Linear Technology DC875 Manuel utilisateur

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 875

HIGH EFFICIENCY BUCK-BOOST FLASH LED DRIVER

LTC3454EDD

DESCRIPTION

Demonstration circuit 875 is a high efficiency buck-

boost flash LED driver featuring the LTC3454EDD.

The

board has two circuits optimized for different applica-

tions: a small footprint solution featuring 350mA,

300ms LED flash current or 150mA constant LED

torch current, and a high current solution featuring

400 / 600 / 1000mA constant-current operation. The

input voltage range of 2.7V to 5.5V is suitable for a

single Lithium-Ion battery or three AAA batteries in

series. The Li-Ion input range, high LED flash current

with separate torch mode current setting, internal

synchronous power switches, minimal and low-

profile external components, and simple design

makes the LTC3454 the top solution for highly effi-

cient, space-constrained cellular telephone camera

flash solutions.

The high current solution is activated by depressing

momentary switches ‘EN1’ and ‘EN2’ in accordance

with the table provided on the silkscreen, or connect-

ing the corresponding terminals to ‘VIN’ with a

jumper wire. The small footprint solution is turned on

and off by pressing the ‘ON/OFF’ button. A ‘TORCH’

button turns the LED on at 150mA for cellular tele-

phone video recording. The ‘FLASH’ button flashes

the LED at 350mA for 300ms for cellular telephone

camera flash operation. The buttons control the state

of the PIC microcontroller IC that is programmed with

DC875 software. The PIC keeps the LED from being

flashed constantly, preventing overheating and burn-

out. The PIC also controls the flash time. When no

buttons are pressed for a short time, the PIC turns off

and the circuit enters low-power mode for battery-life

preservation. A red/green LED indicates the state of

the PIC. The LED is ready to flash or torch if the state

indicator is blipping green. If the indicator is not on,

the PIC is turned off. For a brief time after the flash

has occurred, the state-indicator LED is red, indicat-

ing that another flash cannot happen, even if the but-

ton is pressed, until the red LED is off and the green

LED is blipping again. Alternatively, the torch and

flash modes can be enabled by connecting ‘EN1’

(torch), or’ EN1’ and ‘EN2’ (flash) to ‘VIN’, as indi-

cated in the table.

WARNING- Do not pull down the ‘ENx’ pins on the

small current solution while the PIC is active, as dam-

age to the PIC outputs may result.

WARNING- The small footprint solution LED can be

permanently damaged if the flash mode is engaged for

more than a few seconds.

The PIC control circuit may be used to control the

high current solution circuit by connecting the ‘EN1’

and ‘EN2’ pins of each solution in parallel, and apply-

ing power to both circuits. Typical efficiency for the

demo circuits is presented in Figure 2. Higher effi-

ciency can be obtained by selecting larger inductors

with lower ESR.

The LTC3454 datasheet gives a complete description

of the part, operation and applications information.

The datasheet must be read in conjunction with this

Quick Start Guide for demonstration circuit 875. In

addition, the Lumileds Luxeon III Star datasheet

(www.lumileds.com) and the AOT2015 datasheet

(www.aot.com) must be read to understand thermal

and LED current constraints for varying flash current

pulse-widths and intensity. The LTC3454 is assem-

bled in a small low profile DFN package. Proper board

layout is essential for maximum thermal perform-

ance. See the datasheet section ‘Layout Considera-

tions’.

Design files for this circuit board are available. Call

the LTC factory.

LTC is a trademark of Linear Technology Corporation

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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 875

HIGH EFFICIENCY BUCK-BOOST FLASH LED DRIVER

Table 1.

Performance Summary (T

= 25°C)

PARAMETER CONDITION VALUE

Minimum Input Voltage 2.7V

Maximum Input Voltage 5.5V

Maximum (Guaranteed) Output Voltage I

= 1A

4.95V

Output Current I

OUT

– small footprint solution V

= 2.7V to 5.5V 150 / 200 / 350mA

Output Current I

OUT

– high current solution V

= 2.7V to 5.5V 400 / 600 / 1000mA

Maximum Output Voltage 4.9V

Typical Output Ripple V

OUT

= 4.2V, I

OUT

= 1.0A (20MHz BW) 10mV

P–P

Typical Output Regulation Line ±0.5%

Nominal Switching Frequency 1.0MHz

= 4.2V, I

OUT

= 150mA 92% TypicalEfficiency

= 4.2V, I

OUT

= 1000mA 86% Typical

QUICK START PROCEDURE

Demonstration circuit 875 is easy to set up to evaluate

the performance of the LTC3454EDD. Refer to Figure 1

for proper measurement equipment setup and follow

the procedure below:

Preset a DC source capable of 5.5V at 2.5A to 4V.

With power off, connect the input power supply to

‘VIN’ and ‘GND’ of the circuit solution of interest.

Turn on the power at the input. The input voltage

may be varied between 2.7V and 5.5VDC.

NOTE:

Make sure that the input voltage does not ex-

ceed 5.5V.

SMALL FOOTPRINT SOLUTION:

If the state-indicator LED is not blipping green,

press the ‘ON/OFF’ button to turn on the circuit.

Press ‘FLASH’ to observe a 300ms, 350mA camera

flash.

Press ‘TORCH’ to observe a constant 150mA LED

light for recording video.

Press ‘TORCH’ to turn off the 150mA video mode.

Press ‘ON/OFF’ to turn off the circuit or let the cir-

cuit turn off by not pressing any buttons for ap-

proximately 10 seconds.

Alternatively, the torch and flash modes can be

enabled by connecting ‘EN1’ (torch), or ‘EN1’ and

‘EN2’ (flash) to ‘VIN’, as indicated in the table.

WARNING- The small footprint solution LED can be

permanently damaged if the flash mode is engaged

for more than a few seconds.

HIGH CURRENT SOLUTION:

10.

Press ‘EN1’ to observe a 400mA LED illumination.

11.

Press ‘EN2’ to observe a 600mA LED illumination.

12.

Press ‘EN1’ and ‘EN2’ to observe a 1000mA LED

illumination.

13.

As an alternative to pressing ‘EN1’ and/or ‘EN2’, the

corresponding terminals may be connected to ‘VIN’.

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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 875

HIGH EFFICIENCY BUCK-BOOST FLASH LED DRIVER

Figure 1.

Proper Measurement Equipment Setup

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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 875

HIGH EFFICIENCY BUCK-BOOST FLASH LED DRIVER

Figure 2.

Efficiency

DC875A Ltc3454 Efficie ncy

2 2.5 3 3.5 4 4.5 5 5.5 6

Vin Voltage

Efficiency in %

Iout= 400m A

Iout= 600m A

Iout=1000mA

DC875A Ltc3454 Efficie ncy

2 2.5 3 3.5 4 4.5 5 5.5 6

Vin Voltage

Efficiency %

Iout= 150m A

Iout= 200m A

Iout= 350m A

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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 875

HIGH EFFICIENCY BUCK-BOOST FLASH LED DRIVER

Figure 3.

Schematic: Small Footprint Solution

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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 875

HIGH EFFICIENCY BUCK-BOOST FLASH LED DRIVER

Figure 4.

Schematic: High Current Solutio

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