LTC3453

8

3453fa

APPLICATIO S I FOR ATIO

OPERATIO

amplifier programs the output LED current to

If both enable inputs are logic high, the setting currents are

summed internally and the output LED current will be

given by

Thus three different (nonzero) current levels are program-

mable, optimal for low current LED torch and high current

LED camera flash applications.

LED Current Sources

Each LED pin is driven by a current source specifically

designed for low dropout. The LTC3453 employs a propri-

etary architecture that determines which of the four LEDs

requires the largest forward voltage drop at its pro-

grammed current, and then generates a feedback voltage

based on this one for closing the buck-boost regulation

loop. This results in the lowest output voltage required for

regulating all of the LEDs and thus the highest LED power

efficiency. The voltage present at the LED pin of the

“controlling LED” will be typically 130mV at 75mA of

current.

LED Detect Circuit

If fewer than four LED outputs are required, unused ones

LED detect circuit that disables the output current source

to save power if an output is not needed. A small 30

µA

current is employed to detect the presence of an LED at

startup.

Component Selection

Inductor Selection

The high frequency operation of the LTC3453 allows the

use of small surface mount inductors. The inductor cur-

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

inductor current. For a given ripple the inductance terms

are given as follows:

L

VV

V

f I

Ripple V

L

VV

V

f I

Ripple V

IN MIN

OUT

IN MIN

OUT MAX

OUT

OUT

IN MAX

OUT

OUT MAX

IN MAX

>

()

>

()

()

()

()

()

()

()

•–

•

%

•• %

•

,

•–

•

%

•• %

•

2

2

100

100

where f = operating frequency, Hz

%Ripple = allowable inductor current ripple, %

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

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 radi-

ated noise, use a toroid, pot core or shielded bobbin

inductor. For the white LED application, a 4.7

µH 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

Vishay-Dale

www.vishay.com