Linear Technology LT3755 Quick Start Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
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
HIGH VOLTAGE LED CONTROLLER
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
LT3755, LT3755-1
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