Linear Technology LT3755 User manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT

1268A-A, 1268A-B

HIGH VOLTAGE LED CONTROLLER

LT3755, LT3755-1

DESCRIPTION

Demonstration circuit 1268A-A, 1268A-B is a high

voltage and high current LED driver controller. The

VIN pin input voltage is as high as 40V. The LED out-

put voltage is as high as 60V. The demonstration cir-

cuit is assembled as a boost topology from 8-40V

input and up to 55V LED output at 1A with an open

LED status pin (-A) or a sync input pin (-B).

DC1268A features PWM dimming, analog dimming,

shutdown, and open LED overvoltage protection. Effi-

ciency at 12VIN is as high as 93% for 50W output.

The switching frequency is set at 400kHz to accom-

modate the automotive switching frequency range.

Small ceramic input and output capacitors are used

to save space and cost and a high voltage 60V exter-

nal power switch and a 60V catch diode are used for

up to 55W (55V at 1A) output as assembled. The

open LED overvoltage protection will limit the output

to approximately 56V if the LED string is opened. For

low input voltage operation, the CTRL pin voltage is

reduced as the input voltage drops below 9V, reduc-

ing LED brightness and restraining the peak switch

currents in order to limit inductor and switch size.

UVLO is employed to turn the LEDs off when VIN

drops below 8V.

DC1268A PWM dimming is simple. The boost topol-

ogy LED string returns to the LED- terminal and the

PWM dimming MOSFET turns the LED string on and

off given an input to the PWM dimming terminal. For

the highest pwm dimming ratio, it is recommended to

use 100Hz as a pwm dimming frequency. Information

regarding PWM dimming ratios and performance can

be found in the LT3755/-1 datasheet in the applica-

tions section.

Soft-start is provided with a simple and small capaci-

tor to maintain control of the switch current during

turn on for high power designs.

Modifications can be made to the DC1268A in order

to convert the board from a boost topology LED

driver to any other topology. Buck-boost mode, buck

mode, and SEPIC schematics are provided in Figures

6-8. Please consult the factory or LT3755/-1 data-

sheet for details regarding how to customize the

DC1268A or how to design different topologies for

custom specifications.

The LT3755, LT3755-1 datasheet gives a complete

description of the part, operation and applications

information. The datasheet must be read in conjunc-

tion with this Quick Start Guide for demonstration

circuit 1268A-A, 1268A-B. The LT3755, LT3755-1 is

assembled in a small 16-lead plastic QFN package

with a thermally enhanced ground pad. 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.

LT is a trademark of Linear Technology Corporation

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT

1268A-A, 1268A-B

HIGH VOLTAGE LED CONTROLLER

Table 1.

Typical Performance Summary for DC1268A-A/B

PARAMETER CONDITION VALUE (TYPICAL)

Input Voltage PVIN Range Operating 8V-40V

Switching Frequency R8 = 28.7k 400kHz

LED

Maximum Output Voltage - Open LED Voltage 56V

Efficiency PVIN=12V V

LED

=50V I

LED

=1A 93%

Under Voltage Lockout 8V

Peak Switch Current Limit RS1 = 0.015

Ω

6.6A

QUICK START PROCEDURE

Demonstration circuit 1268A-A, 1268A-B is easy to

set up to evaluate the performance of the LT3755,

LT3755-1. Follow the procedure below:

NOTE:

Make sure that the PVIN DC input voltage

does not exceed 40V.

Connect a string of LEDs with forward voltage

less than 55V, but greater than the PVIN voltage,

to the LED+ and LED- (Boost) terminals on the

PCB as shown in Figure 1.

Connect the shutdown terminal to GND.

With power off, connect the PVIN power supply

to the PVIN and GND terminals within the ranges

specified on the PCB.

Connect the PWM terminal. If PWM is not used,

connect PWM to a 5V source or to IntVcc on the

PCB. PWM must be pulled high to work. For –B,

the SYNC input can be left floating.

Turn the PVIN power supply on.

Release the shutdown/GND connection.

Observe the LED string running at the pro-

grammed LED current.

For PWM dimming, connect a PWM 100Hz or

higher signal to the PWM terminal.

Observe the reduction of brightness in the LED

string when PWM dimming.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT

1268A-A, 1268A-B

HIGH VOLTAGE LED CONTROLLER

Figure 1. Test procedure setup drawing for DC1268A-A and DC1268A-B

TWO ICS, THREE TOPOLOGIES

DC1268A features two build versions. DC1268A-A

uses the LT3755EUD with an open LED status and

DC1268A-B uses the LT3755EUD-1 with sync. Each

board is clearly marked indicating which IC is used.

The schematic for each is attached to this docu-

ment. The open LED indicator terminal has a pull-

up resistor to the internal 7V regulator IntVcc. The –

B version with the sync terminal has an RC filter

between the PWM input pin and the SYNC input

pin. This filter provides a small delay between the

PWM signal and the SYNC input when PWM is

used in order to optimize high PWM dimming ratio

waveforms. If high ratios are not needed, this RC

filter may not be needed. If the sync input is used,

this filter may be removed. Please note that when

PWM and SYNC are both used, optimal PWM dim-

ming occurs when there is at least a 200ns delay

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT

1268A-A, 1268A-B

HIGH VOLTAGE LED CONTROLLER

from the PWM rising edge and the SYNC rising

edge – with the PWM edge occurring first. For more

questions regarding this matter, please consult the

factory.

Although DC1268A-A/B is assembled as a boost

topology LED driver, it can be reconfigured for

buck-boost mode, buck mode and SEPIC LED

driver topologies. A few schematics featuring these

other topologies are attached. Please keep in mind

that the chosen topology should reflect the relation-

ship between the entire input voltage range and the

LED voltage range. Other circuit requirements such

as short circuit between terminals or ground can be

affected by the choice of LED driver topology.

TERMINAL OPTIONS

DC1268A-A/B has both PVIN and VIN terminals. The

schematic shows that the PVIN and VIN terminals are

shorted together with a resistor. As assembled, only

the PVIN terminal needs 8V to 40V power in order for

the board to operate. However, resistor R2 can be re-

moved and the VIN terminal can be connected to a

separate supply from the PVIN terminal. The VIN ter-

minal is tied directly to the VIN pin.

The VOUT terminal is available for the customer to

monitor the state of the output capacitor directly. The

VOUT terminal can also be used as an input for buck

mode topology.

The PWM terminal must be pulled high in order for the

board to operate. It is preferred that the PWM termi-

nal/pin is tied to IntVcc of the controller if PWM dim-

ming is not used, but for board evaluation it can be

tied to an external 5V source as well.

The CTRL terminal is used for analog dimming. The

CTRL pin is tied directly to the terminal and it is al-

ready being used to reduce the LED current when VIN

drops too low. The CTRL pin voltage can easily be

overridden with an external voltage supply and the

resistor divider from VIN can be removed.

The shutdown/undervoltage terminal is used to shut

down the demonstration circuit. Tie the terminal to

GND and the converter will stop switching. The

PWMOUT pin will go low and the PWM dimming

MOSFET will be turned off. As long as the PWM dim-

ming MOSFET is used properly, the LEDs will not turn

on during shutdown.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT

1268A-A, 1268A-B

HIGH VOLTAGE LED CONTROLLER

R18

OPT

R17

22k

22.1k

Si7850DP

LED+

4.7uF

LED-

BOOST

LED-

BUCK / BUCK-BOOST MODES

RS2

0.10

140k

PVIN BUCK M ODE

INTVCC

Si2308DS

1uF

50V

OPENLED

R14

OPT

22uH

PWMD

PDS560

R13

OPT

R10

100k

OPT R6

RS1

0.015

90.9k

0.1uF

CTRL

0 ohm

28.7k

R15

1M U1

LT3755EUD

154

SD/UV

PWM

INTVCC

VCSS

VREF

CTRL

OPENLED

VIN

ISP

ISN

PWMOUT

GATE

SENSE

GND

499k

0 Ohm

Cout

3x 2.2uF

100V

400kHz

2200pF

C1, C10

2x 4.7uF

50V

C9 OPT

OPT

10V

PVIN 8V to 40V

PWMD

HC9-220-R

PWM

Figure 2. DC1268A-A featuring LT3755 and Open LED flag.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT

1268A-A, 1268A-B

HIGH VOLTAGE LED CONTROLLER

80%

82%

84%

86%

88%

90%

92%

94%

96%

98%

100%

5 10 15 20 25 30 35 40

INPUT VOLTAGE (V)

EFFICIENCY (%)

Figure 3. DC1268A-A/B Efficiency with 50V LEDs at 1A

0.0

0.2

0.4

0.6

0.8

1.0

1.2

5 10 15 20 25 30 35 40

INPUT VOLTAGE (V)

LED CURRENT (A)

Figure 4. DC1268A-A/B LED Current Drops at Low VIN (CTRL) and Turns Off at Lower VIN (UVLO)

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT

1268A-A, 1268A-B

HIGH VOLTAGE LED CONTROLLER

C1, C10

2x 4.7uF

50V

R18

10k

PW M

Si7850DP

22.1k

22uH

4.7uF

OPT

LT3755EUD-1

154

SD/UV

PWM

INTVCC

VCSS

VREF

CTRL

SYNC

VIN

ISP

ISN

PWMOUT

GATE

SENSE

GND

LED-

BOOST

LED-

BUCK / BUCK-BOOST M ODES

1uF

50V

HC9-220-R

R13

OPT

PDS560

0.1uF

RS1

0.015

0 ohm

PWMD

Si2308DS

2200pF

LED+

CTRL

Cout

3x 2.2uF

100V

PWMD

R10

OPT

R17

22k

499k

0 Ohm

INTVCC

RS2

0.10

SYNC

400kHz

28.7k

C9 OPT

90.9k

PVIN BUCK M ODE

PVIN 8V to 40V

R15

100pF

R14

OPT

140k

10V

Figure 5. DC1268A-B featuring LT3755-1 and SYNC input.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT

1268A-A, 1268A-B

HIGH VOLTAGE LED CONTROLLER

Si2308DS

0 ohm

R13 510

93.1k

22.1k

28.7k

RS2

0.125

Lfilt1

22uH C2, C3

2x 2.2uF

100V

CDRH127

499k C4

1uF

50V

OPENLED

R14

510

R10

100k

Cfilt1

22uF

PW M

10V

4700pF

22uH

14V

LT3755EUD

154

SD/UV

PWM

INTVCC

VCSS

VREF

CTRL

OPENLED

VIN

ISP

ISN

PWMOUT

GATE

SENSE

GND

RS1

0.025

Si2319DS

R15

549k

0.01uF

R17

4.7k

PDS560

800mA

0 Ohm

C1, C10

2x 2.2uF

50V

PVIN 8V to 36V

4.7uF

Si7850DP

400kHz

75.0k

LED+

LED-

BUCK-BOOST M ODE

Figure 6. DC1268A-A Customized Buck-Boost Mode LED Driver Schematic

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT

1268A-A, 1268A-B

HIGH VOLTAGE LED CONTROLLER

Qovp

FMMT558

4.7uF

10V to 36V

Rx5

510

Si7850DP

RS1

0.033

OPENLED

PVIN BUCK MODE

8.5V

D1 30BQ060

PVIN

R13

1.5A

Rx1

Rx4

365

R17

15k

LT3755EUD

154

SD/UV

PWM

INTVCC

VCSS

VREF

CTRL

OPENLED

VIN

ISP

ISN

PWMOUT

GATE

SENSE

GND

0.01uF

22.1k

LED+

2200pF

28.7k

R10

100k

400kHz

267k

22uF

16V

C2, C3

2x 2.2uF

50V

Rx2

100k

1uF

50V RS2

0.068

Rx3

249k

10V

10uH

PWM

Si1471DH

Qx1

2222

CDRH8D43

Figure 7. DC1268A-A Customized Buck Mode LED Driver Schematic

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT

1268A-A, 1268A-B

HIGH VOLTAGE LED CONTROLLER

1uF

50V

Cout

6x 47uF

Si7454DP

CDRH127/LD

PVIN 9V to 36V

Ccoup

270uF 50V

C1, C10

2x 47uF

30.9k

RS1

0.0165

50V

4.7uF

CDRH8D43

0.01uF

RS2

0.10

50V

35V

LED-

L1 22uH

CTRL

LT3755EUD-1

154

SD/UV

PWM

INTVCC

VCSS

VREF

CTRL

SYNC

VIN

ISP

ISN

PWMOUT

GATE

SENSE

GND

BOOST

R18

10k

PDS3100

22nF

499k

732k

350kHz

Si2318DS

10V

0 ohm

0 Ohm

24.9k

PWM

100pF

33uH

R17

51k

84.5k

LED+

Figure 8. DC1268A-B Customized SEPIC Schematic with PWM Dimming and Aluminum Electrolytic Capacitors

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