LINEAR High Power LED Camera Flash Solutions for Tiny, Efficient Cell Phone Applications – Design Note 1009 Keith Szolusha Quick User Guide and Owner's Manual

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Tiny, Efficient High Power LED Camera Flash Solutions for Cell Phone Applications – Design Note 1009 Keith Szolusha

Introduction LEDs an attractive solution for the now popular 1 megapixel and higher cell phone cameras.

Cell phones and PDAs are approaching the limit for small form factors, but their feature sets are growing with new

Linear Technology offers many simple and extremely

functions such as built-in cameras and even video cameras.

compact solutions for driving the latest high-light-out-

The initial in-phone cameras were basic, low resolution de-

put LEDs, including Lumileds Luxeon Flash LEDs. These

vices with little in the way of usable flash capability. How-

LEDs are up to 12 times brighter than conventional LEDs

ever, cell phone camera resolutions have improved and

(http:/www.lumileds.com/solutions/solution.cfm?id=9).

sophisticated flash technology is expected to keep pace.

Constant Current LED Drivers

There are two flash technologies used in digital still cam-

Switcher-based DC/DC converters are small and efficient,

eras. The first uses a Xenon flash lamp to provide a very

so they are prime candidates for LED driver solutions. The

short, high lumen flash with excellent spectral character-

problem with a traditional switcher is that it is designed to

istics. LTC offers Xenon photoflash solutions that are

produce a regulated voltage for varying load currents. LEDs

simple, compact and efficient (see Design Note 345).

must be driven with constant current to generate predict-

The second flash technology uses LEDs. LED solutions hold able light output. Using a typical switcher for constant the upper hand when simplicity and space is at a premium current involves adding an op amp and additional circuitry, and they are the only choice when ‘torch’ or video-mode which are undesirable in space-constrained applications. lighting is required. LED flash units fit into spaces where

Four new devices solve this problem. The LTC®3216 charge

even the smallest Xenon photoflash cannot. Until recently,

pump, the LTC3453 buck-boost, and the LT®1618 and

LEDs could not compete with Xenon solutions in light

LT3479 boost regulators all incorporate the required cir-

output and spectral performance, relegating them to the

cuitry to generate a constant current output while main-

realm of low resolution (sub 1 megapixel) cameras. How-

taining high efficiency operation.

ever, recent (>10×) performance improvements have made

, LTC and LT are registered trademarks of Linear Technology Corporation.

4.7µH DO3314-472

3.3mm × 3.3mm × 1.4mm 2.2µF 2.2µF

2.7V to 4.2V

C1+ C1– C2+ C2– D1: LUMILEDS LXCL-PWF1

SW1 SW2 VOUTPVINVIN

VIN2.9V TO 4.4V CPO

ILED D1

VC LED1-4 2.2µF

0.1µF LTC3216

EN1 (TORCH) EN1

D1: LUMILEDS LXCL-PWF1 ILED

EN2 (FLASH) EN2 LTC3453 EN1 ILED

EN1 (TORCH)

EN1 EN2 ILED

ISET1 EN2 (FLASH) EN2

8.25k 0 (SHUTDOWN)

ISET1 ISET2 GND EN1 EN2 ILED

1% ISET2 150m A (TORCH)

3.48k 350m A 0 (SHUTDOWN)

GND 500m A (FLASH) 200m A (TORCH)

DN1009 F01

800m A (FLASH)

DN1009 F02

Figure 1. High Performance LED Flash Driver Yields Greater Figure 2. This LXCL-PWF1 Luxeon Flash Driver is than 90% Efficiency. The LTC3453 Buck-Boost Delivers Extremely Small. It Requires Only an LTC3216 Low Noise 500m A to the LXCL-PWF1 LED Flash with Only Six Tiny High Efficiency Charge Pump in a Small DFN Package, External Components Along with Four 0603 Capacitors and Two 0402 Resistors