumida

www.japanlink.com/sumida

Toko

www.toko.com

Vishay-Dale

www.vishay.com

Input Capacitor Selection

Since the VIN pin is the supply voltage for the IC it is recom-

mended to place at least a 2.2μF, low ESR bypass capacitor

to ground. See Table 2 for a list of component suppliers.

Table 2. Capacitor Vendor Information

SUPPLIER

WEB SITE

AVX

www.avxcorp.com

Sanyo

www.sanyovideo.com

Taiyo Yuden

www.t-yuden.com

TDK

www.component.tdk.com

Output Capacitor Selection

The bulk value of the 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)

COUT • VOUT2 • f

• 100%

( ) %Ripple _Buck

=

VIN(MAX) – VOUT • 100%

8 • VIN(MAX) • f2 •L • COUT

where COUT = output filter capacitor, F

The output capacitance is usually many times larger in

order to handle the transient response of the converter. For

a rule of thumb, the ratio of operating frequency to unity-

gain bandwidth of the converter is the amount the output

capacitance will have to increase from the above calcula-

tions in order to maintain desired transient response.

The other component of ripple is due to ESR (equivalent

series resistance) of the output capacitor. Low ESR ca-

pacitors should be used to minimize output voltage ripple.

For surface mount applications, Taiyo Yuden, TDK, AVX

ceramic capacitors, AVX TPS series tantalum capacitors

or Sanyo POSCAP are recommended. For the white LED

application, a 10μF capacitor value is recommended. See

Table 2 for a list of component suppliers.

Optional Schottky Diodes

Schottky diodes across the synchronous switches B and

D are not required, but provide a lower drop during the

break-before-make time (typically 20ns) of the NMOS to

PMOS transition, improving efficiency. Use a Schottky

3454fa

9

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