Linear Technology DC1784A Demo Manual

Description
DC1784A is a triple boost LED driver featuring the
triple boost LED controller. It accepts an input voltage
from 2.5V to 40V (with higher transient) and drives three
independent strings of up to 50V LEDs at 1A. DC1784A
features independent PWM and analog dimming of each
of the three LED strings. Each of the three channels has
its own short-circuit protection, open LED protection, and
FAULT flag output.
Each or every channel can be altered to run as a different
topology such as buck-boost mode, buck mode, or SEPIC
depending upon the relationship between input and output
voltage. Although the boost is set up to power 50V of LEDs
at 1A, the maximum LED string voltage can be changed
to almost 100V and the LED current can be adjusted by
merely changing a few resistors and external components.
DC1784A features high efficiency at 310kHz switching
frequency. At high LED string voltages up to 50V and 1A
of LED current, the triple LED boost controller has 93.5%
efficiency. The switching frequency can be adjusted be-
tween 100kHz and 1MHz with a single resistor. All three
channels run in-phase with each other and an external
SYNC pin can be used to set the switching frequency and
phasing, as well as to optimize PWM dimming.
For low input voltage operation, down to 2.5V, the CTRL
pin voltage is reduced as the input voltage drops below
10.5V, reducing LED brightness and restraining the peak
switch currents in order to limit thermal rise on this PCB.
The LT3797 has a unique, internal buck-boost INTV
supply that powers the gate drivers at 7.8V, regardless
of the state of the input voltage, rather than using an LDO
from the input. UVLO turns the LEDs off when V
below 2.5V. Internal OVLO turns off the switching when
DEMO MANUAL DC1784A
Triple LED Boost Controller
LT3797 the input exceeds 41V, but OVLO can be programmed
externally to a lower voltage if desired. The LT3797 can
withstand transients up to 60V. DC1784A can withstand
transients up to 50V with C3 > 50V rating.
Small ceramic input and output capacitors are used to save
space and cost. The open LED overvoltage protection uses
the IC's constant voltage regulation loop to regulate the
output to approximately 55V if the LED string is opened
although it may reach 59V peak during transient from
running LEDs to open. The unique FBH (feedback high)
pins allow the overvoltage protection of non-grounded
LED driver topologies such as buck-boost mode and buck
mode to be accomplished with just two resistors.
Modifications can be made to DC1784A in order to convert
the independent channels to higher or lower power or from
LED drivers to constant voltage regulators. They can easily
be changed from boost topology to SEPIC, buck mode, or
buck-boost mode LED drivers. Please consult the factory
or the LT3797 data sheet for details.
The LT3797 data sheet gives a complete description of
the device, operation and applications information. The
data sheet must be read in conjunction with this demo
manual for DC1784A. The LT3797EUKG is assembled in
a 52-lead (7mm × 8mm) plastic QFN UKG package with
a thermally enhanced ground pad. Proper board layout
is essential for maximum thermal performance. See the
data sheet section 'Layout Considerations'.
CC
Design files for this circuit board are available at
http://www.linear.com/demo/DC1784A
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Technology Corporation. All other trademarks are the property of their respective owners.
LT3797EUKG
dc1784af
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