Linear Technology 1315 Manuel utilisateur

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1315

DUAL STEP-DOWN 1.5A LED DRIVER

LT3475-1

DESCRIPTION

Demonstration circuit 1315 features the

LT®3475-1 dual step-down 1.5A LED driver in a

16-lead TSSOP thermally enhanced package.

The demonstration circuit is designed to drive

two 1.5A LED strings (one on each channel)

from a high input voltage. The high switching

frequency scheme permits the use of a small

inductor and ceramic capacitors to save space

and cost. The current mode PWM control

scheme provides fast transient response and

cycle-by-cycle current limiting. DC1315 is

specifically designed for high power LED

applications such as automotive and avionic

lighting, architectural detail lighting and display

backlighting. See the LT3475-1 datasheet for

more detailed information on LED brightness

and dimming control using both the PWM pin

function and the V

ADJ

pin function. The PWM

terminals can be used to dim the LEDs with the

addition of a PWM MOSFET and the resistor-

short connection from LED- to GND removed.

The efficiency of DC1315A with 20V input and 4

white LEDs on each 1.5A channel is 91.5%.

NOTE:

The input voltage is limited to 30V during

startup. During normal (non-startup) operation,

the input voltage can be as high as 36V.

DC1315 allows LED string voltages up to 23V.

However, some combinations of VIN, VLED,

and ILED can result in the LT3475-1 getting hot.

Please consult the datasheet for thermal calcu-

lations and details. The voltages, the LED cur-

rent, and the switching frequency can be ad-

justed for optimal thermal performance.

Design files for this circuit board are avail-

able. Call the LTC factory.

LT is a registered trademark of Linear Technology Corporation

QUICK START PROCEDURE

DC1315 is easily set up for evaluating the

LT3475-1 dual step-down 1.5A LED driver

circuit. Follow the test procedure outlined below

and see Figure 1.

1. Connect a bench supply with power OFF

to the V

and GND terminals.

2. Connect a 1.5A LED string with forward

voltage >13V to the LED1+ and LED1-

terminals.

3. Connect a 1.5A LED string with forward

voltage >13V to the LED2+ and LED2-

terminals.

NOTE:

The boost circuitry on DC1315 is de-

signed to work best with VLED >13V since

there is a 9V zener diode limiting the boost

voltage. If lower voltage LED strings are re-

quired, please lower the voltage of the zener

diode or use another boost method. Please

see the datasheet for details.

4. Turn on the input power supply and set

the voltage above the LED string voltage

and below 30V.

5. Observe the LEDs working properly at

1.5A without looking straight at the LEDs.

The brightness is harmful to the eye.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1315

DUAL STEP-DOWN 1.5A LED DRIVER

Figure 1. Proper Measurement Equipment

Setup

INPUT VOLTAGE RANGE

The operating input voltage range for the LT3475-

1 IC is 4V to 36V with an absolute maximum rat-

ing of 40V. Under some conditions, it is not rec-

ommended to run the IC up to 36V non-transient

DC voltage. Please see the datasheet for input

voltage range calculations based on switching fre-

quency, LED output voltage, and compensation

network topology during startup. As assembled,

the DC1315 should not be started up above 30V

DC input without first assembling the C-R-C com-

pensation network as shown in the datasheet or

using a bigger inductor with up to 3.2A capability

in order to maintain control of the switch current.

PWM DIMMING

DC1315 is assembled to show a combination of

the most basic operation of the IC and the small

solution size. The optional PWM dimming MOS-

FETs (Q1, Q2) are not stuffed on DC1315, but

there are two SOT23 spaces designated for these

to be added to the board. The LED- to GND resis-

tor-short connections (R2, R3) must be removed

from the board to enable PWM dimming when the

PWM dimming MOSFETs are stuffed with their

gate pull-down resistors (R6, R7).

EXTENDED PWM DIMMING

The PWM dimming range can be extended by

adding the components Q3, R13, and C7 to the

circuit as shown in the datasheet. Q3 is located on

the back of the PCB near the PWM terminal. The

datasheet describes the calculation of maximum

PWM dimming range with and without the ex-

tended dimming range components. When these

components are added to the circuit, the PWM1

signal is directly connected to the extended dim-

ming range components. If both channel 1 and

channel 2 are being used in PWM dimming mode

and the extended dimming range circuit is used,

the PWM2 signal must be synchronized with the

PWM1 signal. The rising edges must occur at the

same time although their duty cycles may vary.

Failure to do this may result in significant jitter on

the second channel or limited PWM dimming

range. Please note that a C-R-C compensation

network may also help provide the highest pwm

dimming range as shown in the datasheet.

LED STRINGS

DC1315 is assembled with optimized components

for driving four white LEDs at 1.5A on each of its

two channels from an input voltage of 20V. The

LEDs are not assembled on the PCB and should

be attached to the LED terminals as shown in Fig-

ure 1. If the LEDs are not attached, DC1315 will

protect itself and survive an open LED condition.

If a different number of LEDs is attached in series

to either output, the inductor size, switching fre-

quency, and/or input voltage range may need to

be adjusted for proper operation. Consult the

datasheet for proper sizing of the inductor for

given input and output voltage conditions. The

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1315

DUAL STEP-DOWN 1.5A LED DRIVER

maximum LED output voltage is 23V as defined by

the external clamp voltage. This limits the number

of LEDs in series on the output and sets the open

LED maximum output voltage.

Please note the zener diode in the boost path. The

9V zener diode reduces the boost voltage by 9V

from the LED (output) voltage. A minimum of 3V is

required on the boost voltage capacitor for proper

operation. Please consult the datasheet for details

regarding the boost circuit in relation to the input

voltage, output voltage, thermals, and maximum

ratings.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1315

DUAL STEP-DOWN 1.5A LED DRIVER

23V

LT3475EFE-1

1 6

2 1

1 8

1 3

1 5

1 4

V I N

SH DN

GN D

PW M 1

BOOST1

OU T1

SW 1

LED1

REF

VADJ1

V C 1

PW M 2

BOOST2

OU T2

SW 2

LED2

VADJ2

V C 2

GN D

VIN

L1 47uH

E11

LED2-

OPT

2N7002

R13

1.0M

OPT

PWM2

SH DN

1N4448W

220pF

O P T

1k 1%

UP TO 36V

1N4448W

GND

0.1uF

0.22uF

25V

22V

OPT

Si2318DS

1.5A

9.1V

COUT1

2.2uF

50V

VC2

MMBT3904

R18

100k

OPT

23V

REF

600kHz

OPT

R15

100k

R16

10k

VC1

LED1-

GND

EXTENDED DIMMING RANGE

OPT

Si2318DS

R14

OPT

FMMT3904

OPTIONAL

9.1V

B240A

COUT2

2.2uF

50V

VC2

R20

1k 1%

GND

VC1

B24 0A

OPT

CIN1

4.7uF

50V

R17

10k

22V

E10

LED2+

UP TO

1.5A

OPT

L2 47uH

UP TO

R12

24.3k 1%

PWM1

OPT

0.1uF

REF

LED1+

20k

0.22uF

25V

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